The $\Lambda$CDM model provides a good fit to a large span of cosmological data but harbors areas of phenomenology. With the improvement of the number and the accuracy of observations, discrepancies among key cosmological parameters of the model have emerged. The most statistically significant tension is the $4-6\sigma$ disagreement between predictions of the Hubble constant $H_0$ by early time probes with $\Lambda$CDM model, and a number of late time, model-independent determinations of $H_0$ from local measurements of distances and redshifts. The high precision and consistency of the data at both ends present strong challenges to the possible solution space and demand a hypothesis with enough rigor to explain multiple observations--whether these invoke new physics, unexpected large-scale structures or multiple, unrelated errors. We present a thorough review of the problem, including a discussion of recent Hubble constant estimates and a summary of the proposed theoretical solutions. Some of the models presented are formally successful, improving the fit to the data in light of their additional degrees of freedom, restoring agreement within $1-2\sigma$ between {\it Planck} 2018, using CMB power spectra data, BAO, Pantheon SN data, and R20, the latest SH0ES Team measurement of the Hubble constant ($H_0 = 73.2 \pm 1.3{\rm\,km\,s^{-1}\,Mpc^{-1}}$ at 68\% confidence level). Reduced tension might not simply come from a change in $H_0$ but also from an increase in its uncertainty due to degeneracy with additional physics, pointing to the need for additional probes. While no specific proposal makes a strong case for being highly likely or far better than all others, solutions involving early or dynamical dark energy, neutrino interactions, interacting cosmologies, primordial magnetic fields, and modified gravity provide the best options until a better alternative comes along.[Abridged]

https://iopscience.iop.org/article/10.1088/1361-6382/ac086d/pdf

In the realm of the Hubble tension - a review of solutions

Thank you for downloading gravitation and cosmology. As you may know, people have search hundreds times for their chosen novels like this gravitation and cosmology, but end up in malicious downloads. Rather than reading a good book with a cup of coffee in the afternoon, instead they juggled with some malicious bugs inside their laptop. gravitation and cosmology is available in our book collection an online access to it is set as public so you can get it instantly. Our digital library spans in multiple locations, allowing you to get the most less latency time to download any of our books like this one. Merely said, the gravitation and cosmology is universally compatible with any devices to read.

Gravitation And Cosmology

The strong discrepancy between local and early-time (inverse distance ladder) estimates of the Hubble constant H0 could be pointing towards new physics beyond the concordance ΛCDM model. Several attempts to address this tension through new physics rely on extended cosmological models, featuring extra free parameters beyond the six ΛCDM parameters. However, marginalizing over additional parameters has the effect of broadening the uncertainties on the inferred parameters (including H0), and it is often the case that within these models the H0 tension is addressed due to larger uncertainties rather than a genuine shift in the central value of H0. In this paper I consider an alternative viewpoint: what happens if a physical theory is able to fix the extra parameters to a specific set of nonstandard values? In this case, the degrees of freedom of the model are reduced with respect to the standard case where the extra parameters are free to vary. Focusing on the dark energy equation of state w and the effective number of relativistic species Neff, I find that physical theories able to fix w≈-1.3 or Neff≈3.95 would lead to an estimate of H0 from cosmic microwave background, baryon acoustic oscillation, and type Ia supernovae data in perfect agreement with the local distance ladder estimate, without broadening the uncertainty on the former. These two nonstandard models are, from a model-selection perspective, strongly disfavored with respect to the baseline ΛCDM model. However, models that predict Neff≈3.45 would be able to bring the tension down to 1.5σ while only being weakly disfavored with respect to ΛCDM, whereas models that predict w≈-1.1 would be able to bring the tension down to 2σ (at the cost of the preference for ΛCDM being definite). Finally, I estimate dimensionless multipliers relating variations in H0 to variations in w and Neff, which can be used to swiftly repeat the analysis of this paper in light of future more precise local distance ladder estimates of H0, should the tension persist. As a caveat, these results were obtained from the 2015 Planck data release, but these findings would be qualitatively largely unaffected were I to use more recent data.

New physics in light of the $H_0$ tension: an alternative view

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

Snowmass2021 - Letter of interest cosmology intertwined II: The hubble constant tension

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 ΛCDM model, based...

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 $\Lambda$CDM model, based on General Relativity and standard model of particles, are discussed in view of the most recent observational constraints. The motivations for considering extensions and modifications of General Relativity are taken into account, with particular attention to $f(R)$ and $f(T)$ theories of gravity where dynamics is represented by curvature or torsion field respectively. The features of $f(R)$ models are explored in metric and Palatini formalisms. We discuss the connection between $f(R)$ gravity and scalar-tensor theories highlighting the role of conformal transformations in the Einstein and Jordan frames. Cosmological dynamics of $f(R)$ models is investigated through the corresponding viability criteria. Afterwards, the equivalent formulation of General Relativity (Teleparallel Equivalent General Relativity) in terms of torsion and its extension to $f(T)$ gravity is considered. Finally, the cosmographic method is adopted to break the degeneracy among dark energy models. A novel approach, built upon rational Pade 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 $f(R)$ and $f(T)$ functions and to frame the late-time expansion history of the universe with no \emph{a priori} assumptions on its equation of state. A comparison between the $\Lambda$CDM cosmological model with $f(R)$ and $f(T)$ models is reported.

Extended Gravity Cosmography

We apply the holographic principle in the cosmological context through the nonadditive Tsallis entropy used to describe the thermodynamic properties of nonstandard statistical systems such as the gravitational ones. Assuming the future event horizon as the infrared cutoff, we build a dark energy model free from cosmological inconsistencies, which includes standard thermodynamics and standard holographic dark energy as a limiting case. We thus describe the dynamics of Tsallis holographic dark energy in a flat Friedmann-Lema\^{\i}tre-Roberson-Walker background. Hence, we investigate cosmological perturbations in the linear regime on subhorizon scales. We study the growth of matter fluctuations in the case of clustering dark matter and a homogeneous dark energy component. Furthermore, we employ the most recent late-time cosmic data to test the observational viability of our theoretical scenario. We thus obtain constraints on the free parameters of the model by means of Monte Carlo numerical method. We also used Bayesian selection criteria to estimate the statistical preference for Tsallis holographic dark energy compared to the concordance $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ paradigm. Our results show deviations from standard holographic dark energy within the $2\ensuremath{\sigma}$ confidence level. Finally, the analysis of the dark energy equation of state indicates a quintessencelike behavior with no evidence for phantom-divide crossing at the $1\ensuremath{\sigma}$ level.

Holographic dark energy from nonadditive entropy: Cosmological perturbations and observational constraints

Cosmography becomes non-predictive when cosmic data span beyond the red shift limit $z\simeq1 $. This leads to a \emph{strong convergence issue} that jeopardizes its viability. In this work, we critically compare the two main solutions of the convergence problem, i.e. the $y$-parametrizations of the redshift and the alternatives to Taylor expansions based on Pade series. In particular, among several possibilities, we consider two widely adopted parametrizations, namely $y_1=1-a$ and $y_2=\arctan(a^{-1}-1)$, being $a$ the scale factor of the Universe. We find that the $y_2$-parametrization performs relatively better than the $y_1$-parametrization over the whole redshift domain. Even though $y_2$ overcomes the issues of $y_1$, we get that the most viable approximations of the luminosity distance $d_L(z)$ are given in terms of Pade approximations. In order to check this result by means of cosmic data, we analyze the Pade approximations up to the fifth order, and compare these series with the corresponding $y$-variables of the same orders. We investigate two distinct domains involving Monte Carlo analysis on the Pantheon Superovae Ia data, $H(z)$ and shift parameter measurements. We conclude that the (2,1) Pade approximation is statistically the optimal approach to explain low and high-redshift data, together with the fifth-order $y_2$-parametrization. At high redshifts, the (3,2) Pade approximation cannot be fully excluded, while the (2,2) Pade one is essentially ruled out.

High-redshift cosmography: auxiliary variables versus Padé polynomials

Standard sirens are the gravitational wave (GW) analog of the astronomical standard candles and can provide powerful information about the dynamics of the Universe In this work, we simulate a catalog with 1000 standard siren events from binary neutron star mergers, within the sensitivity predicted for the third generation of the ground GW detector called the Einstein Telescope (ET) After correctly modifying the propagation of GWs as input to generate the catalog, we apply our mock dataset on scalar-tensor theories where the speed of GW propagation is equal to the speed of light As a first application, we find new observational bounds on the running of the Planck mass, when considering appropriate values within the stability condition of the theory, and we discuss some consequences on the amplitude of the running of the Planck mass In the second part, we combine our simulated standard sirens catalog with other geometric cosmological tests (supernovae Ia and cosmic chronometer measurements) to constrain the Hu-Sawicki $f(R)$ gravity model We thus find new and non-null deviations from the standard $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model, showing that in the future $f(R)$ gravity can be tested up to 95% confidence level The results obtained here show that the statistical accuracy achievable by future ground-based GW observations, mainly with the ET detector (and planned detectors with a similar sensitivity), can provide strong observational bounds on modified gravity theories

/pdf/probing-observational-bounds-on-scalar-tensor-theories-from-xanp9kiwvu.pdf

Rocco D'Agostino

Papers

Extended gravity cosmography

Extended Gravity Cosmography

Holographic dark energy from nonadditive entropy: Cosmological perturbations and observational constraints

High-redshift cosmography: auxiliary variables versus Padé polynomials

Probing observational bounds on scalar-tensor theories from standard sirens