Revealing Limitation in the Standard Cosmological Model: A Redshift-Dependent Hubble Constant from Fast Radio Bursts

TL;DR

FRB dispersion measures offer a novel late-time probe of $H_0$ but face degeneracies with baryon priors and DM contributions. The authors combine ML-based DM reconstruction with Bayesian inference to test whether $H_0$ is constant in $Λ$CDM, finding redshift dependence that depends on priors on $Ω_b$ or $Ω_b h^2$. They show that joint priors or a dynamical dark-energy framework ($w_0w_a$CDM) can yield a redshift-invariant $H_0$, indicating $Λ$CDM may be incomplete at late times. The results underscore the need for flexible cosmological models and better FRB systematics, with FRBs offering a complementary perspective to SNe Ia and CMB data.

Abstract

A major issue in contemporary cosmology is the persistent discrepancy, known as the Hubble tension, between the Hubble constant ($H_0$) estimates from local measurements and those inferred from early-Universe observations under the standard $Λ$ cold dark matter ($Λ$CDM) paradigm. Recent advances have identified fast radio bursts (FRBs), a class of extragalactic phenomena observable at considerable redshifts, as a promising observational tool for probing late-time cosmology. In this study, we incorporate two complementary methodologies, machine learning algorithms and Bayesian analysis, on a set of localized FRBs to rigorously test the consistency of the $Λ$CDM model at late cosmic epochs. Our results reveal a statistically significant redshift-dependent variation of $H_0$ when using separate priors on baryon density parameters $Ω_\mathrm{b}$ or $Ω_\mathrm{b}h^2$, indicating contradiction to the core postulate of $Λ$CDM. However, when the priors are combined, this redshift dependence disappears, yielding a consistent estimate of $H_0$. We further validate that the redshift dependency of $H_0$ can be removed within the more flexible framework of $w_0w_a$CDM model even without combining the priors. These findings highlight that the redshift evolution of $H_0$ is not merely an artifact of the standard model but an indication of a deeper inadequacy in the $Λ$CDM model, supporting the need for a more flexible cosmological framework.

Figures (4)

• Figure 1: Observed and predicted $\mathrm{DM}'-z_\mathrm{s}$ relation. The green points represent measured redshifts $z_\mathrm{s}$ values for the localized FRBs plotted against the corresponding $\mathrm{DM}' = \mathrm{DM} - \mathrm{DM}_\mathrm{MW}$ including observational uncertainties. The red curve shows the ML prediction, with the shaded region denoting the associated 1$\sigma$ uncertainty. For comparison, we plot the Macquart relation combined with mean host and halo contribution in blue line with $H_0 = 73 \rm\,km\,s^{-1}\,Mpc^{-1}$ and $f_\mathrm{IGM}=0.85$ under $\Lambda$CDM cosmology.
• Figure 2: Variation of $H_0$ with respect to redshift for different $f_\mathrm{IGM}$. Solid and dashed lines denote the results obtained using the trained ML model, while scatter points represent the corresponding values derived from the Bayesian analysis.
• Figure 3: Posterior distribution of $\Omega_\mathrm{b}H_0$ across three redshift bins, derived using the joint prior that combines constraints from DES and DESI+CMB.
• Figure 4: Reduced $\chi^2$ value for different combinations of ($w_0,w_a$). This contour plot is independent of value of $f_\mathrm{IGM}$ and of the choice to fix $\Omega_\mathrm{b}$ or $\Omega_\mathrm{b}h^2$. Different combinations of ($w_0,w_a$) yield $\chi^2_\mathrm{reduced}\ll1$ supporting the statistical consistency of a redshift-independent $H_0$ within these models.