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The $Lambda$ Cold Dark Matter model ($Lambda$CDM) represents the current standard model in cosmology. Within this, there is a tension between the value of the Hubble constant, $H_0$, inferred from local distance indicators and the angular scale of fluctuations in the Cosmic Microwave Background (CMB). We investigate whether the tension is significant enough to warrant new physics in the form of modifying or adding energy components to the standard cosmological model. We find that late time dark energy explanations are slightly disfavoured whereas a pre-CMB decoupling extra dark energy component has a marginally positive Bayesian evidence. A constant equation of state of the additional early energy density is constrained to 0.086$^{+0.04}_{-0.03}$. Although this value deviates significantly from 1/3, valid for dark radiation, the latter is not disfavoured based on the Bayesian evidence. If the tension persists, future estimates of $H_0$ at the 1$%$ level will be able to decisively determine which of the proposed explanations is favoured.
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We use the largest sample to date of spectroscopic SN Ia distances and redshifts to look for evidence in the Hubble diagram of large scale outflows caused by local voids suggested to exist at z<0.15. Our sample combines data from the Pantheon sample with the Foundation survey and the most recent release of lightcurves from the Carnegie Supernova Project to create a sample of 1295 SNe over a redshift range of 0.01<z<2.26. We make use of an inhomogeneous and isotropic Lemaitre-Tolman-Bondi metric to model a void in the SN Ia distance-redshift relation. We conclude that the SN luminosity distance-redshift relation is inconsistent at the 4-5 sigma confidence level with large local underdensities (|delta| > 20%, where the density contrast delta = Delta rho /rho) proposed in some galaxy count studies, and find no evidence of a change in the Hubble constant corresponding to a void with a sharp edge in the redshift range 0.023<z<0.15. With empirical precision of sigma_H_0 = 0.60%, we conclude that the distance ladder measurement is not affected by local density contrasts, in agreement with cosmic variance of sigma_H_0 = 0.42% predicted from simulations of large-scale structure. Given that uncertainty in the distance ladder value is sigma_H_0=2.2%, this does not affect the Hubble tension. We derive a 5 sigma constraint on local density contrasts on scales larger than 69 megaparsec h^-1 of delta < 27%. The presence of local structure does not appear to impede the possibility of measuring the Hubble constant to 1% precision.
The current cosmological probes have provided a fantastic confirmation of the standard $Lambda$ Cold Dark Matter cosmological model, that has been constrained with unprecedented accuracy. However, with the increase of the experimental sensitivity a few statistically significant tensions between different independent cosmological datasets emerged. While these tensions can be in portion the result of systematic errors, the persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the need for new physics. In this Letter of Interest we will focus on the $4.4sigma$ tension between the Planck estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we will list a few interesting new physics models that could solve this tension and discuss how the next decade experiments will be crucial.
The Hubble constant ($H_0$) estimated from the local Cepheid-supernova (SN) distance ladder is in 3-$sigma$ tension with the value extrapolated from cosmic microwave background (CMB) data assuming the standard cosmological model. Whether this tension represents new physics or systematic effects is the subject of intense debate. Here, we investigate how new, independent $H_0$ estimates can arbitrate this tension, assessing whether the measurements are consistent with being derived from the same model using the posterior predictive distribution (PPD). We show that, with existing data, the inverse distance ladder formed from BOSS baryon acoustic oscillation measurements and the Pantheon SN sample yields an $H_0$ posterior near-identical to the Planck CMB measurement. The observed local distance ladder value is a very unlikely draw from the resulting PPD. Turning to the future, we find that a sample of $sim50$ binary neutron star standard sirens (detectable within the next decade) will be able to adjudicate between the local and CMB estimates.
We investigate a generalized form of the phenomenologically emergent dark energy model, known as generalized emergent dark energy (GEDE), introduced by Li and Shafieloo [Astrophys. J. {bf 902}, 58 (2020)] in light of a series of cosmological probes and considering the evolution of the model at the level of linear perturbations. This model introduces a free parameter $Delta$ that can discriminate between the $Lambda$CDM (corresponds to $Delta=0$) or the phenomenologically emergent dark energy (PEDE) (corresponds to $Delta=1$) models, allowing us to determine which model is preferred most by the fit of the observational datasets. We find evidence in favor of the GEDE model for Planck alone and in combination with R19, while the Bayesian model comparison is inconclusive when Supernovae Type Ia or BAO data are included. In particular, we find that $Lambda$CDM model is disfavored at more than $2sigma$ CL for most of the observational datasets considered in this work and PEDE is in agreement with Planck 2018+BAO+R19 combination within $1sigma$ CL.
The Hubble constant ($H_0$) tension between Type Ia Supernovae (SNe Ia) and Planck measurements ranges from 4 to 6 $sigma$. To investigate this tension, we estimate $H_{0}$ in the $Lambda$CDM and $w_{0}w_{a}$CDM models by dividing the Pantheon sample, the largest compilation of SNe Ia, into 3, 4, 20 and 40 bins. We fit the extracted $H_{0}$ values with a function mimicking the redshift evolution: $g(z)={H_0}(z)=tilde{H}_0/(1+z)^alpha$, where $alpha$ indicates an evolutionary parameter and $tilde{H}_0=H_0$ at $z=0$. We set the absolute magnitude of SNe Ia so that $H_0=73.5,, textrm{km s}^{-1},textrm{Mpc}^{-1}$, and we fix fiducial values for $Omega_{0m}^{Lambda CDM}=0.298$ and $Omega_{0m}^{w_{0}w_{a}CDM}=0.308$. We find that $H_0$ evolves with redshift, showing a slowly decreasing trend, with $alpha$ coefficients consistent with zero only from 1.2 to 2.0 $sigma$. Although the $alpha$ coefficients are compatible with 0 in 3 $sigma$, this however may affect cosmological results. We measure locally a variation of $H_0(z=0)-H_0(z=1)=0.4, textrm{km s}^{-1},textrm{Mpc}^{-1}$ in 3 and 4 bins. Extrapolating ${H_0}(z)$ to $z=1100$, the redshift of the last scattering surface, we obtain values of $H_0$ compatible in 1 $sigma$ with Planck measurements independently of cosmological models and number of bins we investigated. Thus, we have reduced the $H_0$ tension from $54%$ to $72%$ for the $Lambda$CDM and $w_{0}w_{a}$CDM models, respectively. If the decreasing trend of $H_0(z)$ is real, it could be due to astrophysical selection effects or to modified gravity.
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