2023ApJ...955..101W

We use the dispersion measure (DM) and redshift measurements of 24 localized fast radio bursts (FRBs) to compare cosmological models and investigate the Hubble tension. Setting a flat prior on the DM contribution from the Milky Way's halo, ${\mathrm{DM}}_{\mathrm{halo}}^{\mathrm{MW}}\in [5,\,80]\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$, the best fit for flat ΛCDM is obtained with a Hubble constant ${H}_{0}={95.8}_{-9.2}^{+7.8}\,\mathrm{km}\,{{\rm{s}}}^{-1}\,{\mathrm{Mpc}}^{-1}$ and a median matter density Ω_m ≈ 0.66. The best fit for the R_h = ct universe is realized with ${H}_{0}={94.2}_{-6.2}^{+5.6}\,\mathrm{km}\,{{\rm{s}}}^{-1}\,{\mathrm{Mpc}}^{-1}$. We emphasize that the H₀ measurement depends sensitively on the ${\mathrm{DM}}_{\mathrm{halo}}^{\mathrm{MW}}$ prior. Since flat ΛCDM has one more free parameter, R_h = ct is favored by the Bayesian Information Criterion (BIC) with a likelihood of ∼73% versus ∼27%. Through simulations, we find that if the real cosmology is ΛCDM, a sample of ∼1150 FRBs in the redshift range 0 < z < 3 would be sufficient to rule out R_h = ct at a 3σ confidence level, while ∼550 FRBs would be necessary to rule out ΛCDM if the real cosmology is instead R_h = ct. The required sample sizes are different, reflecting the fact that the BIC imposes a severe penalty on the model with more free parameters. We further adopt a straightforward method of deriving an upper limit to H₀, without needing to consider the poorly known probability distribution of the DM contributed by the host galaxy. The theoretical DM contribution from the intergalactic medium (DM_IGM) at any z is proportional to H₀. Thus, requiring the extragalactic DM_ext to be larger than DM_IGM delimits H₀ to the upside. Assuming flat ΛCDM, we have H₀ < 89.0 km s^–1 Mpc^–1 at a 95% confidence level.