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arXiv · 2508.14434

Breaking the Baryon Density$\unicode{x2013}$Hubble Constant Degeneracy in Fast Radio Burst Applications with Associated Gravitational Waves

Joscha N. Jahns-Schindler·Laura G. Spitler
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Abstract

Fast Radio Bursts (FRBs) are a unique probe of the cosmos, owing to dispersion caused by free electrons in the intergalactic medium (IGM). Two of the main quantities of interest are degenerate: the density of matter $Ω_\mathrm{b}f_\mathrm{d}$ outside of galaxies and the Hubble constant $H_0$. Here, we present a new possibility of breaking the degeneracy without invoking early Universe priors on $Ω_\mathrm{b}$. Assuming some FRBs originate in compact object mergers, the combination of dispersion and luminosity distance from the gravitational wave (GW) can be used to measure $Ω_\mathrm{b}h^2f_\mathrm{d}$ (where $h$ is the dimensionless Hubble constant). We show that this measurement can be combined with the abundant FRBs that have a redshift measurement. This combination breaks the degeneracy with the Hubble constant. We develop a Bayesian framework and forecast that third-generation GW detectors are required to obtain meaningful constraints. We forecast that one year of Einstein Telescope operations can constrain $H_0$ to $\pm 6\,\mathrm{km}\mathrm{s}^{-1}\mathrm{Mpc}^{-1}$ and $Ω_\mathrm{b}h^2f_\mathrm{d}$ to $^{+0.0015}_{-0.0016}$ (68$\,\%$ credible interval). The method can also be used with luminosity distances obtained through other means than GWs.

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Joscha N. Jahns-Schindler, Laura G. Spitler. 2025-08-20. Breaking the Baryon Density$\unicode{x2013}$Hubble Constant Degeneracy in Fast Radio Burst Applications with Associated Gravitational Waves. https://arxiv.org/abs/2508.14434

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