Decoding the cosmological baryonic fluctuations using localized fast radio bursts

¹ Institute of Astronomy, National Tsing Hua University, 101, Section 2. Kuang-Fu Road, Hsinchu, 30013, Taiwan
² Department of Physics, National Tsing Hua University, 101, Section 2. Kuang-Fu Road, Hsinchu, 30013, Taiwan
³ Department of Physics, National Chung Hsing University, 145, Xingda Road, Taichung, 40227, Taiwan
⁴ Department of Physics and Astronomy, Physics Building, University College London, Gower St, London WC1E 6BT, UK
⁵ Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
⁶ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia
⁷ OzGrav: The Australian Research Council Centre of Excellence for Gravitational Wave Discovery, Hawthorn, VIC 3122, Australia
⁸ ASTRO3D: The Australian Research Council Centre of Excellence for All-sky Astrophysics in 3D, ACT 2611, Australia
⁹ National Institute of Physics, College of Science, University of the Philippines, Diliman, Quezon City, Philippines

^⋆ Corresponding authors: emma30407@gmail.com; tetsuya@phys.nchu.edu.tw

Received: 23 October 2024
Accepted: 23 April 2025

Abstract

Aims. The enigma of the missing baryons poses a prominent and unresolved problem in astronomy. Dispersion measures (DM) serve as a distinctive observable of fast radio bursts (FRBs). They quantify the electron column density along each line of sight and reveal the missing baryons that are described in the Macquart (DM–z) relation. The scatter of this relation is anticipated to be caused by the variation in the cosmic structure. This is not yet statistically confirmed, however. We present statistical evidence that the cosmological baryons fluctuate.

Methods. We measured the foreground galaxy number densities around 14 and 13 localized FRBs with the WISE-PS1-STRM and WISE × SCOS photometric redshift galaxy catalog, respectively. The foreground galaxy number densities were determined through a comparison with measured random apertures with a radius of 1 Mpc.

Results. We found a positive correlation between the excess of DM that is contributed by the medium outside galaxies (DM_cosmic) and the foreground galaxy number density. The correlation is strong and statistically significant, with median Pearson coefficients of 0.6 and 0.6 and median p-values of 0.012 and 0.032 for the galaxy catalogs, respectively, as calculated with Monte Carlo simulations.

Conclusions. Our findings indicate that the baryonic matter density outside galaxies exceeds its cosmic average along the line of sight to regions with an excess galaxy density, but there are fewer baryons along the line of sight to low-density regions. This is statistical evidence that the ionized baryons fluctuate cosmologically on a characteristic scale of ≲6 Mpc.