Resolving Hubble tension and locating missing baryons: Synergies between fast radio bursts and emerging cosmological probes

TL;DR

The paper investigates how fast radio bursts (FRBs), via their dispersion measure, can jointly constrain the Hubble constant $H_0$ and the cosmic baryon density $\Omega_{\rm b}$ when combined with three emerging probes: gravitational wave (GW) standard sirens, strong gravitational lensing (SGL) time delays, and 21 cm intensity mapping (IM). FRBs alone suffer a degeneracy between $H_0$ and $\Omega_{\rm b}$; the authors forecast, using mock data for SKA FRBs, Einstein Telescope GW standard sirens, LSST SGL time delays, and HIRAX 21 cm IM, that FRB+GW, FRB+SGL, and FRB+21 cm IM can simultaneously constrain $H_0$ and $\Omega_{\rm b}$ to sub-percent to a few-percent precision across $\Lambda$CDM, $w$CDM, CPL, and cosmography. The results show FRB+GW provides the strongest $H_0$ and $\Omega_{\rm b}$ constraints in ΛCDM, with substantial improvements over GW alone; FRB+SGL and FRB+21 cm IM also yield competitive gains, with FRB+21 cm IM offering the greatest leverage in dynamical dark energy models. Even in a model-independent cosmographic framework, FRB+GW and FRB+SGL deliver robust, unbiased constraints on $H_0$ and $\Omega_{\rm b}$ at the 1–2% level, highlighting the promise of multi-messenger cosmology to address the Hubble tension and locate missing baryons.

Abstract

Two of the most pressing challenges in cosmology are the persistent discrepancy in measurements of the Hubble constant, referred to as the Hubble tension, and the deficit of baryons in the local Universe, known as the missing baryon problem. Fast radio bursts (FRBs) encode the integrated electron column density along their lines of sight, offering a unique probe of both the cosmic expansion rate ($H_0$) and the baryon density ($Ω_{\rm b}$). However, constraints from FRBs alone suffer from a severe $H_0$-$Ω_{\rm b}$ degeneracy that prevents them from resolving either problem independently. We show that this degeneracy can be broken by combining FRBs with other emerging probes whose degeneracy directions differ in the $H_0$-$Ω_{\rm b}$ plane. Specifically, we quantify three multi-messenger approaches: FRBs paired with gravitational wave (GW) standard sirens, strong gravitational lensing (SGL) time delays, and 21 cm intensity mapping (IM). The combinations FRB+GW, FRB+SGL, and FRB+21 cm IM each deliver simultaneous constraints on $H_0$ and $Ω_{\rm b}$ better than ($1\%$, $1\%$) in the $Λ$CDM model, ($1.5\%$, $2\%$) in the $w$CDM model, and ($2\%$, $3.5\%$) in the CPL model. Moreover, in a model-independent framework, both FRB+GW and FRB+SGL constrain $H_0$ and $Ω_{\rm b}$ to better than ($1\%$, $2\%$) precision. These results demonstrate that the synergy between FRBs and other emerging probes holds great promise for resolving the Hubble tension and locating the missing baryons.

Figures (3)

• Figure 1: Simulated observational data for four emerging cosmological probes: FRBs from SKA (top left); GW standard sirens from ET (top middle); SGL time delays from LSST (top right); and 21 cm IM from HIRAX (bottom three panels). For the FRB, GW, and SGL data, we show not only the measurement uncertainties but also the redshift distributions of the simulated samples.
• Figure 2: Two-dimensional posterior distribution contours in the $H_0-\Omega_{\rm m}$ plane for the $\Lambda$CDM, $w$CDM, and CPL models: constraints from the simulated FRB, GW, SGL, 21 cm IM, FRB+GW, FRB+SGL, and FRB+21cmIM data.
• Figure 3: Constraints on cosmographic parameters derived from FRB, GW, and FRB+GW in the left panel, and from FRB, SGL, and the joint FRB+SGL probe in the right panel.