Finding the missing baryons in the intergalactic medium with localized fast radio bursts

TL;DR

This paper uses localized fast radio bursts and advanced simulations to accurately measure the cosmic baryon density, providing strong evidence that missing baryons are in the intergalactic medium and aligning with other cosmological measurements.

Contribution

It introduces a novel method that accounts for IGM inhomogeneities and host galaxy dispersion measures to precisely determine the universe's baryon content.

Findings

Measured baryon density $\, ext{Ω}_b=0.0490^{+0.0036}_{-0.0033}$ with 7.0% precision

Results are consistent with cosmic microwave background and Big Bang nucleosynthesis

Supports that missing baryons reside in the intergalactic medium

Abstract

The missing baryon problem is one of the major unsolved problems in astronomy. Fast radio bursts (FRBs) are bright millisecond pulses with unknown origins. The dispersion measure of FRBs is defined as the electron column density along the line of sight, and accounts for every ionized baryon. Here we measure the baryon content of the Universe using 22 localized FRBs. Unlike previous works that fixed the value of dispersion measure of FRB host galaxies and ignored the inhomogeneities of the intergalactic medium (IGM), we use the probability distributions of dispersion measures contributed by host galaxies and IGM from the state-of-the-art IllustrisTNG simulations. We derive the cosmic baryon density of $\Omega_b=0.0490^{+0.0036}_{-0.0033}$ (1$\sigma$), with a precision of 7.0%. This value is dramatically consistent with other measurements, such as the cosmic microwave background and Big Bang nucleosynthesis. Our work supports that the baryons are not missing, but residing in the IGM.