Since their discovery in 2002, scientists have struggled to understand Fast Radio bursts — high-energy pulses that originate from galaxies billions of light-years away. Though only a handful of these radio blips have ever been detected, new research suggests they could be a ubiquitous fixture of the cosmos, flashing about once every second throughout the observable universe. It's an intriguing conclusion, but one lacking in observational data that would lend it support.
Artist's depiction of a rupturing magnetar — a rotating neutron star with an extremely strong magnetic field. These exotic objects could be the source of the mysterious cosmic bursts observed by scientists. (Image: NASA's Goddard Space Flight Center/S. Wiessinger)
Scientists still aren't sure what causes these powerful bursts of radio emission. The most popular explanation is that they are caused by rapidly spinning neutron stars with extraordinarily strong magnetic fields, known as magnetars. Much more speculatively, these bursts could be produced by an advanced alien civilisation's antenna ray for lightsail propulsion.
What we do know is that at least some FRBs are not produced by catastrophic events, such as a supernova explosion. We know this because of a Fast Radio Burst source known as FRB 121102 — an object that's producing FRBs with surprising regularity.
Last month, scientists working on the Breakthrough Listen Project — a 10-year mission to search the skies for signs of extraterrestrial intelligences (ETIs) — detected 15 new FRBs from this single source. In total, only 23 FRB sources have been observed so far, pointing to the difficulty of detecting these strange signals.
By applying what we know of FRB 121102 and the other known FRB sources, Anastasia Fialkov and Avi Loeb from the Harvard-Smithsonian Center for Astrophysics have calculated how many FRBs could exist across the entire sky. The title of their new paper, now published in The Astrophysical Journal Letters, pretty much sums up their conclusion: "A Fast Radio Burst Occurs Every Second Throughout the Observable Universe."
That's obviously a lot — a conclusion that, if correct, would upend what we know about FRBs.
"If we are right about such a high rate of FRBs happening at any given time, you can imagine the sky is filled with flashes like paparazzi taking photos of a celebrity," noted Fialkov in a press release. "Instead of the light we can see with our eyes, these flashes come in radio waves."
Fialkov, who led the study, worked under the assumption that FRB 121102 is representative of all FRBs, an object located in a metal-poor dwarf galaxy about three billion light-years away.
"In our paper we calculated the rate of FRBs in the entire volume of the observable universe and found that it can reach once every second," Loeb told Gizmodo.
As for they chose to focus their estimate on FRB121102, "FRB121102 is the only FRB for which a host galaxy and a distance were identified," Loeb said. "It is also the only repeating FRB source from which we detected hundreds of FRBs by now. The radio spectrum of its FRBs is centered on a characteristic frequency and not covering a very broad band. This has important implications for the detectability of such FRBs, because in order to find them the radio observatory needs to be tuned to their frequency."
Loeb says that if we can study even a small fraction of the FRBs like FRB121102, those that occur on a regular basis, we should be able to unravel their origin and answer myriad other questions.
"FRBs can be used to measure the column of free electrons towards their source," he said. "This can be used to measure the density of ordinary matter between galaxies in the present-day universe. In addition, FRBs at early cosmic times can be used to find out when the ultraviolet light from the first stars broke up the primordial atoms of hydrogen left over from the Big Bang into their constituent electrons and protons."
Andrew Siemion, Director of Berkeley Research Center, agrees with the conclusion of the new study, that FRBs are probably happening all the time. He also thinks determining the rate of FRBs could help to unravel celestial mysteries.
"For example, if we hypothesize that a particular phenomena is due to, say, the merger of two stellar mass black holes, but discover that the rate of the phenomena is much less or much greater than the expected rate of mergers, we know either mergers are not responsible or our rate estimate of their occurrence is wrong," said Siemion, who wasn't involved in the new study, in an interview with Gizmodo. "Indeed we have known for some time that FRBs are fairly common... and the fact that we detect them so rarely testifies to how difficult they are to observe and how much real estate we have to cover with our telescopes."
But Emily Petroff, a postdoctoral researcher at the Netherlands Institute for Radio Astronomy (ASTRON), says it's still an open question as to how many FRBs are going off and how often.
"This paper is taking one approach which is to make some estimates about the underlying population and then to see how many are visible. It makes a nice contribution to the overall body of work but I don't think it answers any fundamental questions," Petroff told Gizmodo. "Despite hours and hours of looking, no other FRB has been seen to emit repeat pulses so it may not be right to assume that they all behave like this special source. But in the absence of any additional information it's a fair assumption to make for the purposes of this model."
Petroff says that only a small fraction of FRBs may be like the FRB 121102 repeater, which if true, would bring the numbers estimated in this paper down significantly.
"This paper makes an interesting point about what the sort of maximum number of FRBs going off at any given time might be, but I think the main takeaway is that our knowledge of FRBs is too limited at the moment to know if these distributions might be correct," said Petroff.
Clearly, what's needed are more observations. And as Loeb points out, we're already — or soon will be — in the possession of tools that are capable of findings more FRBs, including the recently deployed Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the soon-to-be-finished Square Kilometer Array (SKA).