First-year DESI BAO data are consistent with flat LambdaCDM and, when combined with CMB, show a 2.5-3.9 sigma preference for evolving dark energy (w0 > -1, wa < 0) that strengthens with certain supernova datasets.
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Everything You Always Wanted To Know About The Cosmological Constant Problem (But Were Afraid To Ask)
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abstract
This article aims at discussing the cosmological constant problem at a pedagogical but fully technical level. We review how the vacuum energy can be regularized in flat and curved space-time and how it can be understood in terms of Feynman bubble diagrams. In particular, we show that the properly renormalized value of the zero-point energy density today (for a free theory) is in fact far from being 122 orders of magnitude larger than the critical energy density, as often quoted in the literature. We mainly consider the case of scalar fields but also treat the cases of fermions and gauge bosons which allows us to discuss the question of vacuum energy in super-symmetry. Then, we discuss how the cosmological constant can be measured in cosmology and constrained with experiments such as measurements of planet orbits in our solar system or atomic spectra. We also review why the Lamb shift and the Casimir effect seem to indicate that the quantum zero-point fluctuations are not an artifact of the quantum field theory formalism. We investigate how experiments on the universality of free fall can constrain the gravitational properties of vacuum energy and we discuss the status of the weak equivalence principle in quantum mechanics, in particular the Collela, Overhausser and Werner experiment and the quantum Galileo experiment performed with a Salecker-Wigner-Peres clock. Finally, we briefly conclude with a discussion on the solutions to the cosmological constant problem that have been proposed so far.
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Stable flat solutions in scale-invariant scalar-tensor theories carry a non-vanishing cosmological constant unless the quartic coupling vanishes, and this vanishing is not radiatively protected, making a residual CC generic.
A generalized Chaplygin-type solid unifies dark matter and dark energy via an early pressureless phase transitioning to a late solid phase that supports acceleration and produces distinct low-redshift perturbation signatures.
Full DES data from SN+BAO+3x2pt yields w0=-0.84, wa=-0.44 with 2.2σ deviation from ΛCDM; adding DESI+CMB reaches 3.0σ while 3x2pt improves figure of merit by ~10%.
Inhomogeneous decelerating models generate effective accelerating FLRW universes at large scales through backreaction that mimics a running dark-energy term.
The f(Q, L_m) gravity model fits observational data from BBN to late-time acceleration, acting as a viable quintessence-like alternative to the standard LambdaCDM model.
A lapse function in a 5D anisotropic gravity theory generates a global constraint that cancels Standard Model vacuum energy radiative corrections at all orders in the 4D effective theory.
Bin-wise uncorrelated reconstruction from DESI/SDSS BAO and Pantheon+/Union3.1/DES-Dovekie supernovae yields dark energy density peaking then declining and equation of state oscillating with phantom crossing near z~0.7, consistent across datasets at moderate significance.
Latent-f and latent-H Gaussian process reconstructions from OHD data both yield f(z), w(z), and Om(z) consistent with Lambda-CDM, with no strong predictive preference and small prior-dependent residuals mainly at high redshift.
Generalized interacting dark energy models with constant or dynamical couplings yield analytical density expressions but are not preferred over LambdaCDM by Bayesian evidence from DESI, Pantheon+, and CMB data.
Dynamical systems analysis of a Palatini k-essence model identifies fixed points for quasi-de-Sitter epochs, scaling solutions, and quintessence phases connected by heteroclinic orbits in flat FLRW cosmology.
Rényi entropic corrections to cosmology are constrained by DESI DR2 BAO and GW data to a viable quintessence-like model that approaches ΛCDM without phantom behavior and satisfies BBN bounds.
A barotropic fluid with ω_s ≈ 0.29 and Ω_s ≈ 1.5×10^{-5} raises the inferred H0 to match SH0ES while remaining consistent with Planck CMB, DESI BAO, and Pantheon data.
Dynamical systems analysis of dimensionally reduced unimodular cosmology yields a line of vacuum equilibria at λ=dH and isolated critical points in the d=1 matter sector under a volume-dilution conservation rule.
Hybrid redshift model with postulated quantum correction fits Pantheon+SH0ES data to recover Planck-consistent expansion H_Λ and eliminates apparent drift in inferred Hubble parameter across bins.
QFT in FLRW spacetime yields a running vacuum energy density ρ_vac(H) that unifies mild dynamical dark energy today with H^4 inflation without an inflaton field.
String theory imposes constraints on dark energy but permits various construction attempts for de Sitter vacua and single-field exponential quintessence models despite obstructions.
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DESI 2024 VI: Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations
First-year DESI BAO data are consistent with flat LambdaCDM and, when combined with CMB, show a 2.5-3.9 sigma preference for evolving dark energy (w0 > -1, wa < 0) that strengthens with certain supernova datasets.
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The unavoidable de Sitter fate of a scale-invariant Universe
Stable flat solutions in scale-invariant scalar-tensor theories carry a non-vanishing cosmological constant unless the quartic coupling vanishes, and this vanishing is not radiatively protected, making a residual CC generic.
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A generalized Chaplygin-type solid unifies dark matter and dark energy via an early pressureless phase transitioning to a late solid phase that supports acceleration and produces distinct low-redshift perturbation signatures.
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Full DES data from SN+BAO+3x2pt yields w0=-0.84, wa=-0.44 with 2.2σ deviation from ΛCDM; adding DESI+CMB reaches 3.0σ while 3x2pt improves figure of merit by ~10%.
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A class of decelerating inhomogeneous cosmological models giving rise to accelerating FLRW universes at large scales
Inhomogeneous decelerating models generate effective accelerating FLRW universes at large scales through backreaction that mimics a running dark-energy term.
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From Big Bang Nucleosynthesis to Late-Time Acceleration in $f(Q,L_m)$ Gravity
The f(Q, L_m) gravity model fits observational data from BBN to late-time acceleration, acting as a viable quintessence-like alternative to the standard LambdaCDM model.
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A Lapse in the Cosmological Constant Problem
A lapse function in a 5D anisotropic gravity theory generates a global constraint that cancels Standard Model vacuum energy radiative corrections at all orders in the 4D effective theory.
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Reconstructing dark energy with fewer assumptions
Bin-wise uncorrelated reconstruction from DESI/SDSS BAO and Pantheon+/Union3.1/DES-Dovekie supernovae yields dark energy density peaking then declining and equation of state oscillating with phantom crossing near z~0.7, consistent across datasets at moderate significance.
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Latent-Space Gaussian Processes for Dark-Energy Reconstruction from Observational \(H(z)\) Data
Latent-f and latent-H Gaussian process reconstructions from OHD data both yield f(z), w(z), and Om(z) consistent with Lambda-CDM, with no strong predictive preference and small prior-dependent residuals mainly at high redshift.
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Generalizing the CPL Parametrization through Dark Sector Interaction
Generalized interacting dark energy models with constant or dynamical couplings yield analytical density expressions but are not preferred over LambdaCDM by Bayesian evidence from DESI, Pantheon+, and CMB data.
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Dynamical system analysis of the cosmological phases in Palatini $k$-essence gravity
Dynamical systems analysis of a Palatini k-essence model identifies fixed points for quasi-de-Sitter epochs, scaling solutions, and quintessence phases connected by heteroclinic orbits in flat FLRW cosmology.
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Exploring Cosmic Evolution in R\'enyi Entropic Cosmology with Constraints from DESI DR2 BAO and GW Data
Rényi entropic corrections to cosmology are constrained by DESI DR2 BAO and GW data to a viable quintessence-like model that approaches ΛCDM without phantom behavior and satisfies BBN bounds.
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A barotropic alternative to Early Dark Energy for alleviating the $H_0$ tension
A barotropic fluid with ω_s ≈ 0.29 and Ω_s ≈ 1.5×10^{-5} raises the inferred H0 to match SH0ES while remaining consistent with Planck CMB, DESI BAO, and Pantheon data.
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Dynamical systems analysis of unimodular cosmology in $D=4+d$ dimensions
Dynamical systems analysis of dimensionally reduced unimodular cosmology yields a line of vacuum equilibria at λ=dH and isolated critical points in the d=1 matter sector under a volume-dilution conservation rule.
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Redshift Duality with Pantheon+SH0ES in a Planck-anchored Flat $\Lambda$CDM Framework: Implications for Hubble Tension and Observational Inference
Hybrid redshift model with postulated quantum correction fits Pantheon+SH0ES data to recover Planck-consistent expansion H_Λ and eliminates apparent drift in inferred Hubble parameter across bins.
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Running Vacuum in the expanding Universe: a unified QFT paradigm for Inflation and Dark Energy
QFT in FLRW spacetime yields a running vacuum energy density ρ_vac(H) that unifies mild dynamical dark energy today with H^4 inflation without an inflaton field.
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Dark energy from string theory: an introductory review
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Testing General Relativity with Present and Future Astrophysical Observations
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