Simultaneous determination of Hubble constant and cosmic baryon density: Forecasts for the synergy between FRBs and emerging probes

TL;DR

This paper explores how combining fast radio bursts with other cosmological probes can precisely determine the Hubble constant and baryon density, addressing key cosmological tensions.

Contribution

It demonstrates that multi-messenger approaches can break degeneracies and constrain key cosmological parameters more accurately than individual probes.

Findings

FRB+GW constrains H_0 and Ω_b to better than 1% and 1.5%.

FRB+SGL constrains H_0 and Ω_b to better than 1.5% and 3%.

Combining probes improves constraints significantly under optimistic scenarios.

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) provide a unique probe of both the Hubble constant $H_0$ and the cosmic baryon density $\Omega_{\rm b}$. However, constraints from FRBs alone suffer from a severe $H_0$-$\Omega_{\rm b}$ degeneracy that prevents them from resolving either problem. We show that this degeneracy can be broken by combining FRBs with other emerging probes whose degeneracy directions differ in the $H_0$-$\Omega_{\rm b}$ 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) surveys. The combinations FRB+GW, FRB+SGL, and FRB+21 cm IM each deliver simultaneous constraints on $H_0$ and $\Omega_{\rm b}$ better than (1%, 1.5%) in the $\Lambda$CDM model, and when dynamical dark energy is introduced, the constraining precision degrades gracefully as model complexity increases. Furthermore, within a model-independent framework, both FRB+GW and FRB+SGL constrain $H_0$ and $\Omega_{\rm b}$ to precisions better than (1.5%, 3%). These precision levels are based on nominal observational expectations and would improve significantly under optimistic observational scenarios.