Are These the Screams of Cosmic Disaster?
Telescopes have spotted hundreds of powerful signals coming from deep space. Who, or what, is behind them?
Nobody seemed to have any idea what caused them. Was it a spy satellite, perhaps, transmitting clandestine signals and fooling radio telescopes? Could they be the dying screams of supermassive stars, on the verge of collapsing forever into a black hole? Or were they messages from aliens, sending bursts of energy across the cosmos?
For years these mysterious signals have puzzled astronomers. Every now and then radio telescopes would pick one up - a sudden bright flash that disappeared as quickly as it came. When astronomers did the careful work of analysing the flash, it would invariably turn out to be incredibly distant, originating half way across the universe. As for the energy involved, calculations said this must be enormous, almost unbelievably intense.
Lacking an easy explanation, astronomers named them Fast Radio Bursts, or FRBs for short, and filed them away as yet another cosmic mystery. Nevertheless, some early studies revealed intriguing facts. They seemed to be common - erupting once every few seconds somewhere in the universe. They appeared, at least for a while, to follow intriguing numerical patterns - sparking speculation they were created by intelligent beings.
Yet despite their power and frequency, astronomers struggled to spot them in time. They could appear anywhere, at any moment, and last fractions of a second. The few we found were by chance - a lucky coincidence where a telescope was pointed in the right direction at the right time.
In fact, the first time we saw one of these mysterious signals it went unnoticed for years, its trace hidden in data archived at the Parkes Radio Telescope in Australia. Duncan Lorimer, an astronomer at West Virginia University, was the first to uncover it. In 2007 he noted it as a five millisecond long pulse, originating at least three billion light years away.
Fortunately, there was an explanation for such signals. Decades ago astronomers discovered rapidly spinning stars they named pulsars. As they spin they sweep the cosmos with a beam of radio waves - rather like a cosmic lighthouse. From Earth this appears as a steady rhythm of flashing radio signals, just like a ship out at sea sees a lighthouse.
Lorimer’s signal, however, didn’t quite fit. For one thing it happened only once. Despite searching through the record, Lorimer and his team could find no evidence it had ever repeated. It was also far more powerful than any other pulsar seen before. As Lorimer wrote in his paper reporting the discovery, that was both intriguing and puzzling.
For some time Lorimer’s signal stood alone, a unique and rather strange event occuring in a far off galaxy. But then astronomers started to find other examples, coming from all over the place. They displayed odd properties and appeared around the same time each day, which made some think they were artificial, rather than natural. Spy satellites maybe - or aliens, manipulating vast amounts of energy.
Then astronomers noticed something embarrassing. Many of the signals turned out to be artificial after all - but it wasn’t aliens causing them, it was microwave ovens. All of a sudden it was clear why the Parkes Radio Telescope only detected them around lunchtime, or why they all seemed to come from the same direction. Hungry astronomers, it turned out, had created their own mystery.
For a while FRBs seemed to be a dead end, a mistaken observation that had led astronomers into flights of fancy. But then other telescopes started to spot them too; including the world class facility at Arecibo, in Puerto Rico. With more data, astronomers could finally make a stab at solving their mystery.
So far, however, increased observation has only added to the puzzle. Astronomers have determined that they seem to come in two different flavours. Some repeat, flashing over and over again. Others appear just once, briefly lighting up the sky before fading away forever. Are they both caused by the same thing? Or are many events capable of causing such powerful flashes?
With more time, astronomers have been able to track down the locations of the FRBs. Most appear to come from extreme distances, originating billions of light years from Earth. That points to their intense power - for something to be visible across half the universe it must be incredibly bright. Some, however, are much closer to home.
Last year, astronomers spotted a fast radio burst coming from our own galaxy. The culprit was a highly magnetic star - something known as a magnetar - about 35,000 light years from Earth. A day before the outburst the star started to act strangely, spitting out huge amounts of radiation. That attracted the attention of telescopes around the planet.
Then, with the eyes of the world watching, the star released an unexpected wave of radio energy - a fast radio burst. The flash lasted mere milliseconds, but it was enough to convince researchers that magentars were the ultimate cause of the bursts.
The problem is, astronomers have also recently spotted FRBs coming from places where magnetars have no business being. Either our theories about magnetars are wrong, or the bursts have several different causes. Either way, the door is once again open to speculation.
One oft suggested idea is that fast radio bursts are caused by colliding neutron stars or black holes. Such events should be powerful - black holes and neutron stars are some of the densest possible objects in existence. Others suggest that intense supernova might be to blame, or that they are somehow related to debris swirling around black holes.
Whatever the cause, astronomers are hunting for more data. A telescope in Canada, CHIME, includes a bit of equipment dedicated towards detecting fast radio bursts. The first data release, covering observations in 2018 and 2019, and announced in June this year, contains more than five hundred FRBs.
That so many events were captured in one year indicates that such bursts really are common. Some astronomers have estimated that thousands light up the sky every day, visible if only we look carefully enough. What’s more, they appear all over the sky - strongly supporting the conclusion that they come from distant regions of the universe.
The data also suggests that there are multiple types of burst - probably with different causes. Those that reoccur may be similar to pulsars, their power magnified by intense magnetic fields. Those that happen once may be more cataclysmic in nature - intense storms erupting on magnetars or colliding neutron stars.
Of course, not everyone is satisfied with such explanations. The idea that powerful radio signals from space are artificial has intrigued many - even if the main tenets of the theory have already been disproven. If aliens really are communicating via radio signals, however, FRBs may be exactly what we would expect to see.
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