Late-time Expansion Research Articles

Early dark energy constraints with late-time expansion marginalization

Early dark energy (EDE) is an extension to the ΛCDM model that includes an additional energy density contribution near recombination. The model was proposed to reduce the tension between the measurements of the Hubble constant H 0 from the cosmic microwave background (CMB) and from the local cosmic distance ladder. Some analyses in the recent literature have shown intriguing hints for EDE. However, this model increases the tension in the derived clustering of galaxies (as measured by the so-called S 8 parameter) between CMB and large scale structure (LSS) measurements. This new tension limits the contribution of EDE during recombination, and thus its effect on the Hubble tension. In this work, we investigate whether the inclusion of a general, smooth late-time dark energy modification can increase back the EDE contribution when LSS data is included in the analysis. In order to generalize the late expansion with respect to the ΛCDM model, we substitute the cosmological constant by a late dark energy fluid model with a piecewise constant equation of state w(z) in redshift bins. We show that, when analysing this generalized model with combinations of CMB, LSS and type Ia supernovae data from several experiments no significant changes on S 8 and EDE parameter constraints is found. The contribution to the EDE fraction constraint with late-time expansion marginalization is f EDE = 0.067+0.019 -0.027 using 3 redshift bins, with similar results for 5 and 10 redshift bins. This work shows that in order to solve simultaneously the Hubble and S 8 tensions, one needs a mechanism for increasing the clustering of matter at late times different from a simple change in the background evolution of late dark energy.

Unified inflation and dark energy with accelerated particle holographic model

In this paper and in the follows of the holographic approach to describe the primary acceleration and the late-time acceleration eras, we have considered and unified inflation and dark energy while accelerated particle holographic (APHM) density is used (Fazlollahi in Chin Phys C 47:035101, 2023). As discussed, establishing holographic model for constant roll inflation during the very early Universe leads one to explicit form of Hubble parameter which satisfies inflationary era. The validity of such holographic constant roll inflation with respect to the Planck data puts a certain bound on the infrared cut-off at the time of horizon crossing. Beside the mere inflation, the Hubble parameter gives explicit form for density of accelerated particle holographic model. It is shown such density due to inflationary bound condition on infrared cut-off could present late-time acceleration epoch. Consequently, inflation and late time expansion era unified in unique infrared cut-off model. Moreover, such corresponding fractional density gives more matter regimes during matter era. Nevertheless, studying matter structure formation during matter era reveals the model is not in conflict with {Lambda }CDM model.

Future instability of FLRW fluid solutions for linear equations of state p=Kρ with 1/3<K<1

Using numerical methods, we examine the dynamics of nonlinear perturbations in the expanding time direction, under a Gowdy symmetry assumption, of FLRW fluid solutions to the Einstein-Euler equations with a positive cosmological constant $\Lambda>0$ and a linear equation of state $p = K\rho$ for the parameter values $1/3<K<1$. This paper builds upon the numerical work in \cite{Marshalloliynyk:2022} in which the simpler case of a fluid on a fixed FLRW background spacetime was studied. The numerical results presented here confirm that the instabilities observed in \cite{Marshalloliynyk:2022} are also present when coupling to gravity is included as was previously conjectured in \cite{Rendall:2004,Speck:2013}. In particular, for the full parameter range $1/3 < K <1$, we find that the density contrast of the nonlinear perturbations develop steep gradients near a finite number of spatial points and becomes unbounded there at future timelike infinity. This instability is of particular interest since it is not consistent with the standard picture for late time expansion in cosmology.

Restoring cosmological concordance with early dark energy and massive neutrinos?

ABSTRACT The early dark energy (EDE) solution to the Hubble tension comes at the cost of an increased clustering amplitude that has been argued to worsen the fit to galaxy clustering data. We explore whether freeing the total neutrino mass Mν, which can suppress small-scale structure growth, improves EDE’s fit to galaxy clustering. Using Planck Cosmic Microwave Background and BOSS galaxy clustering data, a Bayesian analysis shows that freeing Mν does not appreciably increase the inferred EDE fraction fEDE: we find the 95 per cent C.L. upper limits fEDE &amp;lt; 0.092 and $M_{\nu }\lt 0.15\, {\rm eV}$. Similarly, in a frequentist profile likelihood setting (where our results support previous findings that prior volume effects are important), we find that the baseline EDE model (with $M_{\nu }=0.06\, {\rm eV}$) provides the overall best fit. For instance, compared to baseline EDE, a model with $M_\nu =0.24\, {\rm eV}$ maintains the same H0(km/s/Mpc) = (70.08, 70.11, respectively) whilst decreasing S8 = (0.837, 0.826) to the ΛCDM level, but worsening the fit significantly by Δχ2 = 7.5. For the datasets used, these results are driven not by the clustering amplitude, but by background modifications to the late-time expansion rate due to massive neutrinos, which worsen the fit to measurements of the BAO scale.

Holographic dark energy from the anti–de Sitter black hole

The anti-de Sitter (AdS) black hole plays an important role in the holographic principle. In this study, the upper bound in energy corresponding to the mass of the Schwarzschild black hole is modified to be that of the AdS black hole. Via the correspondence between the ultraviolet (UV) and infrared (IR) cutoffs, the constant term in the energy density of the holographic dark energy (HDE) can be obtained from the negative cosmological constant from the black hole. Interestingly, the proposed dark energy model could drive the late-time expansion of the Universe without the causality violation. The cosmic evolution is investigated by choosing the Hubble and particle horizons as the IR length scales. It is found that the accelerated expansion at late time can be obtained for both cases. This result may shed light on the connection between the AdS black hole and the de Sitter (dS) spacetime in the context of cosmology.

Neural network reconstruction of late-time cosmology and null tests

The prospect of nonparametric reconstructions of cosmological parameters from observational data sets has been a popular topic in the literature for a number of years. This has mainly taken the form of a technique based on Gaussian processes but this approach is exposed to several foundational issues ranging from overfitting to kernel consistency problems. In this work, we explore the possibility of using artificial neural networks (ANN) to reconstruct late-time expansion and large scale structure cosmological parameters. We first show how mock data can be used to design an optimal ANN for both parameters, which we then use with real data to infer their respective redshift profiles. We further consider cosmological null tests with the reconstructed data in order to confirm the validity of the concordance model of cosmology, in which we observe a mild deviation with cosmic growth data.

Einstein-Gauss-Bonnet gravity coupled to bumblebee field in four dimensional spacetime

We study Einstein-Gauss-Bonnet gravity coupled to a bumblebee field which leads to a spontaneous Lorentz symmetry breaking in the gravitational sector. We obtain an exact black hole solution and a cosmological solution in four dimensional spacetime by a regularization scheme. We also obtain a Schwarzschild-like bumblebee black hole solution in D-dimensional spacetime. We find that the bumblebee field doesn't affect the locations of the black hole horizon, but only affects the gravitational potential. That is, its gravitational potential has a minimum value (negative) in the black hole interior and has a positive value 1+ℓ at short distance r→0. If the constant ℓ is large enough, then this kind of black hole is practically free from the singularity problem. The thermodynamics and phase transition are also studied. In a cosmological context, it is interesting that the Gauss-Bonnet term has no effect on the conservation of energy equation. A late-time expansion of de Sitter universe can be replicated in an empty space. The Gauss-Bonnet term and the bumblebee field can both actually act as a form of dark energy.

No-go guide for the Hubble tension: Late-time solutions

The Hubble tension, if not caused by any systematics, could be relieved or even resolved from modifying either the early-time or late-time Universe. The early-time modifications are usually in tension with either galaxy clustering or galaxy lensing constraints. The late-time modifications are also in conflict with the constraint from the inverse distance ladder, which, however, is weakened by the dependence on a sound-horizon prior and some particular approximation for the late-time expansion history. To achieve a more general no-go argument for the late-time scenarios, we propose to use a global parametrizationbased on the cosmic age (PAge) to consistently use the cosmic chronometers data beyond the Taylor expansion domain and without the input of a sound-horizon prior. Both the early-time and late-time scenarios are therefore largely ruled out, indicating the possible ways out of the Hubble tension from either exotic modifications of our concordance Universe or some unaccounted systematics.

Detecting the massive bosonic zero-mode in expanding cosmological spacetimes

We examine a quantised massive scalar field in $(1+1)$-dimensional spatially compact cosmological spacetimes in which the early time and late time expansion laws provide distinguished definitions of Fock "in" and "out" vacua, with the possible exception of the spatially constant sector, which may become effectively massless at early or late times. We show, generalising the work of Ford and Pathinayake, that when such a massive zero mode occurs, the freedom in the respective "in" and "out" states is a family with two real parameters. As an application, we consider massive untwisted and twisted scalar fields in the $(1+1)$-dimensional spatially compact Milne spacetime, where the untwisted field has a massive "in" zero mode. We demonstrate, by a combination of analytic and numerical methods, that the choice of the massive "in" zero mode state has a significant effect on the response of an inertial Unruh-DeWitt detector, especially in the excitation part of the spectrum. The detector's peculiar velocity with respect to comoving cosmological observers has the strongest effect in the "in" vacuum of the untwisted field, where it shifts the excitation and de-excitation resonances towards higher values of the detector's energy gap.

Scalar and gravitational hair for extreme Kerr black holes

For scalar perturbations of an extreme Reissner-Nordstr\"om black hole we show numerically that the Ori prefactor equals the Aretakis conserved charge. For a family of scalar or gravitational perturbations of an extreme Kerr black hole, whose members vary only in the radial location of the center of the initial packet, we demonstrate a linear relation of a generalized Ori prefactor---a certain expression obtained from the late-time expansion or the perturbation field at finite distances---and the Aretakis conserved charge. We infer that it can be established that there is an Aretakis conserved charge for scalar or gravitational perturbations of extreme Kerr black holes. This conclusion, in addition to the calculation of the Aretakis charge, can be made from measurements at a finite distance: Extreme Kerr black holes have gravitational hair that can be measured at finite distances and violates the uniqueness theorems. This gravitational hair can in principle be detected by gravitational-wave detectors.

Quantum corrections to the accretion onto a Schwarzschild black hole in the background of quintessence

It is well known that quantum effects may lead to removal of the intrinsic singularity point of back holes. Also, the quintessence scalar field is a candidate model for describing late-time acceleration expansion. Accordingly, Kazakov and Solodukhin considered the existence of back-reaction of the spacetime due to the quantum fluctuations of the background metric to deform a Schwarzschild black hole, which led to a change of the intrinsic singularity of the black hole to a 2-sphere with a radius of the order of the Planck length. Also, Kiselev rewrote the Schwarzschild metric by taking into account the quintessence field in the background. In this study, we consider the quantum-corrected Schwarzschild black hole inspired by Kazakov–Solodukhin’s work, and the Schwarzschild black hole surrounded by quintessence deduced by Kiselev to study the mutual effects of quantum fluctuations and quintessence on the accretion onto the black hole. Consequently, the radial component of the 4-velocity and the proper energy density of the accreting fluid have a finite value on the surface of its central 2-sphere due to the presence of quantum corrections. Also, by comparing the accretion parameters in different kinds of black holes, we infer that the presence of a point-like electric charge in the spacetime is somewhat similar to some quantum fluctuations in the background metric.

Implications of Symmetry and Pressure in Friedmann Cosmology. III. Point Sources of Dark Energy that Tend toward Uniformity

Abstract We show that point sources of dark energy can explain accelerated late-time expansion and, simultaneously, satisfy observational constraints on massive compact objects. Population III stellar collapse into GEneric Objects of Dark Energy (GEODEs) between 8 ≲ z ≲ 20 mimics the observed ΩΛ within the typical concordance cosmology. We determine the appropriate dynamical model of aggregate GEODE flow within covariant linear perturbation theory. We find that all continuum fluid properties, at large scales, are determined by the internal properties and spin of individual GEODEs. For large spin, the spatial distribution of GEODEs becomes uniform on scales ≲200 Mpc. The power spectrum of cold dark matter is essentially unaltered. A Population III GEODE scenario provides an observationally consistent physical origin for accelerated late-time expansion while imposing no new constraints on structure formation.

Cross-correlating Planck with VST ATLAS LRGs: a new test for the ISW effect in the Southern hemisphere

ABSTRACT The integrated Sachs–Wolfe (ISW) effect probes the late-time expansion history of the Universe, offering direct constraints on dark energy. Here, we present our measurements of the ISW signal at redshifts of $\bar{z}=0.35$, 0.55, and 0.68, using the cross-correlation of the Planck cosmic microwave background temperature map with ∼0.5 million luminous red galaxies (LRGs) selected from the VST ATLAS survey. We then combine these with previous measurements based on WMAP and similar SDSS LRG samples, providing a total sample of ∼2.1 million LRGs covering ∼12 000 deg2 of sky. At $\bar{z}=0.35$ and $\bar{z}=0.55$, we detect the ISW signal at 1.2σ and 2.3σ (or 2.6σ combined), in agreement with the predictions of lambda cold dark matter (ΛCDM). We verify these results by repeating the measurements using the BOSS LOWZ and CMASS, spectroscopically confirmed LRG samples. We also detect the ISW effect in three magnitude limited ATLAS + SDSS galaxy samples extending to z ≈ 0.4 at ∼2σ per sample. However, we do not detect the ISW signal at $\bar{z}=0.68$ when combining the ATLAS and SDSS results. Further tests using spectroscopically confirmed eBOSS LRGs at this redshift remain inconclusive due to the current low sky coverage of the survey. If the ISW signal is shown to be redshift dependent in a manner inconsistent with the predictions of ΛCDM, it could open the door to alternative theories such as modified gravity. It is therefore important to repeat the high-redshift ISW measurement using the completed eBOSS sample, as well as deeper upcoming surveys such as DESI and LSST.

Singularities in Reissner–Nordström black holes

We study black holes produced via collapse of a spherically symmetric charged scalar field in asymptotically flat space. We employ a late time expansion and argue that decaying fluxes of radiation through the event horizon imply that the black hole must contain a null singularity on the Cauchy horizon and a central spacelike singularity.

Low-energy modified gravity signatures on the large-scale structures

A large number of dark energy and modified gravity models lead to the same expansion history of the Universe, hence, making it difficult to distinguish them from observations. To make the calculations transparent, we consider $f(R)$ gravity with a pressureless matter without making any assumption about the form of $f(R)$. Using the late-time expansion history realizations constructed by Shafieloo et al~\cite{2018-Shafieloo.etal-PRD}, we explicitly show for any $f(R)$ model that the Bardeen potentials $\Psi$ and $\Phi$ evolve differently. For an arbitrary $f(R)$ model that leads to late-time accelerated expansion, we explicitly show that $|\Psi + \Phi|$ and its time-derivative evolves differently than the $\Lambda$CDM model at lower redshifts. We show that the $\Psi/\Phi$ has a significant deviation from unity for larger wave-numbers. We discuss the implications of the results for the cosmological observations.

Bianchi type I, Schutz perfect fluid and evolutionary quantum cosmology

We study the classical and quantum cosmology of a universe in which the matter content is a perfect fluid and the background geometry is described by a Bianchi type I metric. To write the Hamiltonian of the perfect fluid we use the Schutz representation, in terms of which, after a particular gauge fixing, we are led to an identification of a clock parameter which may play the role of time for the corresponding dynamical system. In view of the classical cosmology, it is shown that the evolution of the universe represents a late time expansion coming from a big-bang singularity. We also consider the issue of quantum cosmology in the framework of the canonical Wheeler-DeWitt (WDW) equation. It is shown that the Schutz formalism leads to the introduction of a momentum that enters linearly into Hamiltonian. This means that the WDW equation takes the form of a Schr\"{o}dinger equation for the quantum-mechanical description of the model under consideration. We find the eigenfunctions and with the use of them construct the closed form expressions for the wave functions of the universe. By means of the resulting wave function we evaluate the expectation values and investigate the possibility of the avoidance of classical singularities due to quantum effects. We also look at the problem through Bohmian approach of quantum mechanics and while recovering the quantum solutions, we deal with the reason of the singularity avoidance by introducing quantum potential.

Dynamical Stability of Bulk Viscous Isotropic and Homogeneous Universe

In this paper, we study the phase space portrait of homogeneous and isotropic universe by taking different coupling functions between dark energy models and bulk viscous dark matter. The dimensionless quantities are introduced to establish an autonomous set of equations. To analyze the stability of the cosmos, we evaluate critical points and respective eigenvalues for different dynamical quantities. For bulk viscous matter and radiation in tachyon coupled field, these points show stable evolution when γ ≫ δ but accelerated expansion of the universe for δ &gt; 1 9 . The stability of the universe increases for some stationary points which may correspond to the late-time expansion for the coupled phantom field.

Extended gravity cosmography

Cosmography can be considered as a sort of a model-independent approach to tackle the dark energy/modified gravity problem. In this review, the success and the shortcomings of the [Formula: see text]CDM model, based on General Relativity (GR) and standard model of particles, are discussed in view of the most recent observational constraints. The motivations for considering extensions and modifications of GR are taken into account, with particular attention to [Formula: see text] and [Formula: see text] theories of gravity where dynamics is represented by curvature or torsion field, respectively. The features of [Formula: see text] models are explored in metric and Palatini formalisms. We discuss the connection between [Formula: see text] gravity and scalar–tensor theories highlighting the role of conformal transformations in the Einstein and Jordan frames. Cosmological dynamics of [Formula: see text] models is investigated through the corresponding viability criteria. Afterwards, the equivalent formulation of GR (Teleparallel Equivalent General Relativity (TEGR)) in terms of torsion and its extension to [Formula: see text] gravity is considered. Finally, the cosmographic method is adopted to break the degeneracy among dark energy models. A novel approach, built upon rational Padé and Chebyshev polynomials, is proposed to overcome limits of standard cosmography based on Taylor expansion. The approach provides accurate model-independent approximations of the Hubble flow. Numerical analyses, based on Monte Carlo Markov Chain integration of cosmic data, are presented to bound coefficients of the cosmographic series. These techniques are thus applied to reconstruct [Formula: see text] and [Formula: see text] functions and to frame the late-time expansion history of the universe with no a priori assumptions on its equation-of-state. A comparison between the [Formula: see text]CDM cosmological model with [Formula: see text] and [Formula: see text] models is reported.

The large proper-time expansion of Yang-Mills plasma as a resurgent transseries

We show that the late-time expansion of the energy density of mathcal{N} = 4 supersymmetric Yang-Mills plasma at infinite coupling undergoing Bjorken flow takes the form of a multi-parameter transseries. Using the AdS/CFT correspondence we find a gravity solution which supplements the well known large proper-time expansion by exponentially-suppressed sectors corresponding to quasinormal modes of the AdS black-brane. The full solution also requires the presence of further sectors which have a natural interpretation as couplings between these modes. The exponentially-suppressed sectors represent nonhydrodynamic contributions to the energy density of the plasma. We use resurgence techniques on the resulting transseries to show that all the information encoded in the nonhydrodynamic sectors can be recovered from the original hydrodynamic gradient expansion.

Kinematic model-independent reconstruction of Palatini f(R) cosmology

A kinematic treatment to trace out the form of $f(R)$ cosmology, within the Palatini formalism, is discussed by only postulating the universe homogeneity and isotropy. To figure this out we build model-independent approximations of the luminosity distance through rational expansions. These approximants extend the Taylor convergence radii computed for usual cosmographic series. We thus consider both Pad\'e and the rational Chebyshev polynomials. They can be used to accurately describe the universe late-time expansion history, providing further information on the thermal properties of all effective cosmic fluids entering the energy momentum tensor of Palatini's gravity. To perform our numerical analysis, we relate the Palatini's Ricci scalar with the Hubble parameter $H$ and thus we write down a single differential equation in terms of the redshift $z$. Therefore, to bound $f(R)$, we make use of the most recent outcomes over the cosmographic parameters obtained from combined data surveys. In particular our clue is to select two scenarios, i.e. $(2,2)$ Pad\'e and $(2,1)$ Chebyshev approximations, since they well approximate the luminosity distance at the lowest possible order. We find that best analytical matches to the numerical solutions lead to $f(R)=a+bR^n$ with free parameters given by the set $(a, b, n)=(-1.627, 0.866, 1.074)$ for $(2,2)$ Pad\'e approximation, whereas $f(R)=\alpha+\beta R^m$ with $(\alpha, \beta, m)=(-1.332, 0.749, 1.124)$ for $(2,1)$ rational Chebyshev approximation. Finally, our results are compared with the $\Lambda$CDM predictions and with previous studies in the literature. Slight departures from General Relativity are also discussed.