“dark energy” · the papers and their links

“Cosmologists then explored the possibility that dark energy was dynamical, e.g., in a form of some light scalar field (Ford 1987; Wetterich 1988; Ratra & Peebles 1988; Peebles & Ratra 1988; Fujii & Nishioka 1990; Chiba et al. 1997; Caldwell et al. 1998; Copeland et al. 1998; Ferreira & Joyce 1998;…”

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• We obtain tight, simultaneous limits on the (constant) equation of state of dark energy and the spatial curvature of the universe: -0.14<1+w<0.12 and -0.0179<Ω_k<0.0081.
• We provide a set of “distance priors,” to test a variety of dark energy models with spatial curvature.
• We test a time-dependent w with a present value constrained as -0.33<1+w_0<0.21 (95% CL).

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• By combining WMAP data with other astronomical data, we constrain the geometry of the universe: Ω_tot = 1.02 ± 0.02, and the equation of state of the dark energy, w < -0.78 (95% confidence limit assuming w ≥ -1.)
• we examine non-flat models, dark energy models in which the properties of the dark energy are parameterized by an effective equation of state

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• Investigates whether phantom dark energy models with w<-1, naively unstable due to negative kinetic terms, could be phenomenologically viable when treated as effective field theories valid only up to a momentum cutoff
• Calculates the tree-level decay rate of a phantom particle into other phantoms and gravitons, showing it is naively infinite but can be rendered finite with a cutoff
• Shows that under optimistic assumptions (approximate shift symmetry), the instability timescale can exceed the age of the universe only if the cutoff is at or below 100 MeV, providing a quantitative bound on viable phantom dark energy models

“This situation is also similar in the context of dark energy – there is a degeneracy as for the potential of the scalar field (“quintessence” [111, 634, 267, 263, 615, 503, 257, 155]) due to the observational degeneracy to the dark energy equation of state around w = −1.”

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• This review puts more weight on observational and experimental aspects of f(R) theories compared to other review articles, which is particularly useful to place constraints on inflation and dark energy models based on f(R) theories.
• The paper reviews and systematizes conditions for cosmological viability of f(R) dark energy models and their compatibility with local gravity constraints, including the chameleon mechanism.
• It reviews viable f(R) dark energy models that satisfy both cosmological and local gravity constraints, distinguishing them from earlier models (e.g., f(R)=R-\alpha/R^n) shown to be unstable or incompatible with matter domination.

“Early examples are Weiss (1987) and Wetterich (1988).51 The former considers a quadratic potential with an ultralight effective mass, an idea that reappears in Frieman et al. (1995).”

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• The paper reviews and synthesizes the physics, astronomy, and history of ideas about the cosmological constant/dark energy, assessing the observational evidence and recent developments in searching for a fundamental theory since Weinberg's 1989 review.
• It discusses the possibility that dark energy is dynamical, evolving toward zero, as an approach to alleviate the fine-tuning problem of the cosmological constant.
• It provides an explicit example (zero-point energy of the 3K background radiation) quantifying the magnitude of the cosmological constant problem.

“The energy associated with the vacuum can be positive, negative, or zero. effectively dynamical (Wetterich 1988; Ratra & Peebles 1988; Frieman et al. 1995; Zlatev, Wang & Steinhardt 1999).”

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• Broadly reviews cosmic acceleration for the astronomy community, synthesizing observational evidence (SNe, CMB, LSS, clusters, weak lensing) and theoretical approaches (cosmological constant problem, dark energy models, modified gravity) into a unified treatment
• Provides updated review of dark energy phenomenology and equation-of-state parameterizations (w0-wa) and their observational discrimination
• Frames the deceleration/acceleration condition (p < - ho/3, w<-1/3) as the defining property of dark energy

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• This is a comprehensive review rather than a novel theoretical proposal, aiming to synthesize and compare a wide range of dark energy models (quintessence, k-essence, tachyon, phantom, dilatonic, coupled dark energy, braneworld, modified gravity) within a unified dynamical systems and perturbation-theory framework.
• The review emphasizes cosmological scaling solutions and tracker behavior as organizing principles for classifying scalar-field dark energy models.
• It presents reconstruction techniques for the dark energy equation of state using CMB, large-scale structure, and Supernovae Ia data.

“The simplest physical model for an appropriate dark energy component is a single slowly-rolling scalar field, sometimes referred to as “quintessence” [74, 266, 189, 208, 267, 120, 94, 92, 91, 86, 43, 132].”

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• The review explicitly generalizes 'cosmological constant' to include time-varying dark energy scenarios ('we shall use the term cosmological constant in a generalized sense including the scenarios in which cosmological "constant" is actually varying in time')
• Provides a unified conceptual treatment linking dark energy to scalar field models (quintessence, tachyonic fields) and to the two distinct 'cosmological constant problems' (smallness and coincidence)
• Synthesizes cosmological constant/dark energy discussion with de Sitter thermodynamics, string theory landscape, and relaxation mechanisms as a coherent review

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• Shows that tiny R^n (n<0) corrections to the Einstein-Hilbert action can produce cosmic acceleration, eliminating the need for dark energy as a separate fluid component
• Provides a unified purely gravitational origin for both early-time inflation (R^n, n>0) and late-time acceleration (R^n, n<0), avoiding invocation of dark energy or an inflaton field
• Identifies self-accelerating vacuum solutions (de Sitter and anti-de Sitter) as alternatives to a cosmological constant, with effective equation-of-state parameters mimicking dark energy (w_DE=-1 or w_DE<-2/3) without actual dark energy

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• The five-year data improve constraints on cosmological parameters including dark energy equation of state, with the neutrino mass limit robust to within 10% to a varying dark energy equation of state.
• Ω_Λ= 0.742±0.030 is measured with improved precision compared to three-year data.
• The paper considers extended models with a constant dark energy equation of state w, bounded by w>-2.5, as part of testing extensions beyond simple ΛCDM.

• Formulates three necessary conditions ('cosmon conditions') on the dynamics of the dilaton for a realistic cosmology, including that the trace anomaly vanish and be purely anomalous at the static vacuum value.
• Establishes a connection between the 'cosmon condition' and the short-distance renormalization-group behavior of the underlying fundamental theory, showing the trace anomaly for static configurations equals μ∂V/∂μ.
• Proposes an anomalous renormalization group equation for the cosmological 'constant' (μ∂V/∂μ = A V) leading to a cosmological constant that evolves with time (V_0 ~ (m/χ)^A χ^4), generalizing earlier work where φ̃ rather than φ̃/χ was held fixed.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Sharpens curvature constraint from WMAP alone (Ω_tot=1.05±0.05) to Ω_tot=1.003±0.010 using LRG power spectrum
• Provides a measurement of the equation of state parameter w=-0.94±0.09 assuming flatness, independent of nonlinear scale complications
• Demonstrates that baryon oscillation distance measurement to z=0.35 is robust and independent of assumptions about dark energy properties

“We acknowledge use of the HEALPix (Gorski et al. 2005), CAMB (Lewis et al. 2000), and CMBFAST (Seljak & Zaldarriaga 1996) packages.”

• We obtain tight, simultaneous limits on the (constant) equation of state of dark energy and the spatial curvature of the universe: -0.14<1+w<0.12 and -0.0179<Ω_k<0.0081.
• We provide a set of “distance priors,” to test a variety of dark energy models with spatial curvature.
• We test a time-dependent w with a present value constrained as -0.33<1+w_0<0.21 (95% CL).

• By combining WMAP data with other astronomical data, we constrain the geometry of the universe: Ω_tot = 1.02 ± 0.02, and the equation of state of the dark energy, w < -0.78 (95% confidence limit assuming w ≥ -1.)
• we examine non-flat models, dark energy models in which the properties of the dark energy are parameterized by an effective equation of state

“We acknowledge use of the HEALPix (Gorski et al. 2005), CAMB (Lewis et al. 2000), and CMBFAST (Seljak & Zaldarriaga 1996) packages.”

• Improved constraints on the dark energy equation-of-state parameter w and its time-dependent parameterization (w0, wa) using the combination of 7-year WMAP data with BAO, H0, supernova, and time-delay distance measurements.
• First use of a lens time-delay distance measurement (D_Δt) in combination with WMAP data to constrain dark energy properties.

• Demonstrates that baryon-induced features in the initial conditions are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with next generation surveys.
• Shows for the first time that baryon-induced oscillations detected in the CMB power spectrum survive in distorted form not only in the nonlinear dark matter power spectrum but also in realistically selected galaxy samples at 0<z<3, providing a basis for future surveys to constrain the equation of state of dark energy.

“We use CMBfast (Seljak & Zaldarriaga 1996; Zaldarriaga et al. 1998; Zaldarriaga & Seljak 2000) to compute the linear power spectra, which we convert to correlation functions with a Fourier transform.”

“For a general cosmology, we begin from the CMBfast linear power spectrum.”

• Provides a geometric method (via the baryon acoustic peak) for measuring cosmological distance and thus constraining dark energy, complementary to luminosity-distance methods like Type Ia supernovae.
• First clear detection of the late-time acoustic peak, enabling independent measurement of Ω_m and constraints on dark energy equation of state (w0) and curvature assuming dark energy is a cosmological constant.

• Demonstrates a fast MCMC approach that allows joint constraints on dark energy equation of state w alongside many other cosmological parameters (6, 9, and 11 parameter analyses) with modest additional computational cost.
• Uses importance sampling to incorporate new data (e.g., 2dF, CBI) and assess consistency of datasets when constraining dark energy and other parameters.
• Provides both marginalized and full n-dimensional confidence limits on w, showing how the constraint on dark energy equation of state changes when allowing curvature and quintessence perturbations.

• The model provides an attractor solution for the scalar field's contribution to the energy density that does not require tuning an energy scale characteristic of late times, unlike other scalar-field (dark-energy-like) cosmologies.
• Because the solution is a true dynamical attractor, it avoids the fine-tuning problems inherent in cosmological-constant and other decaying-cosmological-constant models.
• The required exponential potential and the specific λ value needed for structure formation arise naturally in particle physics theories (e.g., Kaluza-Klein compactifications, supergravity/superstring models), giving a theoretically motivated rather than ad hoc dark-energy-like component.

“We have modified CMBFAST (Seljak & Zaldarriaga 1996) accordingly to match the same convention.”

“We thank Uroš Seljak for his help with modifications to CMBFAST.”

“For this section, since this model predicts sharp features in the angular power spectrum, we had to modify the standard CMBFAST splining resolution, splining at ∆l = 1 for 2 ≤ l 50 and ∆l = 5 for l ≥ 50.”

• The five-year data improve constraints on cosmological parameters including dark energy equation of state, with the neutrino mass limit robust to within 10% to a varying dark energy equation of state.
• Ω_Λ= 0.742±0.030 is measured with improved precision compared to three-year data.
• The paper considers extended models with a constant dark energy equation of state w, bounded by w>-2.5, as part of testing extensions beyond simple ΛCDM.

• We explore dark energy constraints in models with a fairly general time dependence of dark energy equation of state, finding Ω_λ=0.72± 0.02, w(z=0.3)=-0.98^+0.10_-0.12
• One method to constrain the nature of dark energy that has not attracted much attention, yet has the potential to produce results on a relatively short time scale, is comparing measurements of amplitude of fluctuations at high redshift from the Lyα forest and CMB to that at low redshift from galaxy clustering.
• We find no evidence for variation of the equation of state with redshift, w(z=1)=-1.03^+0.21_-0.28.

“We acknowledge use of the HEALPix (Gorski et al. 2005), CAMB (Lewis et al. 2000), and CMBFAST (Seljak & Zaldarriaga 1996) packages.”

“CAMB Model power spectra are computed using the Code for Anisotropies in the Microwave Background (CAMB, Lewis et al. 2000), which is based on the earlier code CMBFAST (Seljak & Zaldarriaga 1996).”

• Nine-year WMAP data combined with high-l CMB, BAO, and H0 determine Ω_bh^2, Ω_ch^2, and Ω_Λ each to ~1.5% precision
• Restricting supernova data use to models examining the dark energy equation of state due to residual systematic errors in SN samples
• Updated methodology for combining WMAP with SPT/ACT/BAO/H0/SNe to constrain dark energy equation of state parameters (w, w0, wa) more tightly than previous WMAP releases

• First systematic technique to discover and measure high-redshift Type Ia supernovae in batches to constrain Ω_M and Ω_Λ independently
• Presents the first joint confidence region on the Ω_M–Ω_Λ plane from high-redshift supernovae rather than relying solely on the deceleration parameter q_0
• Notes that alternative energy density components (e.g., topological defects, per Steinhardt 1996) could in principle affect the luminosity-distance relation differently than a simple cosmological constant, though this paper does not address such models due to limited redshift range

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

“The stretch factors is similar to that described in Perlmutter et al. (1997): it parameterizes the brighter-slower relation, originally described in Phillips (1993), by stretching the time axis ofa unique light-curve template;s = 1 is defined in rest-frameB for the time interval−15 to+35 days using…”

• First systematic technique to discover and measure high-redshift Type Ia supernovae in batches to constrain Ω_M and Ω_Λ independently
• Presents the first joint confidence region on the Ω_M–Ω_Λ plane from high-redshift supernovae rather than relying solely on the deceleration parameter q_0
• Notes that alternative energy density components (e.g., topological defects, per Steinhardt 1996) could in principle affect the luminosity-distance relation differently than a simple cosmological constant, though this paper does not address such models due to limited redshift range

• Presents distance measurements to 71 high redshift Type Ia supernovae from the first year of SNLS with improved systematic control
• Provides cosmological fits yielding w = -1.023 ± 0.090 (stat) ± 0.054 (sys) for a flat cosmology with constant equation of state combined with SDSS baryon acoustic oscillation constraints
• Claims improved systematic uncertainty control via single-instrument, multi-band photometry compared to traditional multi-telescope SN surveys

“While some comparison with the stated results of the Supernova Cosmology Project (Perlmutter et al. is possible, an informed1995, 1997, 1998) combination of the data will have to await a similarly detailed description of their measurements.”

“…the discussion from results limited principally by statistical errors to those limited by our depth of understanding of SNe Ia. Published high-redshift SN Ia data are a small fraction of the data in hand both for our team and for the Supernova Cosmology Project (Perlmutter et al. 1995, 1997, 1998).”

“Application of large-format CCDs and sophisticated image analysis techniques by the Supernova Cosmology Project et al. led to the discovery of SN(Perlmutter 1995) 1992bi (z \ 0.46), followed by six more SNe Ia at z B 0.4 et al. Employing a correction for the(Perlmutter 1997). luminosity/light curve shape relation (but none for host galaxy extinction), compar…”

• First systematic technique to discover and measure high-redshift Type Ia supernovae in batches to constrain Ω_M and Ω_Λ independently
• Presents the first joint confidence region on the Ω_M–Ω_Λ plane from high-redshift supernovae rather than relying solely on the deceleration parameter q_0
• Notes that alternative energy density components (e.g., topological defects, per Steinhardt 1996) could in principle affect the luminosity-distance relation differently than a simple cosmological constant, though this paper does not address such models due to limited redshift range

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Combines HST and ground-based photometry of high-z SNe Ia to derive luminosity distances accurate to 10-20%, allowing joint constraints on Ω_m and Ω_Λ
• First use of HST imaging to cleanly separate SN light from host galaxy for high-z SN Ia photometry, improving light curve precision
• Demonstrates that matter density alone (Ω_m) is insufficient for a flat universe at >95% confidence, implying a nonzero cosmological constant contribution is needed for flatness

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

• Argues that a smoothly distributed, time-varying dark energy component is unphysical because it violates the equivalence principle, unlike prior 'smooth' xCDM treatments
• Shows that including fluctuations in the Q-component breaks the near-degeneracy with ΛCDM found when the component is treated as spatially smooth
• Computes CMB anisotropy and mass power spectra for a wide, representative class of quintessence models including scalar field potentials (exponential, cosine) with self-consistent fluctuations

• By combining WMAP data with other astronomical data, we constrain the geometry of the universe: Ω_tot = 1.02 ± 0.02, and the equation of state of the dark energy, w < -0.78 (95% confidence limit assuming w ≥ -1.)
• we examine non-flat models, dark energy models in which the properties of the dark energy are parameterized by an effective equation of state

“If one generalizes to larger parameter spaces, e.g., adding an unknown dark energy equation of state w(z) (Turner &White 1997; Caldwell et al. 1998) or a non-zero curvature, then a parameter degeneracy opens in the CMB (e.g, Eisenstein, Hu, & Tegmark 1998; Efstathiou & Bond 1999).”

• Argues that a smoothly distributed, time-varying dark energy component is unphysical because it violates the equivalence principle, unlike prior 'smooth' xCDM treatments
• Shows that including fluctuations in the Q-component breaks the near-degeneracy with ΛCDM found when the component is treated as spatially smooth
• Computes CMB anisotropy and mass power spectra for a wide, representative class of quintessence models including scalar field potentials (exponential, cosine) with self-consistent fluctuations

• Provides a geometric method (via the baryon acoustic peak) for measuring cosmological distance and thus constraining dark energy, complementary to luminosity-distance methods like Type Ia supernovae.
• First clear detection of the late-time acoustic peak, enabling independent measurement of Ω_m and constraints on dark energy equation of state (w0) and curvature assuming dark energy is a cosmological constant.

• Argues that a smoothly distributed, time-varying dark energy component is unphysical because it violates the equivalence principle, unlike prior 'smooth' xCDM treatments
• Shows that including fluctuations in the Q-component breaks the near-degeneracy with ΛCDM found when the component is treated as spatially smooth
• Computes CMB anisotropy and mass power spectra for a wide, representative class of quintessence models including scalar field potentials (exponential, cosine) with self-consistent fluctuations

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“52 For recent discussions of this model see Ferreira and Joyce (1998), Ott (2001), Hwang and Noh (2001), and references therein.”

• The model provides an attractor solution for the scalar field's contribution to the energy density that does not require tuning an energy scale characteristic of late times, unlike other scalar-field (dark-energy-like) cosmologies.
• Because the solution is a true dynamical attractor, it avoids the fine-tuning problems inherent in cosmological-constant and other decaying-cosmological-constant models.
• The required exponential potential and the specific λ value needed for structure formation arise naturally in particle physics theories (e.g., Kaluza-Klein compactifications, supergravity/superstring models), giving a theoretically motivated rather than ad hoc dark-energy-like component.

• The paper reviews and synthesizes the physics, astronomy, and history of ideas about the cosmological constant/dark energy, assessing the observational evidence and recent developments in searching for a fundamental theory since Weinberg's 1989 review.
• It discusses the possibility that dark energy is dynamical, evolving toward zero, as an approach to alleviate the fine-tuning problem of the cosmological constant.
• It provides an explicit example (zero-point energy of the 3K background radiation) quantifying the magnitude of the cosmological constant problem.

• The model provides an attractor solution for the scalar field's contribution to the energy density that does not require tuning an energy scale characteristic of late times, unlike other scalar-field (dark-energy-like) cosmologies.
• Because the solution is a true dynamical attractor, it avoids the fine-tuning problems inherent in cosmological-constant and other decaying-cosmological-constant models.
• The required exponential potential and the specific λ value needed for structure formation arise naturally in particle physics theories (e.g., Kaluza-Klein compactifications, supergravity/superstring models), giving a theoretically motivated rather than ad hoc dark-energy-like component.

• This is a comprehensive review rather than a novel theoretical proposal, aiming to synthesize and compare a wide range of dark energy models (quintessence, k-essence, tachyon, phantom, dilatonic, coupled dark energy, braneworld, modified gravity) within a unified dynamical systems and perturbation-theory framework.
• The review emphasizes cosmological scaling solutions and tracker behavior as organizing principles for classifying scalar-field dark energy models.
• It presents reconstruction techniques for the dark energy equation of state using CMB, large-scale structure, and Supernovae Ia data.

“The simplest physical model for an appropriate dark energy component is a single slowly-rolling scalar field, sometimes referred to as “quintessence” [74, 266, 189, 208, 267, 120, 94, 92, 91, 86, 43, 132].”

“To date, investigations have focused on scaling or tracker models of quintessence, in which the scalar field energy density can parallel that of matter or radiation, at least for part of its history [86, 62, 279, 158, 232, 278, 219].”

• The model provides an attractor solution for the scalar field's contribution to the energy density that does not require tuning an energy scale characteristic of late times, unlike other scalar-field (dark-energy-like) cosmologies.
• Because the solution is a true dynamical attractor, it avoids the fine-tuning problems inherent in cosmological-constant and other decaying-cosmological-constant models.
• The required exponential potential and the specific λ value needed for structure formation arise naturally in particle physics theories (e.g., Kaluza-Klein compactifications, supergravity/superstring models), giving a theoretically motivated rather than ad hoc dark-energy-like component.

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

• The model provides an attractor solution for the scalar field's contribution to the energy density that does not require tuning an energy scale characteristic of late times, unlike other scalar-field (dark-energy-like) cosmologies.
• Because the solution is a true dynamical attractor, it avoids the fine-tuning problems inherent in cosmological-constant and other decaying-cosmological-constant models.
• The required exponential potential and the specific λ value needed for structure formation arise naturally in particle physics theories (e.g., Kaluza-Klein compactifications, supergravity/superstring models), giving a theoretically motivated rather than ad hoc dark-energy-like component.

• The review explicitly generalizes 'cosmological constant' to include time-varying dark energy scenarios ('we shall use the term cosmological constant in a generalized sense including the scenarios in which cosmological "constant" is actually varying in time')
• Provides a unified conceptual treatment linking dark energy to scalar field models (quintessence, tachyonic fields) and to the two distinct 'cosmological constant problems' (smallness and coincidence)
• Synthesizes cosmological constant/dark energy discussion with de Sitter thermodynamics, string theory landscape, and relaxation mechanisms as a coherent review

• The model provides an attractor solution for the scalar field's contribution to the energy density that does not require tuning an energy scale characteristic of late times, unlike other scalar-field (dark-energy-like) cosmologies.
• Because the solution is a true dynamical attractor, it avoids the fine-tuning problems inherent in cosmological-constant and other decaying-cosmological-constant models.
• The required exponential potential and the specific λ value needed for structure formation arise naturally in particle physics theories (e.g., Kaluza-Klein compactifications, supergravity/superstring models), giving a theoretically motivated rather than ad hoc dark-energy-like component.

• The five-year data improve constraints on cosmological parameters including dark energy equation of state, with the neutrino mass limit robust to within 10% to a varying dark energy equation of state.
• Ω_Λ= 0.742±0.030 is measured with improved precision compared to three-year data.
• The paper considers extended models with a constant dark energy equation of state w, bounded by w>-2.5, as part of testing extensions beyond simple ΛCDM.

• The model provides an attractor solution for the scalar field's contribution to the energy density that does not require tuning an energy scale characteristic of late times, unlike other scalar-field (dark-energy-like) cosmologies.
• Because the solution is a true dynamical attractor, it avoids the fine-tuning problems inherent in cosmological-constant and other decaying-cosmological-constant models.
• The required exponential potential and the specific λ value needed for structure formation arise naturally in particle physics theories (e.g., Kaluza-Klein compactifications, supergravity/superstring models), giving a theoretically motivated rather than ad hoc dark-energy-like component.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“In order to analyze the stability of the above fixed points it is sufficient to consider only timedependent linear perturbations δxi(t) (i = 1, 2, 3) around them (see [170, 171] for the detail of such analysis).”

• Shows that the scaling solution (where scalar field energy density tracks the barotropic fluid) is the unique late-time attractor whenever λ² > 3γ
• Demonstrates that fluid-dominated solutions are always unstable except in the cosmological constant case, meaning the scalar field's energy density never vanishes relative to other matter
• Applies nucleosynthesis constraints to derive a 'relic density' problem for such scalar fields with λ² < 20

• This review puts more weight on observational and experimental aspects of f(R) theories compared to other review articles, which is particularly useful to place constraints on inflation and dark energy models based on f(R) theories.
• The paper reviews and systematizes conditions for cosmological viability of f(R) dark energy models and their compatibility with local gravity constraints, including the chameleon mechanism.
• It reviews viable f(R) dark energy models that satisfy both cosmological and local gravity constraints, distinguishing them from earlier models (e.g., f(R)=R-\alpha/R^n) shown to be unstable or incompatible with matter domination.

• Shows that the scaling solution (where scalar field energy density tracks the barotropic fluid) is the unique late-time attractor whenever λ² > 3γ
• Demonstrates that fluid-dominated solutions are always unstable except in the cosmological constant case, meaning the scalar field's energy density never vanishes relative to other matter
• Applies nucleosynthesis constraints to derive a 'relic density' problem for such scalar fields with λ² < 20

• This is a comprehensive review rather than a novel theoretical proposal, aiming to synthesize and compare a wide range of dark energy models (quintessence, k-essence, tachyon, phantom, dilatonic, coupled dark energy, braneworld, modified gravity) within a unified dynamical systems and perturbation-theory framework.
• The review emphasizes cosmological scaling solutions and tracker behavior as organizing principles for classifying scalar-field dark energy models.
• It presents reconstruction techniques for the dark energy equation of state using CMB, large-scale structure, and Supernovae Ia data.

“To date, investigations have focused on scaling or tracker models of quintessence, in which the scalar field energy density can parallel that of matter or radiation, at least for part of its history [86, 62, 279, 158, 232, 278, 219].”

• Shows that the scaling solution (where scalar field energy density tracks the barotropic fluid) is the unique late-time attractor whenever λ² > 3γ
• Demonstrates that fluid-dominated solutions are always unstable except in the cosmological constant case, meaning the scalar field's energy density never vanishes relative to other matter
• Applies nucleosynthesis constraints to derive a 'relic density' problem for such scalar fields with λ² < 20

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

• Shows that the scaling solution (where scalar field energy density tracks the barotropic fluid) is the unique late-time attractor whenever λ² > 3γ
• Demonstrates that fluid-dominated solutions are always unstable except in the cosmological constant case, meaning the scalar field's energy density never vanishes relative to other matter
• Applies nucleosynthesis constraints to derive a 'relic density' problem for such scalar fields with λ² < 20

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“Ignoring the mass of neutrinos and modifications to gravity, one can obtain the growth rate by solving a single differential equation (Wang & Steinhardt 1998; Linder & Jenkins 2003).33 The 7-year normalization prior is ∆2R(kWMAP ) = (2.208 ± 0.078) × 10−9 (68% CL), where kWMAP = 0.027 Mpc −1.”

• Derivation of a general expression for γ (in σ_8 Ω_m^γ) applicable to a wide range of models including sCDM, ΛCDM, and QCDM with quintessence, including dependence on spectral index n, Hubble constant h, and equation-of-state w
• New treatment of the mass-temperature relation and virial theorem generalized to quintessence models where the equation-of-state affects background evolution but the component does not cluster
• Derivation of a growth index formula f_g = Ω_m(z)^α with an explicit expression for α depending on w, extending growth factor calculations beyond ΛCDM to general quintessence

• Improved constraints on the dark energy equation-of-state parameter w and its time-dependent parameterization (w0, wa) using the combination of 7-year WMAP data with BAO, H0, supernova, and time-delay distance measurements.
• First use of a lens time-delay distance measurement (D_Δt) in combination with WMAP data to constrain dark energy properties.

• Derivation of a general expression for γ (in σ_8 Ω_m^γ) applicable to a wide range of models including sCDM, ΛCDM, and QCDM with quintessence, including dependence on spectral index n, Hubble constant h, and equation-of-state w
• New treatment of the mass-temperature relation and virial theorem generalized to quintessence models where the equation-of-state affects background evolution but the component does not cluster
• Derivation of a growth index formula f_g = Ω_m(z)^α with an explicit expression for α depending on w, extending growth factor calculations beyond ΛCDM to general quintessence

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Introduces and generalizes 'tracker fields' as a form of quintessence that avoids the coincidence problem via attractor-like convergence from a huge range of initial conditions
• Derives a general condition (Γ ≡ V''V/(V')^2 > 1 and nearly constant) for potentials to admit tracker solutions, independent of solving the equation of motion directly
• Predicts a distinguishing Ω_Q-w_Q relation for tracker quintessence models versus a cosmological constant

• We explore dark energy constraints in models with a fairly general time dependence of dark energy equation of state, finding Ω_λ=0.72± 0.02, w(z=0.3)=-0.98^+0.10_-0.12
• One method to constrain the nature of dark energy that has not attracted much attention, yet has the potential to produce results on a relatively short time scale, is comparing measurements of amplitude of fluctuations at high redshift from the Lyα forest and CMB to that at low redshift from galaxy clustering.
• We find no evidence for variation of the equation of state with redshift, w(z=1)=-1.03^+0.21_-0.28.

• Introduces and generalizes 'tracker fields' as a form of quintessence that avoids the coincidence problem via attractor-like convergence from a huge range of initial conditions
• Derives a general condition (Γ ≡ V''V/(V')^2 > 1 and nearly constant) for potentials to admit tracker solutions, independent of solving the equation of motion directly
• Predicts a distinguishing Ω_Q-w_Q relation for tracker quintessence models versus a cosmological constant

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“…dozen years have passed since combined observations of nearby and distant Type Ia Supernovae (SNe Ia) demonstrated that the expansion of the Universe is accelerating at the current epoch (Perlmutter et al. 1998; Garnavich et al. 1998; Schmidt et al. 1998; Riess et al. 1998; Perlmutter et al. 1999).”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Adding these supernovae improves the best combined constraint on dark energy density, ρ_DE(z), at redshifts 1.0 < z < 1.6 by 18% (including systematic errors).
• Nearly doubling the statistical weight of HST-discovered SNe Ia beyond z=1.
• Corrects for the recently identified correlation between luminosity and host galaxy mass and corrects the NICMOS zeropoint at count rates appropriate for very distant SNe Ia.

“It also reproduces the present acceleration of the cosmic expansion inferred from super- E-mail: darren@astro.berkeley.edu nova observations (Riess et al. 1998; Perlmutter et al. 1999), and it is consistent with the mass budget inferred for the present Universe from the dynamics of large-scale…”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

“While highredshift SNe Ia reveal that the universe is now accelerating (Riess et al. 1998; Perlmutter et al. 1999), nearby ones provide the most precise measurements of the present expansion rate, H0.”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Provides an improved, more precise measurement of H0 (3.3% uncertainty) that, combined with WMAP7 data, yields a tighter constraint on the dark energy equation-of-state parameter w than previous determinations.
• Uses this improved H0 measurement to rule out the best-fitting gigaparsec-scale void models posited as an alternative to dark energy.

“…of State: SupErNovae trace Cosmic Expansion (ESSENCE) survey (Wood-Vasey et al. 2007), as well as those used in the original papers of the discovery of the acceleration of the universe (Riess et al. 1998; Perlmutter et al. 1999), and the samples from Barris et al. (2004); Tonry et al. (2003).”

“The issue is the following: when the luminosity distances out to Type Ia supernovae (Riess et al. 1998; Perlmutter et al. 1999) and the angular diameter distances measured from the BAO (Eisenstein et al. 2005) as well as CMB (Bennett et al. 2003b) are put together in the context of homogeneous and…”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• We obtain tight, simultaneous limits on the (constant) equation of state of dark energy and the spatial curvature of the universe: -0.14<1+w<0.12 and -0.0179<Ω_k<0.0081.
• We provide a set of “distance priors,” to test a variety of dark energy models with spatial curvature.
• We test a time-dependent w with a present value constrained as -0.33<1+w_0<0.21 (95% CL).

“They have been the key element in the discovery that the universe is accelerating and dominated by dark energy (e.g., Riess et al. 1998; Perlmutter et al. 1999; Knop et al. 2003; Tonry et al. 2003; Barris et al. 2004; Astier et al. 2006; Riess et al. 2007; Wood-Vasey et al. 2007; Davis et al. 2007;…”

“For the other three fitters, we do not use all the high-redshift SN Ia samples, specifically excluding ∼ 100 objects from the SCP and HighZ objects from Riess et al. (1998), Perlmutter et al. (1999), Knop et al. (2003), Tonry et al. (2003), and Barris et al. (2004).”

“For example, the ESSENCE team modifies the Galactic Line of Sight (glos) prior (Riess et al. 1998; Hatano, Branch, & Deaton 1998; Commins 2004; Riess et al. 2005; Riello & Patat 2005) into the redshift-dependent “glosz” to take into account the redshiftdependent detection probabilities of their…”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Combines the new CfA3 SN Ia sample with literature samples (forming the 'Constitution' set) to produce improved, more precise constraints on the dark energy equation of state parameter w
• Uses four independent light-curve fitters (SALT, SALT2, MLCS2k2 with two R_V values) to test for systematic differences affecting dark energy constraints, for the first time on a sample not used to train them
• Identifies and quantifies specific systematic effects (host-galaxy extinction overestimation, Hubble residual trends, population differences by host morphology) that limit current dark energy measurements, reducing statistical uncertainty to the point where systematics dominate

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• By combining WMAP data with other astronomical data, we constrain the geometry of the universe: Ω_tot = 1.02 ± 0.02, and the equation of state of the dark energy, w < -0.78 (95% confidence limit assuming w ≥ -1.)
• we examine non-flat models, dark energy models in which the properties of the dark energy are parameterized by an effective equation of state

“…luminosity distances out to high-z Type Ia supernovae have been the most powerful data for first discovering the existence of dark energy (Riess et al. 1998; Perlmutter et al. 1999) and then constraining the properties of dark energy, such as the equation of state parameter, w (see…”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Improved constraints on the dark energy equation-of-state parameter w and its time-dependent parameterization (w0, wa) using the combination of 7-year WMAP data with BAO, H0, supernova, and time-delay distance measurements.
• First use of a lens time-delay distance measurement (D_Δt) in combination with WMAP data to constrain dark energy properties.

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Demonstrates that baryon-induced features in the initial conditions are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with next generation surveys.
• Shows for the first time that baryon-induced oscillations detected in the CMB power spectrum survive in distorted form not only in the nonlinear dark matter power spectrum but also in realistically selected galaxy samples at 0<z<3, providing a basis for future surveys to constrain the equation of state of dark energy.

“5.1. mlcs2k2 Fitting Method The Multicolor Light Curve Shape method, known as mlcs2k2 in its current incarnation (JRK07), has been in use for more than a decade; the original MLCS version (Riess et al. 1998) was used by the High-z Supernova Team in the discovery of cosmic acceleration.”

“Ten years ago, measurements of the Hubble diagram of Type Ia supernovae (SNe) provided the first direct evidence for cosmic acceleration (Riess et al. 1998; Perlmutter et al. 1999).”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Use of detailed Monte Carlo simulations of all surveys to account for selection biases, including spectroscopic targeting, as a new feature in the analysis
• Filling in the redshift 'desert' between low- and high-redshift SN Ia surveys with SDSS-II SN Survey data
• Detailed comparison and systematic error analysis of mlcs2k2 vs salt-ii light-curve fitting methods, tracing discrepancies to rest-frame UV modeling and luminosity-color corrections

“…densities inthe Universe", Bartlett J., Dumarchez J. eds.,Editions Frontieres, Paris - astro-ph/0006052S. Perlmutter et al., ApJ, 517, 565(1999)Riess et al., 1998, AJ, 116, 1009Rao, S.M., D. R. Wilton, and A. W. Glis-son,\Electromagnetic scattering by surfaces ofarbitrary shape", IEEE Trans.”

“The LSS (Bond & Ja e 1999) and SN1a supernovae (Riess et al. 1998; Perlmutter et al. 1999) priors are as described in Langeet al. (2001).”

“Considered hereis the impact of applying a stronger constrainton h, constraints derived from measurements oflarge scale structure (LSS)(Bond & Ja e 1999),results from recent measurements of type Ia super-novae (Riess et al. 1998; Perlmutter et al. 1999),and the theoretical bias that tot = 1.”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Presents a new, larger BOOMERANG dataset (four 150 GHz channels, full flight) with improved beam and pointing characterization, enabling detection of multiple acoustic peaks and tighter constraints on Ω_tot, Ω_bh^2, Ω_ch^2, n_s, and by extension Ω_Λ.
• Combines CMB data with LSS and SN1a priors to jointly constrain Ω_Λ and other cosmological parameters, showing consistency with a Λ-dominated, low-curvature adiabatic CDM model.
• Provides refined determination of Ω_Λ (~0.5–0.7 depending on priors) as part of a 7-dimensional parameter extraction using new analysis pipeline (MASTER-based power spectrum estimation).

“About a decade ago, combined observations of nearby and distant SNe Ia led to the discovery of the accelerating universe (Perlmutter et al. 1998; Garnavich et al. 1998; Schmidt et al. 1998; Riess et al. 1998; Perlmutter et al. 1999).”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Presents new light curves and spectra of six SNe Ia discovered in 2001, including ground-based J-band photometry for two SNe with z>1
• Combines these new SNe with other recent data into an improved compilation called Union2, consisting of 557 supernovae
• Refits all light curves with the SALT2 fitter and improves handling of systematic errors compared to the earlier Union compilation

“…can provide precise and robust constraints (Blake & Glazebrook 2003; Hu & Haiman 2003; Linder 2003; Seo & Eisenstein 2003; Amendola et al. 2004; Dolney, Jain, & Takada 2004; Matsubara 2004) on the acceleration of the expansion rate of the universe (Riess et al. 1998; Perlmutter et al. 1999).”

“The acoustic peak method could provide a geometric complement to the usual luminosity-distance methods such as those based on type Ia supernovae (e.g. Riess et al. 1998; Perlmutter et al. 1999; Knop et al. 2003; Tonry et al. 2003; Riess et al. 2004).”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Provides a geometric method (via the baryon acoustic peak) for measuring cosmological distance and thus constraining dark energy, complementary to luminosity-distance methods like Type Ia supernovae.
• First clear detection of the late-time acoustic peak, enabling independent measurement of Ω_m and constraints on dark energy equation of state (w0) and curvature assuming dark energy is a cosmological constant.

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• When combined with CMB constraints, we measure a constant dark-energy equation of state parameter w=-1.018 ± 0.057(stat+sys) for a flat universe.
• Our supernova measurements provide the most stringent constraints to date on the nature of dark energy.
• Section uses additional astrophysical probes in combination with SNe Ia to break degeneracies and constrain dark energy in more generic models.

“It is worth comparing the measurements from future redshift surveys to those inferred from the observations of type Ia supernovae (hereafter SNe) (Riess et al. 1998; Perlmutter et al. 1999; Riess et al. 2001; Tonry et al. 2003).”

“Recent observations of distant type Ia supernovae have reached the startling conclusion that the expansion of the Universe is accelerating (Perlmutter et al. 1999; Riess et al. 1998, 2001; Tonry et al. 2003).”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Demonstrates that baryonic acoustic oscillations in large high-redshift galaxy surveys offer a precision route to measuring dark energy via H(z) and D_A(z)
• Uses a full Fisher matrix formalism to treat cosmological constraints from large-scale structure, CMB anisotropies, and supernova data simultaneously, including a time-variable equation of state, extending prior work
• Provides an explicit treatment of survey data sets combined with dark energy parameter estimation, differing from previous studies

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• The five-year data improve constraints on cosmological parameters including dark energy equation of state, with the neutrino mass limit robust to within 10% to a varying dark energy equation of state.
• Ω_Λ= 0.742±0.030 is measured with improved precision compared to three-year data.
• The paper considers extended models with a constant dark energy equation of state w, bounded by w>-2.5, as part of testing extensions beyond simple ΛCDM.

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“Making the light curves public, as was done for the results of the HZT and its successors Riess et al. (1998); Tonry et al. (2003); Barris et al. (2004); Krisciunas et al. (2005); Clocchiatti et al. (2006), by Knop et al. (2003), by Riess et al. (2004) for the very high redshift HST supernova program, and for the low-z data of Hamuy et al. (1996), Riess et a…”

“The first SN Ia cosmology results using 7 high-redshift SNe Ia(Perlmutter et al. 1997) found a Universe consistent with ΩM= 1 but subsequent work by the SCP (Perlmutter et al. 1998) and by the HZT (Garnavich et al. 1998) revised this initial finding to favor a lower value of ΩM.”

“Making the light curves public, as was done for the results of the HZT and its successors Riess et al. (1998); Tonry et al. (2003); Barris et al. (2004); Krisciunas et al. (2005); Clocchiatti et al. (2006), by Knop et al. (2003), by Riess et al. (2004) for the very high redshift HST supernova…”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Presents first cosmological results from the ESSENCE survey constraining the dark energy equation-of-state parameter w with supernova data over redshift 0.15–0.70
• Combines ESSENCE with SNLS to obtain a joint, tighter constraint on w and Ω_M consistent with a cosmological constant
• Develops a redshift-dependent host-galaxy extinction prior ('glosz') derived from detailed Monte Carlo simulations of the ESSENCE selection function to reduce systematic bias in dark energy inference

“Type Ia Supernovae The first direct evidence for acceleration in the expansion of the universe came from measurements of luminosity distance as a function of redshift using Type Ia supernovae as standard candles (Riess et al. 1998; Schmidt et al. 1998; Perlmutter et al. 1999).”

• Expands the high-redshift SN Ia sample from 4 to 16 objects combined with 34 nearby SNe to place tighter constraints on Ω_M, Ω_Λ, q_0, and t_0.
• Provides detailed methodology (light curve fitting via MLCS and Δm15(B) template methods, K-corrections, photometric calibration) allowing independent verification of results.
• Reports that spectroscopically confirmed SNe Ia are statistically consistent with q_0<0 and Ω_Λ>0 at high confidence (up to 9σ for flat-universe prior), providing strong evidence for a currently accelerating universe.

• Nine-year WMAP data combined with high-l CMB, BAO, and H0 determine Ω_bh^2, Ω_ch^2, and Ω_Λ each to ~1.5% precision
• Restricting supernova data use to models examining the dark energy equation of state due to residual systematic errors in SN samples
• Updated methodology for combining WMAP with SPT/ACT/BAO/H0/SNe to constrain dark energy equation of state parameters (w, w0, wa) more tightly than previous WMAP releases

“While the standard model predicts CTBl = 0 and hence 〈Ur〉 = 0, models in which the global parity symmetry is violated can create CTBl = sin(2∆α)C TE l (Lue et al. 1999; Carroll 1998; Feng et al. 2005).”

“Parity Violation While the TB and EB correlations vanish in a parity-conserving universe, they may not vanish when global parity symmetry is broken on cosmological scales (Lue et al. 1999; Carroll 1998).”

• Explores whether an approximate global symmetry can suppress couplings of the quintessence field to ordinary matter, allowing it to evade existing constraints on long-range forces and time variation of constants.
• Identifies a derivative coupling of quintessence to the electromagnetic pseudoscalar F_{\mu\nu}\tilde F^{\mu\nu} as an allowed, symmetry-respecting interaction that could rotate polarization of light from distant sources, offering a potential observational signature of quintessence.

• Improved constraints on the dark energy equation-of-state parameter w and its time-dependent parameterization (w0, wa) using the combination of 7-year WMAP data with BAO, H0, supernova, and time-delay distance measurements.
• First use of a lens time-delay distance measurement (D_Δt) in combination with WMAP data to constrain dark energy properties.

• Explores whether an approximate global symmetry can suppress couplings of the quintessence field to ordinary matter, allowing it to evade existing constraints on long-range forces and time variation of constants.
• Identifies a derivative coupling of quintessence to the electromagnetic pseudoscalar F_{\mu\nu}\tilde F^{\mu\nu} as an allowed, symmetry-respecting interaction that could rotate polarization of light from distant sources, offering a potential observational signature of quintessence.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Introduces the notion of a "tracker field," a form of quintessence, to explain the cosmic coincidence problem
• Shows that tracker solutions are attractor-like but not fixed points, unlike previously studied self-adjusting quintessence solutions
• Demonstrates that tracker models are extremely insensitive to initial conditions (variations by ~100 orders of magnitude) while matching CMB, large-scale structure, and supernova acceleration data

• Provides tight, robust constraints on the dark energy equation of state w=-0.97±0.10 for a constant dark energy equation of state by combining BAO, supernova, and WMAP5 data.
• Shows that the BAO distance constraint on Ω_m and H_0 is independent of the behaviour of dark energy at redshifts greater than those probed by the BAO and supernova measurements.

“…Carroll, Press, & Turner 1992) has a constant equation of state of −1, while general quintessence models (Caldwell et al. 1998) and other theories (Zlatev et al. 1999; Bucher & Spergel 1999; Armendariz-Picon et al. 2000; Boyle et al. 2001; Gu & Hwang 2001; Kasuya 2001; Bilic et al. 2002;…”

• Introduces the notion of a "tracker field," a form of quintessence, to explain the cosmic coincidence problem
• Shows that tracker solutions are attractor-like but not fixed points, unlike previously studied self-adjusting quintessence solutions
• Demonstrates that tracker models are extremely insensitive to initial conditions (variations by ~100 orders of magnitude) while matching CMB, large-scale structure, and supernova acceleration data

• Demonstrates that baryonic acoustic oscillations in large high-redshift galaxy surveys offer a precision route to measuring dark energy via H(z) and D_A(z)
• Uses a full Fisher matrix formalism to treat cosmological constraints from large-scale structure, CMB anisotropies, and supernova data simultaneously, including a time-variable equation of state, extending prior work
• Provides an explicit treatment of survey data sets combined with dark energy parameter estimation, differing from previous studies

• Introduces the notion of a "tracker field," a form of quintessence, to explain the cosmic coincidence problem
• Shows that tracker solutions are attractor-like but not fixed points, unlike previously studied self-adjusting quintessence solutions
• Demonstrates that tracker models are extremely insensitive to initial conditions (variations by ~100 orders of magnitude) while matching CMB, large-scale structure, and supernova acceleration data

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“The first study on the equation of state produced a 95%-confidence limit of 1+w < 0.3, assuming ΩM ∼ 0.2 and zero possibility of 1 + w < 0 (Garnavich et al. 1998).”

• Places new observational constraints on the equation of state parameter of the unknown dark energy component using an expanded Type Ia supernova sample
• Rules out topological defects (strings, textures) and disfavors domain walls as the dominant dark energy component at high confidence
• Combines supernova data with CMB first acoustic peak measurements to jointly constrain matter density and the unknown energy component, providing evidence for a flat universe

• Combines the new CfA3 SN Ia sample with literature samples (forming the 'Constitution' set) to produce improved, more precise constraints on the dark energy equation of state parameter w
• Uses four independent light-curve fitters (SALT, SALT2, MLCS2k2 with two R_V values) to test for systematic differences affecting dark energy constraints, for the first time on a sample not used to train them
• Identifies and quantifies specific systematic effects (host-galaxy extinction overestimation, Hubble residual trends, population differences by host morphology) that limit current dark energy measurements, reducing statistical uncertainty to the point where systematics dominate

• Places new observational constraints on the equation of state parameter of the unknown dark energy component using an expanded Type Ia supernova sample
• Rules out topological defects (strings, textures) and disfavors domain walls as the dominant dark energy component at high confidence
• Combines supernova data with CMB first acoustic peak measurements to jointly constrain matter density and the unknown energy component, providing evidence for a flat universe

• By combining WMAP data with other astronomical data, we constrain the geometry of the universe: Ω_tot = 1.02 ± 0.02, and the equation of state of the dark energy, w < -0.78 (95% confidence limit assuming w ≥ -1.)
• we examine non-flat models, dark energy models in which the properties of the dark energy are parameterized by an effective equation of state

“About a decade ago, combined observations of nearby and distant SNe Ia led to the discovery of the accelerating universe (Perlmutter et al. 1998; Garnavich et al. 1998; Schmidt et al. 1998; Riess et al. 1998; Perlmutter et al. 1999).”

• Places new observational constraints on the equation of state parameter of the unknown dark energy component using an expanded Type Ia supernova sample
• Rules out topological defects (strings, textures) and disfavors domain walls as the dominant dark energy component at high confidence
• Combines supernova data with CMB first acoustic peak measurements to jointly constrain matter density and the unknown energy component, providing evidence for a flat universe

• Presents new light curves and spectra of six SNe Ia discovered in 2001, including ground-based J-band photometry for two SNe with z>1
• Combines these new SNe with other recent data into an improved compilation called Union2, consisting of 557 supernovae
• Refits all light curves with the SALT2 fitter and improves handling of systematic errors compared to the earlier Union compilation

• Places new observational constraints on the equation of state parameter of the unknown dark energy component using an expanded Type Ia supernova sample
• Rules out topological defects (strings, textures) and disfavors domain walls as the dominant dark energy component at high confidence
• Combines supernova data with CMB first acoustic peak measurements to jointly constrain matter density and the unknown energy component, providing evidence for a flat universe

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“The Sheth–Tormen mass function [535] and the Navarro–Frenk–White halo profile [449] are usually employed in GR.”

• This review puts more weight on observational and experimental aspects of f(R) theories compared to other review articles, which is particularly useful to place constraints on inflation and dark energy models based on f(R) theories.
• The paper reviews and systematizes conditions for cosmological viability of f(R) dark energy models and their compatibility with local gravity constraints, including the chameleon mechanism.
• It reviews viable f(R) dark energy models that satisfy both cosmological and local gravity constraints, distinguishing them from earlier models (e.g., f(R)=R-\alpha/R^n) shown to be unstable or incompatible with matter domination.

“To obtain the comoving number density of halos per logarithmic interval in the virial mass, and the linear bias, they use the Sheth-Tormen method [1137], while they use the Navarro-Frenk-White [946] form for halo profiles.”

• We explore dark energy constraints in models with a fairly general time dependence of dark energy equation of state, finding Ω_λ=0.72± 0.02, w(z=0.3)=-0.98^+0.10_-0.12
• One method to constrain the nature of dark energy that has not attracted much attention, yet has the potential to produce results on a relatively short time scale, is comparing measurements of amplitude of fluctuations at high redshift from the Lyα forest and CMB to that at low redshift from galaxy clustering.
• We find no evidence for variation of the equation of state with redshift, w(z=1)=-1.03^+0.21_-0.28.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• We find all the critical points of the system for constant equations of state in that range.
• We consider further several background quantities that can distinguish the models with different w_X values.
• Using a simple toy model with a varying equation of state, we show that even a large variation of w_X at small redshifts is very difficult to observe with d_L(z) measurements up to z∼1.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• We obtain tight, simultaneous limits on the (constant) equation of state of dark energy and the spatial curvature of the universe: -0.14<1+w<0.12 and -0.0179<Ω_k<0.0081.
• We provide a set of “distance priors,” to test a variety of dark energy models with spatial curvature.
• We test a time-dependent w with a present value constrained as -0.33<1+w_0<0.21 (95% CL).

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Investigates whether phantom dark energy models with w<-1, naively unstable due to negative kinetic terms, could be phenomenologically viable when treated as effective field theories valid only up to a momentum cutoff
• Calculates the tree-level decay rate of a phantom particle into other phantoms and gravitons, showing it is naively infinite but can be rendered finite with a cutoff
• Shows that under optimistic assumptions (approximate shift symmetry), the instability timescale can exceed the age of the universe only if the cutoff is at or below 100 MeV, providing a quantitative bound on viable phantom dark energy models

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• This review puts more weight on observational and experimental aspects of f(R) theories compared to other review articles, which is particularly useful to place constraints on inflation and dark energy models based on f(R) theories.
• The paper reviews and systematizes conditions for cosmological viability of f(R) dark energy models and their compatibility with local gravity constraints, including the chameleon mechanism.
• It reviews viable f(R) dark energy models that satisfy both cosmological and local gravity constraints, distinguishing them from earlier models (e.g., f(R)=R-\alpha/R^n) shown to be unstable or incompatible with matter domination.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• The paper reviews and synthesizes the physics, astronomy, and history of ideas about the cosmological constant/dark energy, assessing the observational evidence and recent developments in searching for a fundamental theory since Weinberg's 1989 review.
• It discusses the possibility that dark energy is dynamical, evolving toward zero, as an approach to alleviate the fine-tuning problem of the cosmological constant.
• It provides an explicit example (zero-point energy of the 3K background radiation) quantifying the magnitude of the cosmological constant problem.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Proposes that dark energy and dark matter could be reinterpreted as 'shortcomings' of General Relativity rather than as real exotic components, and that extending gravity (via f(R) and scalar-tensor Extended Theories of Gravity) can reproduce accelerated expansion and other 'dark' phenomena as curvature effects.
• Claims that dark energy and quintessence issues can be addressed as curvature effects within Extended Theories of Gravity, offering cosmological models fitted to observational data without adding unknown exotic ingredients to the cosmic pie.
• Argues for changing the gravitational side of Einstein's equations (via non-linear Lagrangians) rather than the matter side, as a simpler alternative to invoking dark energy/dark matter.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• This is a comprehensive review rather than a novel theoretical proposal, aiming to synthesize and compare a wide range of dark energy models (quintessence, k-essence, tachyon, phantom, dilatonic, coupled dark energy, braneworld, modified gravity) within a unified dynamical systems and perturbation-theory framework.
• The review emphasizes cosmological scaling solutions and tracker behavior as organizing principles for classifying scalar-field dark energy models.
• It presents reconstruction techniques for the dark energy equation of state using CMB, large-scale structure, and Supernovae Ia data.

“Here is an incomplete list of additional possibilities; see also [48, 58, 218].”

“Some earlier discussions include [85, 50, 221], and subsequent reviews include [58, 218, 246].”

“The classic discussion of the physics of the cosmological constant is by Weinberg [264], with more recent work discussed by [58, 218].”

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Shows that tiny R^n (n<0) corrections to the Einstein-Hilbert action can produce cosmic acceleration, eliminating the need for dark energy as a separate fluid component
• Provides a unified purely gravitational origin for both early-time inflation (R^n, n>0) and late-time acceleration (R^n, n<0), avoiding invocation of dark energy or an inflaton field
• Identifies self-accelerating vacuum solutions (de Sitter and anti-de Sitter) as alternatives to a cosmological constant, with effective equation-of-state parameters mimicking dark energy (w_DE=-1 or w_DE<-2/3) without actual dark energy

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Proposes the Chaplygin gas equation of state p = -A/ρ as an alternative to scalar-field quintessence models for explaining cosmic acceleration.
• Shows that a single exotic fluid (Chaplygin gas) can interpolate between dust-dominated and de Sitter (cosmological-constant-dominated) phases without invoking separate dark matter and dark energy components.
• Predicts that the effective cosmological constant increases with time in this model, offering a distinctive observational signature.

• Introduces the 'cosmic triangle' as a unified geometric representation (Ω_m, Ω_Λ, Ω_k) synthesizing multiple independent observational constraints (cluster mass, baryon fraction, supernovae acceleration, CMB curvature) to pinpoint the universe's current state and trace its past and future trajectory.
• Synthesizes diverse recent observations (cluster mass-to-light, baryon fraction, cluster abundance evolution, supernova Hubble diagrams, gravitational lensing statistics, CMB anisotropy) into a coherent case for a low-density, accelerating, flat universe requiring dark energy.
• Clarifies the distinction between geometric curvature-based definitions of open/closed/flat and the dynamical fate of the universe (expand forever vs. recollapse), noting these need not coincide when dark energy is present.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“Available data for the microwave background anisotropies on degree scales (de Bernardis et al. 2000; Hanany et al. 2000; Netterfield et al. 2002; Lee et al. 2001) and the Hubble diagram of Type Ia supernovae (Riess et al. 1998; Perlmutter et al. 1999) indicate that the universe has a flat geometry…”

• Provides an independent CMB-based constraint on Ω_m and Ω_Λ from the location of the first acoustic peak, complementing prior supernova-based measurements of Λ
• Places the tightest constraint yet on Ω_0=Ω_m+Ω_Λ (0.88<Ω_0<1.12) using resolved CMB structure, supporting a flat, Euclidean universe consistent with a nonzero cosmological constant as suggested by supernova surveys

• Proposes a new differential galaxy age method (using dz/dt from spectroscopic dating of passively-evolving galaxies) that measures the equation-of-state history w_Q(z) directly, offering much better sensitivity than the standard luminosity distance method used by supernova surveys like SNAP.
• Shows that the second derivative d^2z/dt^2 depends explicitly on w_Q without additional integrations, making it a more direct probe of dark energy's equation of state than luminosity distance measurements.
• Demonstrates via simulated spectra and statistical analysis that ~70 pairs of passively-evolving galaxies observed at S/N~30 can distinguish between constant and variable w_Q(z) histories, a novel observational strategy for constraining dark energy from the ground.

• Provides an independent CMB-based constraint on Ω_m and Ω_Λ from the location of the first acoustic peak, complementing prior supernova-based measurements of Λ
• Places the tightest constraint yet on Ω_0=Ω_m+Ω_Λ (0.88<Ω_0<1.12) using resolved CMB structure, supporting a flat, Euclidean universe consistent with a nonzero cosmological constant as suggested by supernova surveys

• We find all the critical points of the system for constant equations of state in that range.
• We consider further several background quantities that can distinguish the models with different w_X values.
• Using a simple toy model with a varying equation of state, we show that even a large variation of w_X at small redshifts is very difficult to observe with d_L(z) measurements up to z∼1.

• Provides an independent CMB-based constraint on Ω_m and Ω_Λ from the location of the first acoustic peak, complementing prior supernova-based measurements of Λ
• Places the tightest constraint yet on Ω_0=Ω_m+Ω_Λ (0.88<Ω_0<1.12) using resolved CMB structure, supporting a flat, Euclidean universe consistent with a nonzero cosmological constant as suggested by supernova surveys

• Proposes that dark energy and dark matter could be reinterpreted as 'shortcomings' of General Relativity rather than as real exotic components, and that extending gravity (via f(R) and scalar-tensor Extended Theories of Gravity) can reproduce accelerated expansion and other 'dark' phenomena as curvature effects.
• Claims that dark energy and quintessence issues can be addressed as curvature effects within Extended Theories of Gravity, offering cosmological models fitted to observational data without adding unknown exotic ingredients to the cosmic pie.
• Argues for changing the gravitational side of Einstein's equations (via non-linear Lagrangians) rather than the matter side, as a simpler alternative to invoking dark energy/dark matter.

• Provides an independent CMB-based constraint on Ω_m and Ω_Λ from the location of the first acoustic peak, complementing prior supernova-based measurements of Λ
• Places the tightest constraint yet on Ω_0=Ω_m+Ω_Λ (0.88<Ω_0<1.12) using resolved CMB structure, supporting a flat, Euclidean universe consistent with a nonzero cosmological constant as suggested by supernova surveys

• The review explicitly generalizes 'cosmological constant' to include time-varying dark energy scenarios ('we shall use the term cosmological constant in a generalized sense including the scenarios in which cosmological "constant" is actually varying in time')
• Provides a unified conceptual treatment linking dark energy to scalar field models (quintessence, tachyonic fields) and to the two distinct 'cosmological constant problems' (smallness and coincidence)
• Synthesizes cosmological constant/dark energy discussion with de Sitter thermodynamics, string theory landscape, and relaxation mechanisms as a coherent review

• Provides an independent CMB-based constraint on Ω_m and Ω_Λ from the location of the first acoustic peak, complementing prior supernova-based measurements of Λ
• Places the tightest constraint yet on Ω_0=Ω_m+Ω_Λ (0.88<Ω_0<1.12) using resolved CMB structure, supporting a flat, Euclidean universe consistent with a nonzero cosmological constant as suggested by supernova surveys

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

“Time-dependent Equation of State As for a time-dependent equation of state, we shall find constraints on the present-day value of the equation of state and its derivative using a linear form, w(a) = w0 + wa(1−a) (Chevallier & Polarski 2001; Linder 2003).”

• We find all the critical points of the system for constant equations of state in that range.
• We consider further several background quantities that can distinguish the models with different w_X values.
• Using a simple toy model with a varying equation of state, we show that even a large variation of w_X at small redshifts is very difficult to observe with d_L(z) measurements up to z∼1.

• Improved constraints on the dark energy equation-of-state parameter w and its time-dependent parameterization (w0, wa) using the combination of 7-year WMAP data with BAO, H0, supernova, and time-delay distance measurements.
• First use of a lens time-delay distance measurement (D_Δt) in combination with WMAP data to constrain dark energy properties.

• We find all the critical points of the system for constant equations of state in that range.
• We consider further several background quantities that can distinguish the models with different w_X values.
• Using a simple toy model with a varying equation of state, we show that even a large variation of w_X at small redshifts is very difficult to observe with d_L(z) measurements up to z∼1.

• Presents first cosmological results from the ESSENCE survey constraining the dark energy equation-of-state parameter w with supernova data over redshift 0.15–0.70
• Combines ESSENCE with SNLS to obtain a joint, tighter constraint on w and Ω_M consistent with a cosmological constant
• Develops a redshift-dependent host-galaxy extinction prior ('glosz') derived from detailed Monte Carlo simulations of the ESSENCE selection function to reduce systematic bias in dark energy inference

“21 When w is allowed to vary with the scale factor according to w(a) = w0 + wa(1 − a) (Chevallier & Polarski 2001; Linder 2003), we find, for a flat universe20 4.6.1.”

• We find all the critical points of the system for constant equations of state in that range.
• We consider further several background quantities that can distinguish the models with different w_X values.
• Using a simple toy model with a varying equation of state, we show that even a large variation of w_X at small redshifts is very difficult to observe with d_L(z) measurements up to z∼1.

• Nine-year WMAP data combined with high-l CMB, BAO, and H0 determine Ω_bh^2, Ω_ch^2, and Ω_Λ each to ~1.5% precision
• Restricting supernova data use to models examining the dark energy equation of state due to residual systematic errors in SN samples
• Updated methodology for combining WMAP with SPT/ACT/BAO/H0/SNe to constrain dark energy equation of state parameters (w, w0, wa) more tightly than previous WMAP releases

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

• Demonstrates that baryonic acoustic oscillations in large high-redshift galaxy surveys offer a precision route to measuring dark energy via H(z) and D_A(z)
• Uses a full Fisher matrix formalism to treat cosmological constraints from large-scale structure, CMB anisotropies, and supernova data simultaneously, including a time-variable equation of state, extending prior work
• Provides an explicit treatment of survey data sets combined with dark energy parameter estimation, differing from previous studies

“A natural explanation could be a vacuum energy density (Carroll et al. 1992), but if so, we are faced with the fine-tuning problem to explain its observed value, 120 orders of magnitude smaller than expected from field theory arguments.”

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

• The five-year data improve constraints on cosmological parameters including dark energy equation of state, with the neutrino mass limit robust to within 10% to a varying dark energy equation of state.
• Ω_Λ= 0.742±0.030 is measured with improved precision compared to three-year data.
• The paper considers extended models with a constant dark energy equation of state w, bounded by w>-2.5, as part of testing extensions beyond simple ΛCDM.

• Provides a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy, focusing on recent developments not fully covered in earlier reviews.
• Modifies the conventional dominant energy condition (using only null vectors) to avoid ruling out a negative cosmological constant, which the author argues has no physical justification for exclusion.

• Provides a comprehensive review of observational and theoretical aspects of a small cosmological Λ-term
• Reviews more recent attempts to generate a small cosmological constant using field theoretic techniques or dynamical Λ-term modeled by scalar fields
• Provides a comprehensive bibliography of recent work on Λ

• Presents a new, larger BOOMERANG dataset (four 150 GHz channels, full flight) with improved beam and pointing characterization, enabling detection of multiple acoustic peaks and tighter constraints on Ω_tot, Ω_bh^2, Ω_ch^2, n_s, and by extension Ω_Λ.
• Combines CMB data with LSS and SN1a priors to jointly constrain Ω_Λ and other cosmological parameters, showing consistency with a Λ-dominated, low-curvature adiabatic CDM model.
• Provides refined determination of Ω_Λ (~0.5–0.7 depending on priors) as part of a 7-dimensional parameter extraction using new analysis pipeline (MASTER-based power spectrum estimation).

• The review explicitly generalizes 'cosmological constant' to include time-varying dark energy scenarios ('we shall use the term cosmological constant in a generalized sense including the scenarios in which cosmological "constant" is actually varying in time')
• Provides a unified conceptual treatment linking dark energy to scalar field models (quintessence, tachyonic fields) and to the two distinct 'cosmological constant problems' (smallness and coincidence)
• Synthesizes cosmological constant/dark energy discussion with de Sitter thermodynamics, string theory landscape, and relaxation mechanisms as a coherent review

• Presents a new, larger BOOMERANG dataset (four 150 GHz channels, full flight) with improved beam and pointing characterization, enabling detection of multiple acoustic peaks and tighter constraints on Ω_tot, Ω_bh^2, Ω_ch^2, n_s, and by extension Ω_Λ.
• Combines CMB data with LSS and SN1a priors to jointly constrain Ω_Λ and other cosmological parameters, showing consistency with a Λ-dominated, low-curvature adiabatic CDM model.
• Provides refined determination of Ω_Λ (~0.5–0.7 depending on priors) as part of a 7-dimensional parameter extraction using new analysis pipeline (MASTER-based power spectrum estimation).

• Shows that tiny R^n (n<0) corrections to the Einstein-Hilbert action can produce cosmic acceleration, eliminating the need for dark energy as a separate fluid component
• Provides a unified purely gravitational origin for both early-time inflation (R^n, n>0) and late-time acceleration (R^n, n<0), avoiding invocation of dark energy or an inflaton field
• Identifies self-accelerating vacuum solutions (de Sitter and anti-de Sitter) as alternatives to a cosmological constant, with effective equation-of-state parameters mimicking dark energy (w_DE=-1 or w_DE<-2/3) without actual dark energy

• Presents a new, larger BOOMERANG dataset (four 150 GHz channels, full flight) with improved beam and pointing characterization, enabling detection of multiple acoustic peaks and tighter constraints on Ω_tot, Ω_bh^2, Ω_ch^2, n_s, and by extension Ω_Λ.
• Combines CMB data with LSS and SN1a priors to jointly constrain Ω_Λ and other cosmological parameters, showing consistency with a Λ-dominated, low-curvature adiabatic CDM model.
• Provides refined determination of Ω_Λ (~0.5–0.7 depending on priors) as part of a 7-dimensional parameter extraction using new analysis pipeline (MASTER-based power spectrum estimation).

• Presents a new, larger BOOMERANG dataset (four 150 GHz channels, full flight) with improved beam and pointing characterization, enabling detection of multiple acoustic peaks and tighter constraints on Ω_tot, Ω_bh^2, Ω_ch^2, n_s, and by extension Ω_Λ.
• Combines CMB data with LSS and SN1a priors to jointly constrain Ω_Λ and other cosmological parameters, showing consistency with a Λ-dominated, low-curvature adiabatic CDM model.
• Provides refined determination of Ω_Λ (~0.5–0.7 depending on priors) as part of a 7-dimensional parameter extraction using new analysis pipeline (MASTER-based power spectrum estimation).

• Proposes a modified gravity model L=R+R^m+1/R^n that unifies early-time inflation (via positive powers of curvature) and late-time cosmic acceleration (via negative powers of curvature) without invoking dark energy
• Claims the model avoids the instabilities present in the simpler R+1/R modified gravity model
• Claims the equivalent scalar-tensor formulation can pass solar system tests, unlike the original 1/R model which is ruled out by such tests