Diving into the Darkest Corners of the Universe with Fast Radio Bursts

Even the darkest corners of the universe are not completely empty. A recent study has used fast radio bursts to provide the first estimate of the baryon content in cosmic voids.

What’s in a Void?

Cosmological studies have revealed the large-scale filamentary structure of the universe known as the cosmic web. Chains of galaxies and galaxy clusters fall into spindly strands (or filaments) of matter overdensities, opening up cavernous cosmic voids that comprise the darkest corners of the universe. These seemingly vacant voids serve as critical test beds for cosmology, allowing researchers to constrain things like the nature of dark energy and the matter density of the universe.

Despite their name, voids are not completely empty but are instead underdensities with considerably less matter compared to the bustling filaments they border. The baryon (normal matter) content of cosmic voids has important implications for cosmological models and large-scale galactic feedback processes, but assessing how many baryons actually occupy voids has only recently become observationally accessible.

Fast radio bursts — brief, intense flashes of radio waves emitted by compact objects — have proven to be versatile tools for measuring both underdensities and overdensities throughout the universe. The dispersion measure of fast radio bursts, which tells us how many free electrons fall between the radio emitter and observer, can be directly used to map the baryon content along the lines of sight. Fast radio burst sight lines passing through overdensities will produce a dispersion measure excess, and those passing through underdensities will produce a dispersion measure deficit relative to the cosmic mean. Thus far, fast radio bursts have been used extensively to study overdense galaxy and cluster filaments, but their sensitivity to underdense regions like voids has yet to be explored.

Chiming In on the Baryon Search

In concept, directly stacking fast radio burst sight lines that coincide with the positions of known cosmic voids provides a direct measurement of the baryon underdensity inside voids. With no previous observational exploration of this quantity, Kritti Sharma (California Institute of Technology) and collaborators leveraged the second catalog of the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Bursts (CHIME/FRB) sample in conjunction with the Sloan Digital Sky Survey (SDSS) Baryon Oscillation Spectroscopic Survey (BOSS) void catalog to provide the first observational assessment of the baryon underdensity in cosmic voids.

Stacking thousands of fast radio burst sight lines that spatially align with thousands of known voids, the authors measured a statistically significant dispersion measure deficit toward void centers. This deficit indicates that, as theory predicts, baryons inhabit cosmic voids at a suppressed level. The team used cosmological models to quantify the electron density contrast between the interior and exterior of voids, finding an approximate baryon underdensity of 60% ± 30% relative to the cosmic mean. Combining this result with existing thermal scattering measurements of the cosmic microwave background, the authors estimated a mean void gas temperature of about 1.1 million Kelvin — voids are occupied by a suppressed amount of warm-hot diffuse gas.

This study provides a unique approach to baryon mapping and opens up the door for future investigation of cosmic voids. While not physically containing much, voids hold vast astrophysical and cosmological significance. With the astronomical stage primed with galaxy-mapping powerhouses like the Dark Energy Spectroscopic Instrument, Vera Rubin Observatory, Nancy Grace Roman Space Telescope, and Euclid Space Telescope, cosmic voids will become a cornerstone of cosmological studies.

Citation

“Baryons in the Darkest Sites of the Universe,” Kritti Sharma et al 2026 ApJL 1006 L3. doi:10.3847/2041-8213/ae81ac