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Solving the greatest mystery in astronomy
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Solving the greatest mystery in astronomy Fast radio bursts from across the cosmos have perplexed us for over a decade. Now we might know what they are, says Daniel Cossins
A
MINOR point of interest regarding the Spitler Burst.” The subject line Paul Scholz had chosen for his email was deliberately dry, but the recipients knew instantly what its contents meant. He was sitting on a revelation that would blow open the biggest mystery in astronomy. It was 5 November 2015 and Scholz, then a graduate student at McGill University in Montreal, Canada, had spent years scouring data from the world’s largest radio telescope. Staring back from his computer screen was the usual parade of curving lines, each a potential flash in the night sky. Suddenly, Scholz realised one of them looked familiar. A millisecond pulse of radio waves was blasting from a faraway galaxy with the intensity of 500 million suns – and not for the first time. “It was immediately clear this was something staggeringly important,” says Shami Chatterjee, an astronomer at Cornell University in New York. For a decade after the first discovery of these signals, known as fast radio bursts (FRBs), we had no idea what could be producing them. Suggestions ranged from colliding neutron stars to black holes turning themselves inside out to lasers from alien spacecraft. Until a few months ago, we had more ideas than detections. Since Scholz’s email, however, the hunt for the source of FRBs has been moving briskly along, with new clues pointing the finger at an unusual suspect and the latest radio telescopes promising fresh leads. Even if the exact cause isn’t identified soon, these mysterious blasts can still help illuminate the universe, giving us a glimpse at what lurks in the darkest, most distant voids of the cosmos. FRBs aren’t rare. They are raining down on us all the time from all directions. We only missed them for so long because they disappear almost as soon as they appear. Duncan Lorimer at West Virginia University
“
34 | New Scientist | 11 May 2019
and student David Narkevic were the first to find one, in 2007. This was actually six years after it was inadvertently detected. The two were searching archived radio telescope data for an entirely different phenomenon – rotating stars known as pulsars – when they saw a signal that struck them as odd. Lasting less than 5 milliseconds, the so-called Lorimer burst is estimated to have released as much energy as the sun spits out in a month. That wasn’t the only curious thing about this signal. Its higher-frequency waves arrived a fraction of a second earlier than its lower-frequency waves, giving it a strange smeared-out appearance, like a rainbow of light emerging from a prism. This smearing, caused by the scattering of light off electrons and other particles, is known as dispersion. The more of it you measure, the more material the radio signal has passed through and the further it has travelled. It is essentially an inbuilt distance marker. Based on the dispersion measure for their signal, Lorimer and his colleagues estimated it had arrived from a galaxy several billion light years away. If you think that sounds ludicrous, you are in good company. The signal was so incredibly powerful, and apparently so well-travelled, that many astronomers had a hard time believing it was real. But in the past 12 years, radio telescopes have picked up dozens more of these FRBs, all with dispersion measures indicating extragalactic origins. And yet there has been precious little consensus about what generates them. “It’s rare to be presented with a mystery like this,” says Emily Petroff, an astronomer at ASTRON, the Netherlands Institute for Radio Astronomy. “These things are so short and seem to be coming from so far away that the engine behind them must be something we have no analogue for in our galaxy. Whatever it
Daniel is a features writer at New Scientist specialising in the physical sciences
is, we haven’t seen anything like it before.” With so few examples to draw on, there is only so much we can know. While a dispersion measure gives you a rough estimate of distance, it doesn’t allow you to pinpoint host galaxies. That is why Scholz’s discovery in 2015 was such a big deal. With a repeating signal (a “repeater” in FRB astronomy circles) you can go back to the position with higherresolution telescopes and more accurately locate the source. That is what Chatterjee and his colleagues did in the wake of the email sent by Scholz, who is now at the Dominion Radio Astrophysical Observatory near Penticton, Canada. Having convinced the Very Large Array radio telescope in New Mexico to take the gamble of giving them observing time, they staked out the relevant patch of the sky. For the best part of 100 hours they watched and waited and saw nothing. And then, FLASH! It came again, another signal from the same spot. This was the Spitler burst, named after astronomer Laura Spitler, who first saw it on 2 November 2012. It is officially known as FRB121102. Since we began regular monitoring of it four years ago, it has made hundreds of >
ANDREA UCINI
11 May 2019 | New Scientist | 35
Hear more about fast radio bursts
Emily Petroff will be speaking at New Scientist Live, which runs from 10 to 13 October in London. For more details newscientistlive.com
Time to CHIME Even with the Spitler burst flaring again and again, there wasn’t enough to go on. So in early 2019, astronomers the world over were thrilled when it was announced that a new kind of telescope called the Canadian Hydrogen Intensity Mapping Experiment (CHIME) had turned up a fresh lead, even before it had properly switched on. CHIME was designed to create threedimensional maps of hydrogen gas in the early universe, which glows faintly at radio frequencies. While it was being erected in a wooded valley near Penticton, FRB chasers realised that it would also be perfect for hunting their quarry. What sets CHIME apart is its wide field of view. Whereas most radio telescopes study a small portion of the sky, this one scans the entire northern celestial hemisphere every day. That generates an unprecedented avalanche of data, one that is impossible to sift through without the help of cuttingedge algorithms. By the summer of 2018, the telescope was in a pre-commissioning phase, meaning it wasn’t fully operational. Components were 36 | New Scientist | 11 May 2019
still being installed and the system was operating at a fraction of its design capabilities. Hence everyone’s surprise when, amid the organised chaos, FRBs showed up. Thirteen of them, taking the running total to 65. “That was a big celebration,” says Victoria Kaspi at McGill University, who leads the CHIME collaboration. Then at the start of 2019, the champagne corks were popping again. “We kept track of what we were seeing and then somebody noticed there were two in the same position,” says Kaspi. Out of nowhere, we had a second repeater. The new repeating signal, known as FRB180814, showed that the first repeater wasn’t a fluke. Given we had found another
Mysterious origins Five theories for where the enigmatic space signals known as fast radio bursts (FRBs) might come from:
Cosmic shrapnel
in the very first batch of bursts CHIME had detected, repeaters might even be common. More important, however, was a striking similarity between the two signals. Each had a series of sub-pulses that shifted down from higher to lower frequencies as they passed through the detectors. This downshift was so similar that when CHIME researchers presented their results at a recent conference, they briefly fooled their audience into believing they were looking at the wrong signal. That similarity makes it more likely that the high magnetic fields Hessels had detected were a product of the FRBs’ origin, rather than the journey they had taken. Right now the prime suspects for that origin are magnetars, young neutron ANDRE RECNIK,DUNLAP INSTITUE,CHIME
appearances. The timing of its bursts seems to be random. Even so, astronomers have been able to use them to figure out where the signals originate. As Chatterjee and others reported in January 2017, they come from a dwarf galaxy some 3 billion light years from Earth. More clues to its true nature followed a year later, when Jason Hessels at the University of Amsterdam and his colleagues looked more closely at the way the repeater’s radio waves twisted as they propagated through space. Known as Faraday rotation, this effect is caused by magnetic fields. Initially they found nothing. But when they widened the search to look for more extreme effects, they struck gold: the rotation measure for FRB121102 was so absurdly large, it suggested the involvement of magnetic fields many times stronger than anything in our own galaxy, including the supermassive black hole at its centre. Astronomers were now eyeing the usual suspects, including incredibly powerful black holes (see “Mysterious origins”, right). But there was no way to know which of them was responsible, or if they were somehow in cahoots. Then there was the nagging doubt that the extreme magnetic fields the FRBs encountered might have come from something on their route to us, rather than their origin.
Although collisions between hefty astronomical objects can’t explain repeating FRBs (see main story), one-off events could potentially be traced back to such smash-ups.
Black holes Every galaxy has at its centre a supermassive black hole. As they gobble gas and dust, they sometimes fire out beams of radiation that could interact with particles to produce FRBs.
Lost comets It is possible that a comet caught up in the gravitational pull of a small and incredibly dense star could break apart and emit radio waves.
Aliens Some theorists have suggested FRBs could be the result of radio beams used to power light sails on alien spacecraft.
White holes If black holes eventually turn inside out, spewing out matter rather than sucking it up, they might release trapped matter in a way that generates FRBs.
The CHIME telescope in Canada will revolutionise FRB astronomy
stars that are the universe’s most powerful magnets, generating fields millions of billions of times stronger than Earth’s. They spew out electrons and other charged particles, accumulating a vast cloud of orbiting debris. According to a model developed by Brian Metzger at Columbia University, New York, and his colleagues, this is a perfect recipe for FRBs. As the material in every fresh flare collides with the highly magnetised cloud, shock waves excite electrons at the cloud’s outer edge in such a way as to produce brief flashes of radio waves.
Incredible magnetism This scenario not only produces randomly timed repeating bursts like those we have seen, it can also account for the distinctive downshift seen within individual signals. “As each shock wave slows down, so the radio signal we see shifts to lower frequencies,” says Metzger. But there is more. We don’t yet have a rotation measure for the second repeater, but the first one’s absurdly high reading fits nicely with the magnetar theory. Taken together, this is pushing many researchers in the same direction. “The magnetar model is the one people are feeling good about,” says Petroff. Well, not all people. Although most think the substructure of the repeating FRBs is telling us something about the emission mechanism,
Some theorists believe fast radio bursts (FRBs) are produced by high strength magnetic fields. That points the finger squarely at a class of neutron stars known as magnetars, with magnetic field strengths millions of billions of times stronger than the Earth’s
Earth’s magnetic field:
0.5
Gauss
Commercial magnets:
100
Gauss
MRI machines:
70,000
Gauss
Strongest artificial magnets:
10,000,000
Gauss
Typical neutron stars:
1,000,000,000,000
Gauss
Magnetars:
1,000,000,000,000,000 others still believe it could be the result of material the bursts pass through as they zip across the universe. It is also possible that there are several sources – that one-off bursts have different origins to repeaters. “People are getting more comfortable with the idea that you don’t need one theory,” says Petroff. In any case, the nice thing about the magnetar model is that it makes observational predictions. First, any FRBs we see in future should carry the same frequency downshift pattern. Second, they should be coming from the sorts of galaxies that are known to be producing lots of young stars and fresh magnetars. “If we look at these sorts of places and constantly find FRBs, that would give us confidence,” says Chatterjee.
Gauss
What we need now are more bursts, and we are about to get them in spades. Once it is properly up and running this year, CHIME should spot several per day. Then there is the Australian Square Kilometre Array Pathfinder, a network of 36 radio dishes capable of pinpointing host galaxies even for one-off FRBs. “The enterprise of FRB searching is entering a new phase,” says Chatterjee. “It is going to be amazing.” But the story won’t end there. If we can gather a sufficiently large sample, the hope is that FRBs will be able to answer some intriguing fundamental questions about the history and structure of the cosmos. One of these is called the missing baryon problem – the apparent absence from the
universe of a large chunk of ordinary matter. Made of particles called baryons, this matter should make up 5 per cent of the universe. The rest is dark matter and dark energy. But so far, we have only been able to spot half of it. Most people think the rest is hiding in vast expanses of empty space between galaxies. The trouble is, we don’t have instruments sensitive enough to probe these voids, particularly as whatever they contain must be extremely wispy. But because FRBs’ epic journeys across space take them through some of the universe’s darkest corners, they should be able to act as probes into these mysterious cosmic voids. Herein lies the beauty of the radio bursts. By encoding information about the medium through which they pass, they can help us to figure out how much ordinary matter these voids contain – a measurement no other probe can take. “Diagnosis of the baryonic distribution of the intergalactic medium will be the pièce de résistance of FRB science,” says Jean-Pierre Macquart at Curtin University in Perth, Western Australia. Just as tantalising is the prospect that detailed study of FRBs’ twisted radio waves will tell us about the strength of magnetic fields in distant voids. At present, our knowledge of these fields is virtually nil. An accurate measurement may tell us whether they were present in the earliest moments of the universe and, if so, what role they played in shaping how it looks today. “If we can prove that magnetic fields were there during inflation, this period of expansion immediately after the big bang, then they must become an essential ingredient of any cosmological theory,” says Franco Vazza at the University of Bologna in Italy. Not that such measurements will be trivial. FRBs are produced by an incredibly powerful cosmic object and encounter all manner of matter along their way, all of which contributes to the dispersion and rotation measures we read off when they reach Earth. The challenge will be to disentangle the different components, which will only become possible once the new generation of radio telescopes detects a deluge of FRBs. “These things have been going off under our noses for years, and we deduce that there are a few thousand of them raining down on Earth every day,” says Macquart. “They’re an amazing cosmic whodunnit. But even if we never figure out what is producing them, they give us a whole new way to study the universe. It’s a good time to be an astrophysicist!” ❚ 11 May 2019 | New Scientist | 37
Solving the greatest mystery in astronomy Fast radio bursts from across the cosmos have perplexed us for over a decade. Now we might know what they are, says Daniel Cossins
A
MINOR point of interest regarding the Spitler Burst.” The subject line Paul Scholz had chosen for his email was deliberately dry, but the recipients knew instantly what its contents meant. He was sitting on a revelation that would blow open the biggest mystery in astronomy. It was 5 November 2015 and Scholz, then a graduate student at McGill University in Montreal, Canada, had spent years scouring data from the world’s largest radio telescope. Staring back from his computer screen was the usual parade of curving lines, each a potential flash in the night sky. Suddenly, Scholz realised one of them looked familiar. A millisecond pulse of radio waves was blasting from a faraway galaxy with the intensity of 500 million suns – and not for the first time. “It was immediately clear this was something staggeringly important,” says Shami Chatterjee, an astronomer at Cornell University in New York. For a decade after the first discovery of these signals, known as fast radio bursts (FRBs), we had no idea what could be producing them. Suggestions ranged from colliding neutron stars to black holes turning themselves inside out to lasers from alien spacecraft. Until a few months ago, we had more ideas than detections. Since Scholz’s email, however, the hunt for the source of FRBs has been moving briskly along, with new clues pointing the finger at an unusual suspect and the latest radio telescopes promising fresh leads. Even if the exact cause isn’t identified soon, these mysterious blasts can still help illuminate the universe, giving us a glimpse at what lurks in the darkest, most distant voids of the cosmos. FRBs aren’t rare. They are raining down on us all the time from all directions. We only missed them for so long because they disappear almost as soon as they appear. Duncan Lorimer at West Virginia University
“
34 | New Scientist | 11 May 2019
and student David Narkevic were the first to find one, in 2007. This was actually six years after it was inadvertently detected. The two were searching archived radio telescope data for an entirely different phenomenon – rotating stars known as pulsars – when they saw a signal that struck them as odd. Lasting less than 5 milliseconds, the so-called Lorimer burst is estimated to have released as much energy as the sun spits out in a month. That wasn’t the only curious thing about this signal. Its higher-frequency waves arrived a fraction of a second earlier than its lower-frequency waves, giving it a strange smeared-out appearance, like a rainbow of light emerging from a prism. This smearing, caused by the scattering of light off electrons and other particles, is known as dispersion. The more of it you measure, the more material the radio signal has passed through and the further it has travelled. It is essentially an inbuilt distance marker. Based on the dispersion measure for their signal, Lorimer and his colleagues estimated it had arrived from a galaxy several billion light years away. If you think that sounds ludicrous, you are in good company. The signal was so incredibly powerful, and apparently so well-travelled, that many astronomers had a hard time believing it was real. But in the past 12 years, radio telescopes have picked up dozens more of these FRBs, all with dispersion measures indicating extragalactic origins. And yet there has been precious little consensus about what generates them. “It’s rare to be presented with a mystery like this,” says Emily Petroff, an astronomer at ASTRON, the Netherlands Institute for Radio Astronomy. “These things are so short and seem to be coming from so far away that the engine behind them must be something we have no analogue for in our galaxy. Whatever it
Daniel is a features writer at New Scientist specialising in the physical sciences
is, we haven’t seen anything like it before.” With so few examples to draw on, there is only so much we can know. While a dispersion measure gives you a rough estimate of distance, it doesn’t allow you to pinpoint host galaxies. That is why Scholz’s discovery in 2015 was such a big deal. With a repeating signal (a “repeater” in FRB astronomy circles) you can go back to the position with higherresolution telescopes and more accurately locate the source. That is what Chatterjee and his colleagues did in the wake of the email sent by Scholz, who is now at the Dominion Radio Astrophysical Observatory near Penticton, Canada. Having convinced the Very Large Array radio telescope in New Mexico to take the gamble of giving them observing time, they staked out the relevant patch of the sky. For the best part of 100 hours they watched and waited and saw nothing. And then, FLASH! It came again, another signal from the same spot. This was the Spitler burst, named after astronomer Laura Spitler, who first saw it on 2 November 2012. It is officially known as FRB121102. Since we began regular monitoring of it four years ago, it has made hundreds of >
ANDREA UCINI
11 May 2019 | New Scientist | 35
Hear more about fast radio bursts
Emily Petroff will be speaking at New Scientist Live, which runs from 10 to 13 October in London. For more details newscientistlive.com
Time to CHIME Even with the Spitler burst flaring again and again, there wasn’t enough to go on. So in early 2019, astronomers the world over were thrilled when it was announced that a new kind of telescope called the Canadian Hydrogen Intensity Mapping Experiment (CHIME) had turned up a fresh lead, even before it had properly switched on. CHIME was designed to create threedimensional maps of hydrogen gas in the early universe, which glows faintly at radio frequencies. While it was being erected in a wooded valley near Penticton, FRB chasers realised that it would also be perfect for hunting their quarry. What sets CHIME apart is its wide field of view. Whereas most radio telescopes study a small portion of the sky, this one scans the entire northern celestial hemisphere every day. That generates an unprecedented avalanche of data, one that is impossible to sift through without the help of cuttingedge algorithms. By the summer of 2018, the telescope was in a pre-commissioning phase, meaning it wasn’t fully operational. Components were 36 | New Scientist | 11 May 2019
still being installed and the system was operating at a fraction of its design capabilities. Hence everyone’s surprise when, amid the organised chaos, FRBs showed up. Thirteen of them, taking the running total to 65. “That was a big celebration,” says Victoria Kaspi at McGill University, who leads the CHIME collaboration. Then at the start of 2019, the champagne corks were popping again. “We kept track of what we were seeing and then somebody noticed there were two in the same position,” says Kaspi. Out of nowhere, we had a second repeater. The new repeating signal, known as FRB180814, showed that the first repeater wasn’t a fluke. Given we had found another
Mysterious origins Five theories for where the enigmatic space signals known as fast radio bursts (FRBs) might come from:
Cosmic shrapnel
in the very first batch of bursts CHIME had detected, repeaters might even be common. More important, however, was a striking similarity between the two signals. Each had a series of sub-pulses that shifted down from higher to lower frequencies as they passed through the detectors. This downshift was so similar that when CHIME researchers presented their results at a recent conference, they briefly fooled their audience into believing they were looking at the wrong signal. That similarity makes it more likely that the high magnetic fields Hessels had detected were a product of the FRBs’ origin, rather than the journey they had taken. Right now the prime suspects for that origin are magnetars, young neutron ANDRE RECNIK,DUNLAP INSTITUE,CHIME
appearances. The timing of its bursts seems to be random. Even so, astronomers have been able to use them to figure out where the signals originate. As Chatterjee and others reported in January 2017, they come from a dwarf galaxy some 3 billion light years from Earth. More clues to its true nature followed a year later, when Jason Hessels at the University of Amsterdam and his colleagues looked more closely at the way the repeater’s radio waves twisted as they propagated through space. Known as Faraday rotation, this effect is caused by magnetic fields. Initially they found nothing. But when they widened the search to look for more extreme effects, they struck gold: the rotation measure for FRB121102 was so absurdly large, it suggested the involvement of magnetic fields many times stronger than anything in our own galaxy, including the supermassive black hole at its centre. Astronomers were now eyeing the usual suspects, including incredibly powerful black holes (see “Mysterious origins”, right). But there was no way to know which of them was responsible, or if they were somehow in cahoots. Then there was the nagging doubt that the extreme magnetic fields the FRBs encountered might have come from something on their route to us, rather than their origin.
Although collisions between hefty astronomical objects can’t explain repeating FRBs (see main story), one-off events could potentially be traced back to such smash-ups.
Black holes Every galaxy has at its centre a supermassive black hole. As they gobble gas and dust, they sometimes fire out beams of radiation that could interact with particles to produce FRBs.
Lost comets It is possible that a comet caught up in the gravitational pull of a small and incredibly dense star could break apart and emit radio waves.
Aliens Some theorists have suggested FRBs could be the result of radio beams used to power light sails on alien spacecraft.
White holes If black holes eventually turn inside out, spewing out matter rather than sucking it up, they might release trapped matter in a way that generates FRBs.
The CHIME telescope in Canada will revolutionise FRB astronomy
stars that are the universe’s most powerful magnets, generating fields millions of billions of times stronger than Earth’s. They spew out electrons and other charged particles, accumulating a vast cloud of orbiting debris. According to a model developed by Brian Metzger at Columbia University, New York, and his colleagues, this is a perfect recipe for FRBs. As the material in every fresh flare collides with the highly magnetised cloud, shock waves excite electrons at the cloud’s outer edge in such a way as to produce brief flashes of radio waves.
Incredible magnetism This scenario not only produces randomly timed repeating bursts like those we have seen, it can also account for the distinctive downshift seen within individual signals. “As each shock wave slows down, so the radio signal we see shifts to lower frequencies,” says Metzger. But there is more. We don’t yet have a rotation measure for the second repeater, but the first one’s absurdly high reading fits nicely with the magnetar theory. Taken together, this is pushing many researchers in the same direction. “The magnetar model is the one people are feeling good about,” says Petroff. Well, not all people. Although most think the substructure of the repeating FRBs is telling us something about the emission mechanism,
Some theorists believe fast radio bursts (FRBs) are produced by high strength magnetic fields. That points the finger squarely at a class of neutron stars known as magnetars, with magnetic field strengths millions of billions of times stronger than the Earth’s
Earth’s magnetic field:
0.5
Gauss
Commercial magnets:
100
Gauss
MRI machines:
70,000
Gauss
Strongest artificial magnets:
10,000,000
Gauss
Typical neutron stars:
1,000,000,000,000
Gauss
Magnetars:
1,000,000,000,000,000 others still believe it could be the result of material the bursts pass through as they zip across the universe. It is also possible that there are several sources – that one-off bursts have different origins to repeaters. “People are getting more comfortable with the idea that you don’t need one theory,” says Petroff. In any case, the nice thing about the magnetar model is that it makes observational predictions. First, any FRBs we see in future should carry the same frequency downshift pattern. Second, they should be coming from the sorts of galaxies that are known to be producing lots of young stars and fresh magnetars. “If we look at these sorts of places and constantly find FRBs, that would give us confidence,” says Chatterjee.
Gauss
What we need now are more bursts, and we are about to get them in spades. Once it is properly up and running this year, CHIME should spot several per day. Then there is the Australian Square Kilometre Array Pathfinder, a network of 36 radio dishes capable of pinpointing host galaxies even for one-off FRBs. “The enterprise of FRB searching is entering a new phase,” says Chatterjee. “It is going to be amazing.” But the story won’t end there. If we can gather a sufficiently large sample, the hope is that FRBs will be able to answer some intriguing fundamental questions about the history and structure of the cosmos. One of these is called the missing baryon problem – the apparent absence from the
universe of a large chunk of ordinary matter. Made of particles called baryons, this matter should make up 5 per cent of the universe. The rest is dark matter and dark energy. But so far, we have only been able to spot half of it. Most people think the rest is hiding in vast expanses of empty space between galaxies. The trouble is, we don’t have instruments sensitive enough to probe these voids, particularly as whatever they contain must be extremely wispy. But because FRBs’ epic journeys across space take them through some of the universe’s darkest corners, they should be able to act as probes into these mysterious cosmic voids. Herein lies the beauty of the radio bursts. By encoding information about the medium through which they pass, they can help us to figure out how much ordinary matter these voids contain – a measurement no other probe can take. “Diagnosis of the baryonic distribution of the intergalactic medium will be the pièce de résistance of FRB science,” says Jean-Pierre Macquart at Curtin University in Perth, Western Australia. Just as tantalising is the prospect that detailed study of FRBs’ twisted radio waves will tell us about the strength of magnetic fields in distant voids. At present, our knowledge of these fields is virtually nil. An accurate measurement may tell us whether they were present in the earliest moments of the universe and, if so, what role they played in shaping how it looks today. “If we can prove that magnetic fields were there during inflation, this period of expansion immediately after the big bang, then they must become an essential ingredient of any cosmological theory,” says Franco Vazza at the University of Bologna in Italy. Not that such measurements will be trivial. FRBs are produced by an incredibly powerful cosmic object and encounter all manner of matter along their way, all of which contributes to the dispersion and rotation measures we read off when they reach Earth. The challenge will be to disentangle the different components, which will only become possible once the new generation of radio telescopes detects a deluge of FRBs. “These things have been going off under our noses for years, and we deduce that there are a few thousand of them raining down on Earth every day,” says Macquart. “They’re an amazing cosmic whodunnit. But even if we never figure out what is producing them, they give us a whole new way to study the universe. It’s a good time to be an astrophysicist!” ❚ 11 May 2019 | New Scientist | 37