Mysterious fast radio bursts from space: Five explanations for what they could be

Scientists have long been puzzled by what appears to be a strange phenomenon from space known as fast radio bursts (FRBs): short, bright flashes of radio light that appear to be coming from almost halfway across the universe.

Are aliens behind mysterious radio bursts? Probably not, say scientists

Artist impression of a Fast Radio Burst (FRB) reaching Earth. The colours represent the burst arriving at different radio wavelengths, with long wavelengths (red) arriving several seconds after short wavelengths (blue). This delay is called dispersion and occurs when radio waves travel through cosmic plasma. (Jingchuan Yu, Beijing Planetarium)
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Scientists have long been puzzled by what appears to be a strange phenomenon from space known as fast radio bursts (FRBs). These short, bright flashes of radio light appear to be coming from almost halfway across the universe. Our observatories can spot them, but until a new generation of telescopes comes online, scientists are furiously theorizing and compiling a catalogue of potential explanations

"When we start to get more observations, we expect to start seeing an awful lot very quickly. So to have a catalogue, we'll be able to quickly efficiently and rigorously challenge these theories," said Emma Platts, a PhD candidate in applied mathematics at the University of Cape Town in South Africa, who helped develop the catalogue.

So far, scientists have detected about 60 single fast radio bursts and one that repeats. They theorize there could be as many as a thousand fast radio bursts in the sky every day. As far as they know with the limited observations we've had, they could all be repeating. 

Scientists have grouped more than 45 theories into five main categories that might explain the fast radio bursts they've detected so far — plus one more theory that's a bit of an oddball.

A plot of the arrival of the (dispersed) Lorimer burst, the first fast radio burst ever detected, with frequency versus time shown. (Swinburne)

Asteroid collision

If an asteroid is falling towards a neutron star — the collapsed core of a giant star, and the densest known object in the universe — the interaction between its incredibly strong magnetic field and the the asteroid can generate a spark, like lightning.

"This radiation will travel across the universe to us," said Platts.

"One of the things that this theory predicts is that you would have repeated interactions between the asteroid and the neutron star," said Shriharsh Tendulkar, a post-doctoral researcher in the department of physics at the McGill Space Institute in Montreal, who also worked on the database.

If this theory is correct, then he says the first thing you'd expect is repetition and the second thing would be some sort of periodicity in the repetition, "because the neutron star is on an orbit, so you would expect it to interact again and again with some asteroids. So we haven't seen that yet. But if we see something like that that would be interesting."

Active galactic nuclei

Imagine a supermassive black hole in the centre of a galaxy — also known as an active galactic nuclei. The interaction between this black hole and all the dust and gas around it could be what's behind these fast radio bursts. 

"If we find that many FRBs come from centres of galaxies, which have these active galactic nuclei and we can precisely locate them to be coming from those centres, then we would probably link the fast radio bursts to these active galactic nuclei," said Tendulkar.

An artist’s impression of fast radio bursts in the sky above Australian radio telescopes. The circles around the fast radio bursts in space denote the ASKAP's field of view. (OzGrav, Swinburne University of Technology)

Collapse of a neutron star

"If we consider a neutron star, say a pulsar, and it's rotating extremely rapidly, this is going to slow down over time to the point that the neutron star will no longer be able to support itself and it will collapse," said Platts.

A pulsar is a highly magnetized neutron star that emits a beam of electromagnetic radiation as it rotates. If it collapses, it could explode, potentially resulting in a fast radio burst we see on Earth. 

Tendulkar says if this is a collapsing neutron star, he'd expect these to be fairly old astrophysical objects. And if they're old, they often end up going to the fringes of the galaxies.

"If we find that a lot of FRBs are located at the fringes of galaxies, with old stellar populations, that might be a very interesting scenario for this theory," said Tendulkar.

The only catch to this theory is that it'd only work for single FRBs.

"It is a cataclysmic model in the sense that it will happen absolutely once and then the object is basically destroyed," said Tendulkar. "So it doesn't work for the repeating FRB. And if we find later that other FRB and the repeating FRB are the same population, then we can rule out."

Compact bodies like neutron stars merging

"If we consider two compact bodies merging, such as two neutron stars, we can think of them as giant magnets," said Platts.

"When these magnets come close together, the magnetic field lines are rearranged. And when this happens, enormous amounts of energy are released and it could be this that we're seeing as fast radio bursts."

If this theory of two neutron stars merging is correct, according to Tendulkar, we should also be able to also detect gravitational waves and gamma ray bursts from the event.

The challenge to developing this theory is gravitational waves are extremely hard to detect. "The hope is that as the years progress, in the next year in fact, we are going to start the detector with a much higher sensitivity," said Tendulkar. "So we might be able to see more of these."

Some of the debris blasts away in particle jets moving at nearly the speed of light when 2 neutron stars merge, producing a brief burst of gamma-rays. (NASA's Goddard Space Flight Center/CI Lab)

Newly born magnetar​

Magnetars are a rare form of neutron star that have an extremely strong magnetic field, which might be behind the luminous bursts. 

"If we consider a massive star that collapses into a magnetar," said Platts. "Some of that star (...) is going to be ejected out in a shell of dust and gas. And this is going to surround your magnetar. We then might have flares that take place on the magnetar. And these flares will burst out from the surface of the star and interact with this expanding Nebula. And this interaction could cause the big bursts of energy that we're seeing."

This could be the most likely scenario because these "millisecond magnetars" are most commonly found in dwarf galaxies, which Tendulkar and his colleagues were able to trace back to such a galaxy.

"So we think that if FRBs come from these millisecond magnetars, we would find them in these kinds of dwarf galaxies."

Alien solar sail theory

The wackiest theory that's been put forward to explain these fast radio bursts, Platts says, is the idea of an extra-terrestrial solar sail spaceship.

"You can imagine, you've got this giant sail where you have your photons or your light particles and these will push your sail and that would accelerate your vessel through space. And it's this burst of photons that propels our spaceship that could be the fast radio burst that we are seeing."

According to Tendulkar, he says, for this to be true, "You need a lot of extraordinary evidence to claim something that is this extraordinary."

Narrowing down the theories

The CHIME telescope will search our universe for phenomena such as fast radio bursts, pulsars and more. (CHIME, Andre Renard, Dunlap Institute for Astronomy & Astrophysics, University of Toronto)

The search for even more fast radio bursts is beginning to ramp up very quickly with new telescopes coming online with a wider field.

"We have built the Canadian Hydrogen Intensity Mapping Experiment. It is a big telescope in British Columbia. And we are now calibrating the telescope. We are already seeing fast radio bursts," said Tendulkar. "And secondly, there is another telescope called HIREX, which is coming up in South Africa. It also has Canadian collaborators. And that will also be fantastic in discovering fast radio bursts."

Platts says she hopes in the next few years the scientific community will reach a consensus, "on either what the repeating burst might be and what the single bursts might be or a unified theory that can explain what both of these are."

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