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Monthly Notices of the Royal Astronomical Society

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

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

We use the largest sample to date of spectroscopic supernova (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 light curves 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 4–5σ confidence level with large local underdensities (, where the density contrast δ = Δρ/ρ) 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 an empirical precision of , we conclude that the distance ladder measurement is not affected by local density contrasts, in agreement with a cosmic variance of , predicted from simulations of large-scale structure. Given that uncertainty in the distance ladder value is , this does not affect the Hubble tension. We derive a 5σ constraint on local density contrasts on scales larger than of . The presence of local structure does not appear to impede the possibility of measuring the Hubble constant to 1% precision.

https://iopscience.iop.org/article/10.3847/1538-4357/ab0ebf/pdf

The Local Perspective on the Hubble Tension: Local Structure Does Not Impact Measurement of the Hubble Constant

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 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.

Separating the components of redshift due to expansion and motion in the nearby universe ($z<0.1$) is critical for using Type Ia Supernovae (SNe Ia) to measure the Hubble constant ($H_0$) and the equation-of-state parameter of dark energy ($w$). Here, we study the two dominant 'motions' contributing to nearby peculiar redshifts: large-scale, coherent-flow (CF) motions and small-scale motions due to gravitationally-associated galaxies deemed to be in a galaxy group. We use a set of 585 low-$z$ SNe from the Pantheon+ sample, and evaluate the efficacy of corrections to these motions by measuring the improvement of SN distance residuals. We study multiple methods for modeling the large and small-scale motions and show that while group assignments and CF corrections individually contribute to small improvements in Hubble residual scatter, the greatest improvement comes from the combination of the two (relative standard deviation of the Hubble residuals RSD improves from 0.167 mag to 0.157 mag). We find the optimal flow corrections derived from various local density maps significantly reduce Hubble residuals while raising $H_0$ by $\sim0.4$ km s$^{-1}$ Mpc$^{-1}$ as compared to using CMB redshifts, disfavoring the hypothesis that unrecognized local structure could resolve the Hubble tension. We estimate that the systematic uncertainties in cosmological parameters after optimally correcting redshifts are 0.08-0.17 km s$^{-1}$ Mpc$^{-1}$ in $H_0$ and 0.02-0.03 in $w$ which are smaller than the statistical uncertainties for these measurements: 1.5 km s$^{-1}$ Mpc$^{-1}$ for $H_0$ and 0.04 for $w$.

W. D'Arcy Kenworthy

Papers

The Local Perspective on the Hubble Tension: Local Structure Does Not Impact Measurement of the Hubble Constant

The Local Perspective on the Hubble Tension: Local Structure Does Not Impact Measurement of the Hubble Constant

The Pantheon+ Analysis: Evaluating Peculiar Velocity Corrections in Cosmological Analyses with Nearby Type Ia Supernovae