Since 2007, astronomers have recorded about 20 mysterious radio pulses from far beyond our Galaxy. The BBC Earth columnist decided to find out more about this phenomenon.
There is no shortage of strange and not fully understood phenomena in the Universe - from black holes to outlandish planets. Scientists have something to puzzle over.
But one mystery lately has been especially interesting for astronomers - mysterious bursts of radio emission in space, known as fast radio pulses.
They last only a few milliseconds, but they release about a million times more energy than the Sun produces over the same period of time.
Since the discovery of the first such pulse in 2007, astronomers have managed to register less than 20 such cases - all of their sources were located outside our Galaxy and were evenly distributed over the sky.
However, telescopes tend to observe small portions of the sky at any given time.
If we extrapolate the obtained data to the entire sky, then, as astronomers assume, the number of such radio pulses can reach 10 thousand per day.
And nobody knows the reason for this phenomenon.
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Astronomers, of course, have plenty of possible explanations, some of which sound very exotic: collisions of neutron stars, explosions of black holes, breaks of cosmic strings, and even the results of the activity of extraterrestrial intelligence.
“There are now more theories trying to explain the nature of fast radio pulses than there are actually pulses,” says Duncan Lorimer, a researcher at the American University of West Virginia and leader of the research team that discovered the very first fast radio pulse (also called the Lorimer pulse). “This is fertile ground for theorists.”
But even if the explanation of the nature of fast radio pulses turns out to be much more commonplace, they can still be of great benefit to science.
They will undoubtedly revolutionize our understanding of the universe.
These radio signals are like laser beams piercing the Universe and encountering magnetic fields, plasma and other cosmic phenomena in their path.
In other words, they capture information about intergalactic space along the way and can represent a unique tool for exploring the Universe.
"They will undoubtedly revolutionize our understanding of the universe, because they can be used to make very accurate measurements," says Peng Wee-Li, an astrophysicist at the University of Toronto.
But before that happens, scientists need to better understand the nature of fast radio pulses.
Astronomers have made promising progress in this area over the past few months.
The first thing that struck Lorimer about the pulse he discovered was its intensity.
Lorimer and his colleagues reviewed archival datasets collected with the Parks Radio Telescope in Australia. They looked for radio pulses, such as those emitted by rapidly rotating neutron stars, so-called pulsars.
I was so excited that night that I could not sleep
Matthew Bales, astronomer
These stars, each with a diameter of a large city, have the density of an atomic nucleus and can rotate at speeds in excess of 1000 revolutions per second.
At the same time, they emit narrowly directed streams of radio emission, in connection with which they are also called space beacons.
The radio signals emitted by pulsars look like pulsations to an observer from Earth.
But the signal detected by Lorimer's team was very strange.
“It was so intense that it overwhelmed the telescope's electronic components,” recalls Lorimer. "This is extremely unusual for a radio source."
The pulse lasted for about 5 milliseconds, after which its intensity dropped.
“I remember the first time I saw a momentum diagram,” said Lorimer's team member Matthew Bales, an astronomer at Swinburne University of Technology, Australia. "I was so excited that night that I could not sleep."
For about five years after the discovery of Lorimer's impulse, it remained an unexplained anomaly.
Some scholars believed that it was just an instrumental interference. And in a study published in 2015, it is said that pulses with similar parameters are recorded during the operation of microwaves installed in the economic part of the Parks Observatory.
Their sources are outside of our Galaxy, possibly billions of light years from Earth.
However, since 2012, astronomers working on other telescopes have detected several more similar radio pulses, thus confirming that the signals are actually coming from space.
And not just from space - their sources are outside our Galaxy, perhaps billions of light years from Earth. This assumption was made based on measurements of a phenomenon known as the dispersion effect.
During their journey through the Universe, radio waves interact with the electrons of the plasma that they meet on their way. This interaction causes a slowdown in wave propagation, depending on the frequency of the radio signal.
Higher frequency radio waves arrive at the observer slightly faster than lower frequency radio waves.
By measuring the difference in these values, astronomers can calculate how much plasma the signal had to pass on its way to the observer, which gives them an approximate idea of the distance of the radio pulse source.
Radio waves coming to us from other galaxies are nothing new. It's just that before the discovery of fast radio pulses, scientists did not observe signals of such high intensity.
The existence of a signal, the intensity of which is a million times greater than anything previously detected, excites the imagination
Thus, quasars - active galactic nuclei, inside which, as scientists believe, are massive black stars - radiate a huge amount of energy, including in the radio range.
But quasars located in other galaxies are so far from us that the radio signals received from them are extremely weak.
They could easily be drowned out even by a radio signal from a mobile phone placed on the surface of the moon, Bailes notes.
Fast radio pulses are another matter. “The existence of a signal that is a million times stronger than anything previously detected is exciting,” says Bales.
Especially considering the fact that fast radio pulses may indicate new, unexplored physical phenomena.
One of the most ambiguous explanations for their origin has to do with the so-called cosmic strings - hypothetical one-dimensional folds of space-time that can stretch for at least tens of parsecs.
Some of these strings may have superconducting properties, and an electric current can flow through them.
According to a hypothesis proposed in 2014, cosmic strings sometimes break, resulting in a burst of electromagnetic radiation.
Or, says Penh, the explanation for these outbursts could be explosions of black holes.
The gravitational field of a black hole is so massive that even light hitting it is not able to escape back.
If we assume that at the early stage of the development of the Universe small black holes were formed in it, then now they may just evaporate
However, in the 1970s. the famous British theoretical physicist Stephen Hawking suggested that energy can evaporate from the surface of aging black holes.
If we assume that at an early stage in the development of the Universe, small black holes were formed in it, then now they may just evaporate and ultimately explode, which leads to an instantaneous emission of radio emission.
In February 2016, astronomers announced that they may have made a breakthrough in research.
A team of scientists led by Evan Keehan working at the headquarters of the Square Kilometer Array radio interferometer at the British Jodrell Bank Astrophysical Center, analyzed the parameters of one fast radio pulse recorded in April 2015.
According to the conclusions of astronomers, the source of the radio pulse was in a galaxy located 6 billion light years from us and consisting of old stars.
In this case, the parameters of the observed radio pulse indicated the likelihood of at least one scenario: collisions of paired neutron stars
For the first time, researchers were able to determine the location of the radio emission source with an accuracy of the galaxy, which was perceived in the scientific community as an extremely important discovery.
“Identifying the galaxy that contains the source of the fast radio pulse is a piece of the puzzle,” says Bailes, who also worked on Keehan's team. "If we can determine the galaxy, we can find out how far from us the source is."
After that, you can accurately measure the amount of pulse energy and begin to discard the most implausible theories regarding its origin.
In this case, the parameters of the observed radio pulse indicated the likelihood of at least one scenario: collisions of paired neutron stars revolving around each other.
It seemed that the mystery of the nature of fast radio pulses was almost solved. “I was very excited about the results of this study,” says Lorimer.
But just a few weeks later, scientists Edo Berger and Peter Williams of Harvard University questioned the theory.
The findings of Keehan's team were based on observation of the phenomenon, which scientists interpreted as the attenuation of the radio signal after the end of a fast radio pulse.
The source of the fading signal was reliably located in a galaxy located 6 billion light-years from Earth, and the researchers believed that the fast radio pulse came from there.
However, according to Berger and Williams, what Kian took for a residual - fading - radio signal had nothing to do with a fast radio pulse.
They carefully analyzed the characteristics of the residual signal by pointing the American Very Large Array radio telescope at a distant galaxy.
Collisions of neutron stars occur several orders of magnitude less often than the probable frequency of fast radio pulses, so that all registered cases cannot be explained by this phenomenon alone.
It was found that we are talking about a separate phenomenon caused by fluctuations in the brightness of the galaxy itself due to the fact that in its center is a supermassive black hole, absorbing cosmic gases and dust.
In other words, the twinkling galaxy was not the place from which the fast radio pulse was emitted. It's just that it happened to be in the field of view of the telescope - either behind the true source, or in front of it.
And if the radio pulse was not sent from this galaxy, then perhaps it was not caused by the collision of two neutron stars.
The neutron scenario has another weak point. “The frequency of emission of fast radio pulses is much higher than the frequency of radiation expected from collisions of neutron stars,” says Maxim Lyutikov of the American University of Purdue.
In addition, collisions of neutron stars occur several orders of magnitude less often than the probable frequency of fast radio pulses, so that all registered cases cannot be explained by this phenomenon alone.
And soon, new scientific evidence reduced the likelihood of such an explanation even more.
In March 2016, a group of astronomers reported a stunning discovery. They studied a radio pulse recorded in 2014 by the Arecibo Observatory in Puerto Rico. It turned out that this was not a single event - the impulse was repeated 11 times over 16 days.
“This was the biggest discovery since the first fast radio burst,” says Penh. "It puts an end to the huge number of hypotheses proposed so far."
All previously recorded fast radio pulses were single - repetitions of signals from the same sector of the sky were not recorded.
Therefore, scientists assumed that they could be a consequence of cosmic cataclysms, in each case occurring only once - for example, explosions of black holes or collisions of neutron stars.
But this theory does not explain the possibility (in some cases) of repeating radio pulses in rapid succession. Whatever the cause of such a series of impulses, the conditions for their occurrence must be maintained for a certain time.
This circumstance significantly narrows the list of possible hypotheses.
One of them, which Buttercup is researching, says that young pulsars - neutron stars rotating at speeds of up to one revolution per millisecond - can be sources of fast radio pulses.
Buttercup calls such objects pulsars on steroids.
Over time, the rotation of pulsars slows down, and some of the rotational energy can be ejected into space in the form of radio emission.
It is not entirely clear how exactly pulsars can emit fast radio pulses, but it is known that they are capable of emitting short pulses of radio waves.
So, the pulsar located in the Crab Nebula is supposedly about 1000 years old. It is relatively young and is one of the most powerful pulsars known to us.
The younger the pulsar, the faster it rotates and the more energy it has. Buttercup calls such objects "steroid-based pulsars."
And although the pulsar in the Crab Nebula now does not have enough energy to emit fast radio pulses, it is possible that it could do this immediately after its appearance.
Another hypothesis says that the source of energy for fast radio pulses is not the rotation of a neutron star, but its magnetic field, which can be a thousand trillion times stronger than Earth's.
Neutron stars with extremely strong magnetic fields, the so-called magnetars, can emit fast radio pulses through a process similar to that resulting in solar flares.
There are a lot of magnetars in the Universe
As the magnetar rotates, the magnetic fields in its corona - the thin outer layer of the atmosphere - change configuration and become unstable.
At some point, the lines of these fields behave as if you clicked a whip. A stream of energy is released, which accelerates the charged particles, which emit radio pulses.
“There are a lot of magnetars in the universe,” says Bailes. "They are unstable, which probably explains the occurrence of fast radio pulses."
Hypotheses related to neutron stars are more conservative and based on relatively well-studied phenomena, therefore, they seem more likely.
“All hypotheses of the occurrence of fast radio pulses, which I consider to be any serious and which I am seriously discussing with my colleagues, have to do with neutron stars,” says Bales.
However, he admits that this approach can be somewhat one-sided. Many astronomers who study fast radio pulses also study neutron stars, so their tendency to view the former through the prism of the latter is understandable.
It may be that we are dealing with unexplored aspects of physics
There are also more unconventional explanations. For example, a number of researchers have suggested that fast radio pulses arise as a result of collisions of pulsars with asteroids.
It is possible that several hypotheses are correct at once, and each of them explains a certain case of the occurrence of fast radio pulses.
It is possible that some impulses repeat, while others do not, which does not completely rule out the hypothesis of collisions of neutron stars and other cataclysms of a cosmic scale.
“It may turn out that the answer is very simple,” says Lyutikov. "But it may also happen that we are dealing with unexplored aspects of physics, with new astrophysical phenomena."
Regardless of what fast radio pulses actually turn out to be, they can be of great benefit to space science.
For example, they could be used to measure the volume of matter in the universe.
As already mentioned, radio waves meet intergalactic plasma on their way, which slows down their speed depending on the frequency of the wave.
In addition to being able to measure the distance to the signal source, the difference in wave velocity also gives an idea of how many electrons are between our galaxy and the radiation source.
“Radio waves are encoded with information about the electrons that make up the universe,” says Bailes.
Previously, scientists were mainly engaged in this topic in their free time from basic research.
This gives scientists the opportunity to roughly estimate the amount of ordinary matter in space, which will help them in the future when calculating models for the emergence of the Universe.
The uniqueness of fast radio pulses is that they are a kind of cosmic laser beams, says Penh.
They pierce space in a specific direction and are intense enough to provide superior measurement accuracy.
“This is the most accurate measurement tool available to us for studying distant objects within line of sight,” he explains.
So, according to him, fast radio pulses can tell about the structure of plasma and magnetic fields near the radiation source.
As the plasma passes through, radio pulses can flicker, just as stars twinkle when viewed through the earth's atmosphere.
Measuring the characteristics of this scintillation will allow astronomers to measure the dimensions of plasma regions with an accuracy of several hundred kilometers. Due to the high scientific potential, and not least because of the inexplicability of the phenomenon, in the past few years, scientists' interest in fast radio pulses has grown significantly.
“Previously, scientists were mainly engaged in this topic in their free time from mainstream research,” says Lorimer.
Now astronomers are intensely looking for fast radio pulses in the as yet unexplored regions of the sky and continue to observe the sectors of the sky where these phenomena have already been recorded - in the hope of registering them.
At the same time, the powers of telescopes around the world are used, since when one pulse is observed from several observatories, the probability of a more accurate calculation of the source coordinates increases significantly.
So, in the next few years, radio telescopes like the Canadian CHIME (Canadian Hydrogen Intensity Mapping Experiment, or Canadian Hydrogen Intensive Mapping Experiment) will be able to observe vast areas of the sky and register hundreds of fast radio pulses.
The more data is collected, the more understandable the phenomenon of fast radio pulses will become. Perhaps someday their secret will be revealed.