Sign up for CNN’s Wonder Theory science newsletter to explore the universe with news on fascinating discoveries, scientific advancements, and more. Recent research has revealed that astronomers are utilizing fast radio bursts (FRBs)—mysterious millisecond-long bright flashes of radio waves from space—to locate some of the elusive missing matter in the universe.
The universe is primarily composed of dark matter and dark energy, which together account for most of its mass. According to NASA, dark matter is an enigmatic substance that influences the structure of the cosmos, while dark energy drives the universe's accelerated expansion. Although both substances cannot be directly observed, their presence can be inferred from their gravitational effects. In contrast, the remaining universe is made up of cosmic baryons, or ordinary matter, which consists of familiar particles like protons and neutrons.
“If you add up all the stars, planets, and cold gas visible through telescopes, it amounts to less than 10% of the ordinary matter in the universe,” explained Liam Connor, an assistant professor of astronomy at Harvard University. For years, astronomers believed that most of this ordinary matter existed in the intergalactic medium—the space between galaxies—or within the extended halos of galaxies. However, measuring this elusive “foglike” matter proved challenging.
The difficulty in detecting approximately half of the cosmos' ordinary matter has led to a long-standing challenge in cosmology known as the missing baryon problem. However, Connor and his team have made significant strides in this area by utilizing the unique properties of fast radio bursts to map out previously unseen matter. Their groundbreaking findings were published in a study in the journal Nature Astronomy.
“The FRBs shine through the fog of the intergalactic medium, and by precisely measuring how the light slows down, we can weigh that fog, even when it’s too faint to see,” Connor stated, serving as the lead author of the paper. Much of the research was conducted while Connor was a research assistant at the California Institute of Technology (Caltech).
Since their discovery in 2007, more than a thousand fast radio bursts have been detected, but only about 100 have been traced back to their originating galaxies. The causes behind these bursts remain largely unknown, but their increasing frequency may shed light on their mysterious origins. The new analysis included a combination of previously observed fast radio bursts and newly detected flashes, highlighting the dynamic nature of this research.
The study focused on 69 fast radio bursts, which were located between 11.74 million and nearly 9.1 billion light-years away from Earth. Notably, the farthest burst, named FRB 20230521B, was discovered during the research, setting a record as the most distant fast radio burst ever observed.
The research team utilized the Deep Synoptic Array, a network of 110 radio telescopes located near Bishop, California, to identify 39 of the fast radio bursts. The W. M. Keck Observatory in Hawaii and Palomar Observatory near San Diego assisted in measuring the distances between these radio bursts and Earth. The remaining 30 bursts were discovered using the Australian Square Kilometre Array Pathfinder and various telescopes worldwide.
As fast radio bursts travel toward Earth, their light can be measured in different wavelengths, which spread out due to the matter they encounter. The study measured how much each fast radio burst slowed down as it passed through space, effectively illuminating the gas it encountered. The speed of these bursts can vary based on the medium they travel through, with longer red wavelengths arriving later than shorter blue wavelengths.
“We can measure very precisely how much the radio pulse is slowed down at different wavelengths; this effectively counts all the baryons,” Connor explained. “For continuous light from a star, we cannot measure this ‘dispersion’ effect. It must be impulsive, short, and at radio wavelengths.” The technological advancements in measuring dispersion have allowed astronomers to map and quantify the matter along the pathways of fast radio bursts.
“It’s like we’re seeing the shadow of all the baryons, with FRBs as the backlight,” highlighted Vikram Ravi, a coauthor of the study. “If you see a person in front of you, you can learn a lot about them. But if you only see their shadow, you still know they’re there and have a rough idea of their size.”
By mapping out the fast radio bursts and the matter they illuminated, the team concluded that 76% of cosmic matter exists as hot, low-density gas in the space between galaxies. An additional 15% is located in galactic halos, while the remainder can be found within galaxies themselves, in the form of stars, planets, or cold gas. These findings align with prior predictions made using simulations, affirming their significance in the field of cosmology.
Understanding the distribution of ordinary matter is crucial for comprehending galaxy formation and evolution. “Baryons are pulled into galaxies by gravity, but supermassive black holes and exploding stars can blow them back out—acting like a cosmic thermostat,” Connor noted. “Our results indicate that this feedback mechanism is efficient, expelling gas from galaxies into the intergalactic medium.”
Moreover, fast radio bursts may serve as valuable tools for mapping the cosmic web, a structure predominantly composed of dark matter and serving as the universe's backbone. Caltech is currently planning to construct another radio telescope in the Nevada desert, aiming to detect and trace up to 10,000 fast radio bursts annually, further building upon these groundbreaking findings.
“It’s a triumph of modern astronomy,” Ravi stated. “We are beginning to see the universe’s structure and composition in a whole new light, thanks to fast radio bursts. These brief flashes allow us to trace the otherwise invisible matter that fills the vast spaces between galaxies.”
