Astronomers Solve Mystery of Missing Ordinary Matter in the Universe with Fast Radio Bursts

Astronomers have long struggled to measure the distribution of ordinary matter in the universe, due to its diffuse nature and emission of light at different wavelengths. This has led to a decades-long challenge known as the missing baryon problem. However, a new study published in Nature Astronomy has shed light on this issue by using fast radio bursts (FRBs) to map out the previously unseen matter.

Led by Liam Connor, an assistant professor of astronomy at Harvard University, a team of researchers has directly observed the missing matter by using the flashing of FRBs. The study used a combination of previously observed FRBs and new observations to measure the amount of matter along the pathway of the FRBs.

The team used the Deep Synoptic Array, a network of 110 radio telescopes located near Bishop, California, to find and identify 39 FRBs in the study. The farthest FRB discovered during the research, named FRB 20230521B, holds the record for the most distant FRB ever observed. The study also relied on observations from the W. M. Keck Observatory in Hawaii and Palomar Observatory near San Diego to measure distances between the FRBs and Earth.

By measuring how much each FRB signal slowed down as it passed through space before reaching Earth, the team was able to illuminate the gas it encountered along the way. The short pulses of FRBs are crucial for this measurement because they act like flashing cosmic beacons.

The team determined that 76% of cosmic matter exists as hot, low-density gas in the space between galaxies. Another 15% can be found in galactic halos, while the remainder is located within galaxies themselves as stars, planets, or cold gas. This observation-based finding aligns with prior predictions made using simulations.

Understanding the distribution of ordinary matter can help researchers understand how galaxies grow and evolve. "Baryons are pulled into galaxies by gravity, but supermassive black holes and exploding stars can blow them back out — like a cosmic thermostat cooling things down if the temperature gets too high," Connor said. "Our results show this feedback must be efficient, blasting gas out of galaxies and into the intergalactic medium."

Fast radio bursts may also be able to help map the cosmic web in detail, which serves as the backbone of the universe and is largely made of dark matter. Caltech is currently planning to build another radio telescope in the Nevada desert that could build upon the findings from this study by finding and tracing up to 10,000 FRBs per year.

"It's a triumph of modern astronomy," said Vikram Ravi, an assistant professor of astronomy at Caltech and co-author of the study. "We're beginning to see the Universe's structure and composition in a whole new light, thanks to FRBs. These brief flashes allow us to trace the otherwise invisible matter that fills the vast spaces between galaxies."