Powerful millisecond flashes of radiation known as fast radio bursts (FRBs) have baffled scientists since their discovery in 2007. And although their exact causes remain uncertain, astronomers are now beginning to use bursts as tools for exploring the universe — from untangling the nature of the cosmic web to measuring the expansion of the universe.
“I am very optimistic about FRBs becoming an essential astronomical instrument in the near future,” says Di Li, an astronomer at the National Astronomical Observatories in Beijing.
FRBs are thought to come from sources such as dead pulsars or catastrophic events, mostly outside the Milky Way. These events are compact yet incredibly powerful, releasing 500 million times more energy than the Sun over a given duration. The gas each FRB passes through on its way to Earth—whether as it leaves its home galaxy or as it traverses intergalactic space—leaves telltale fingerprints on the signal.
Until now, astronomers have largely used available FRB data to locate and identify the main blasting galaxies, but they are beginning to make broader conclusions about the distribution and structure of hard-to-study matter.
Observatories are detecting FRBs at an unprecedented rate and can increasingly identify which galaxies they come from, says Kiyoshi Masui, a radio astronomer at MIT in Cambridge. “We will have statistically robust samples much sooner than we expected,” he says.
A treasure trove of information
FRBs make good cosmic probes because their radio waves interact with whatever medium they cross. The total amount of matter, as well as the density fluctuations within it, leave an effect on the signal, as do magnetic fields. Xavier Prochaska, an astrophysicist at the University of California, Santa Cruz, says the FRB’s ability to reveal information about its flight has “great scientific strength,” even if separating the signature from the different phases is a challenge.
For example, a magnetic field rotates the polarization of radio waves, which is the direction in which their electric fields oscillate. Astronomers want to know about magnetic fields because they affect how matter flows and how galaxies form. in pre-print1 Prochaska and colleagues published on arXiv last September, and showed that nine FRBs came from galaxies with magnetic fields similar in strength to the Milky Way. Once astronomers can identify the source galaxies for about 100 FRBs, they will be able to explore broader trends, such as whether mass or galaxy type correlates with magnetic field strength, Prochaska says. “At that point, we can really inform the galaxy formation models,” he says.
First and foremost, FRBs can reveal the total amount of matter they encounter as they travel. Low-frequency wave components slow down more than high-frequency ones, leaving a choppy signal. The greater the smearing – known as dispersion – the more material the wave will travel through.
In 2020, astronomers used scattering measures of five local FRBs to shed light on a long-standing mystery — the location of more than half of the ordinary matter in the universe scattered as gas.2rather than concentrating in galaxies. Cosmology predicts that matter must exist, but because the gas has such a low density, it has proven extremely difficult to fully explain it. The team, led by Jean-Pierre Macquart of Curtin University in Perth, Australia, showed that the relationship between the distance of FRBs and measures of scattering roughly matches the expected amount of matter lost.
However, not all FRBs fit the correlation precisely. That’s because the missing matter is not evenly distributed across space, but swirls around and between galaxies in filaments known as the cosmic web. Astronomers working on the FLIMFLAM survey3 Now you want to use a sample of 30 local FRBs to more accurately map the cosmic web. For each FRB, they plan to subtract estimates of the scattering caused by different stages of the journey—the host galaxy, other galaxies, the signal skirts, and the Milky Way—so they can better constrain how much matter should sit in the cosmic web. between.
The extent of FRB staining can also tell astronomers about gas properties on a smaller scale. Vikram Ravi, an astronomer at the California Institute of Technology in Pasadena, and his colleagues use FRBs as skewers to observe the gas that sits around galaxies in halos, known as the perigalactic medium. In a research paper published on arXiv this month4, the team used the FRB’s scatterometer that skimmed the Milky Way’s own halo to put an upper limit on the amount of gas out there, and it turned out to be much lower than expected. The researchers say the findings support the idea that matter is regularly ejected from galaxies in a process known as feedback. Supernovae and stellar winds eject matter as gravity pulls it in—a process that is difficult to model on a computer simulation. “FRBs can help a lot” in understanding this process, which is key to understanding how galaxies form, says Yin-Zhe Ma, an astronomer at the University of Kwazulu-Natal in Durban, South Africa.
Astronomers are even trying to use FRB scattering to measure how fast galaxies are flying away from each other due to the expansion of the universe, which is described by the Hubble constant. Cosmological theories—along with the Hubble constant—predict a specific relationship between the distance of the FRB and how stretched it will appear. By entering observed values for the scatterometer and distance, astronomers can invert the equation to give the best fit value for the Hubble constant instead.
The constant was measured with great accuracy, but different methods yielded contradictory results. So far, the values calculated by the FRB researchers have so much uncertainty, around 10%, that they can’t help solving the question.5. But the number will become more accurate with more localized FRBs, says Esanmouli Ghosh, a student at the Indian Institute of Science Education and Research Mohali in Manoli, who presented one of the studies.5 At the 2022 International Astronomical Union General Assembly in Busan, South Korea, in August.
The method may be promising, says astronomer Adam Riess of Johns Hopkins University in Baltimore, Maryland, but the main challenge is figuring out how much the FRB is scattering due to related matter in intergalactic space and how much comes from the host galaxy and the Milky Way. This problem is common to all efforts to use FRBs as probes, but astronomers are working on several ways to estimate how much scattering comes from each part of the flight, Prochaska says. “I feel optimistic,” he says.