Fast Radio Bursts Poised to Help with Biggest Cosmic Mysteries

Intense, brief flashes of radio light called fast radio bursts (FRBs) travel across billions of light-years to reach Earth, passing through a fog of matter along the way. The bursts' origins are unclear but are thought to possibly come from highly magnetized dead stars called magnetars. The denser the fog the FRBs travel through, the more their signals will become dispersed—similar to the way a prism splits white light into a rainbow of colors.

Thanks to this dispersing trait, FRBs make excellent tracers of how ordinary matter is distributed in the universe; ordinary matter is the same stuff that makes up people, planets, stars, and anything made of subatomic particles called baryons. As the FRB radio beams pass through this matter in our universe, they can essentially map out how much is present and how clumpy it is.

In a new Nature Astronomy study, researchers show how these FRB measurements can help to solve some of the biggest questions in cosmology.

"We've established that FRBs are a leading probe of the distribution of matter in the universe," says Kritti Sharma (MS '24), lead author of the new study and a graduate student working with Vikram Ravi, a professor of astronomy at Caltech and a coauthor of the paper. "These FRB data can be used to enhance cosmology experiments that are trying to answer questions about dark matter, dark energy, and the mass of neutrinos."

Many questions persist about the nature of dark energy, a repulsive force or substance that is causing our universe to fly apart at increasing speeds, and about dark matter, a substance that far outweighs matter in our universe but cannot be seen. Mysteries about neutrinos, ghostly particles that pass freely through ordinary matter, also endure—including the particles' mass, a measurement that could help reveal how large-scale galactic structures in the universe formed.

Dark energy, dark matter, and neutrinos are all predicted to influence how matter clumps together, so scientists use sky surveys to map this clumping and gain clues to the nature of these cosmological phenomena. The problem is that feedback processes inside galaxies can also affect how smooth or clumpy matter is, muddying the researchers' ability to precisely measure the cosmological effects.

All galaxies harbor supermassive black holes at the centers, which voraciously feed on nearby matter while also ejecting winds of hot, ionized—or charged—gas into their surroundings. Exploding stars can also expel energy into the galactic neighborhoods. This feedback has a role in smoothing out the material outside the galaxies, making it less clumpy.

"The feedback process thins the gas around the galaxies, redistributing matter across vast distances. It smooths out clumps of matter in a way that looks astonishingly similar to what massive neutrinos do, or what dark energy or dark matter theories predict," Ravi says. "Unless scientists can independently measure this contribution from feedback, they can't tell these effects apart."

The new study, which analyzed a sample of about 100 FRBs, is the first to directly measure the impact of feedback on clumpy matter in the large-scale regions around and between galaxies. The results show that galactic feedback does indeed smooth surrounding material, making it less clumpy. However, it does so less than what has been measured previously by state-of-the-art surveys, including the eROSITA X-ray telescope, and the former microwave-based Atacama Cosmology Telescope in Chile, which ended in 2022.

"Our analysis of FRBs reveals how gas ejected by astrophysical feedback suppresses cosmic structure, delivering constraints competitive with X-ray and microwave surveys," says coauthor Elisabeth Krause (PhD '12), a professor of astronomy and physics at The University of Arizona. "This is amazing considering we only had about 100 FRBs in our sample. It's only the beginning."

Caltech's Deep Synoptic Array (DSA), a powerful radio telescope scheduled to be built by 2029 in a remote valley in Nevada, is expected to find tens of thousands of FRBs, vastly enhancing the cosmic events' power to improve cosmology measurements. Data from the DSA, which is funded by Schmidt Sciences, will work synergistically with several cosmology experiments, including the European Euclid mission, in which NASA's Jet Propulsion Laboratory and Caltech's IPAC astronomy center play key roles (Caltech manages JPL for NASA); the Dark Energy Spectroscopic Instrument (DESI) in Arizona; the Vera Rubin Observatory in Chile; and NASA's newly launched Nancy Grace Roman Telescope, in which JPL and Caltech's IPAC also play roles.

"The DSA will be a game changer for the field," says Ravi, who is the co-principal investigator on the DSA project.

The Nature Astronomy paper is titled "Signatures of Suppressed Matter Clustering revealed by Fast Radio Bursts." This work builds on a series of related studies from the team, including "Baryons in the Darkest Sites of the Universe," "Backlighting the Cosmic Web with Fast Radio Bursts: An Anthology of Dispersion Measure Cross-Correlations with Large-Scale Structure and Baryon Tracers," "Quantifying the Impact of Selection Effects on FRB DM–z Relation Cosmological Inference," "Probing Baryonic Feedback and Cosmology with the 3×2-point Statistic of FRBs and Galaxies," and "A Hydrodynamical Simulations-based Model that Connects the FRB DM-Redshift Relation to Suppression of the Matter Power Spectrum via Feedback."

Coauthors on the study include Pranjal R. S. (University of Arizona); Dhayaa Anbajagane (University of Chicago); and Liam Connor (Harvard and Smithsonian Center for Astrophysics). Coauthors on related studies include Assistant Professor Kimmy Wu and staff scientist Casey Law (Caltech); Simone Ferraro (UC Berkeley); Sebastian Grandis (University of Innsbruck); David Alonso and William Coulton (University of Oxford); Yi-Kuan Chiang (Academia Sinica Institute of Astronomy and Astrophysics); Samuel McCarty (Harvard and Smithsonian Center for Astrophysics); Nico Schuster (Aix-Marseille University); Alice Pisani (Princeton University); Shivam Pandey (University of Arizona); Nico Hamaus (University Observatory Munich); and Robert Reischke (University of Bonn).

This research was supported by Schmidt Sciences; the National Science Foundation; the Kavli Institute for Theoretical Physics; the David and Lucile Packard Foundation; the European Research Council; Aix-Marseille University's French Initiative of Excellence; the Austrian Research Promotion Agency; Austria's Federal Ministry of Innovation, Mobility, and Infrastructure; and the Austrian Space Applications Program.; and the Austrian Space Applications Program.