They are blasts from the past, shrieks from the black abyss of the universe unlike anything ever found before. And they are deeply mysterious.
Are these some new cosmic phenomenon, an odd habit of nature that we never knew? Or could they be the deliberate wails of societies howling from the farthest corners of space?
In 2007, Duncan Lorimer at West Virginia University was digging through historical records from the Parkes radio telescope in Australia when he and his colleagues stumbled across an unusual signal.
It was no more than a hiccup, a burst of radio energy as fleeting as ocean foam. No one had ever found anything similar, and probably for that reason it was overlooked even by those who had first recorded the data.
Lorimer published this discovery, and the astronomical community scratched its collective pate. What the heck was this? Was it even for real? Some had doubts. Regular visitors to the Parkes antenna, which squats in the fly-filled sheep country west of Sydney, Australia know that there's a kitchen microwave just below the control room. In the 1990s, I frequently used it myself to heat up lunch. So maybe the radio hoots weren't heavenly, but merely ovenly.
However, when other, similar bursts were later found by the Arecibo radio telescope in Puerto Rico, the Parkes microwave was exonerated. Clearly, these quick radio blasts — christened "fast radio bursts," or FRBs — are for real.
The most obvious thing about them, other than being short, is that they chirp, which is to say they quickly decrease in frequency — somewhat like a slide whistle, going from a high note to a low one in a few thousandths of a second.
Astronomers had an off-the shelf explanation for the pitch change: a bit of physics involving the breathlessly thin, hot gas that fills the space between galaxies. The slide whistle behavior of the FRBs suggested that whatever was belching this radio energy had to be far away — significantly beyond the familiar neighborhoods of our own galaxy.
Tracking down the source of the bursts was hard. But thanks to a lucky break, it has now been solved in one case. A source with the lilting name FRB 121102, discovered at Arecibo, was burping over and over. Unlike others, it was making more than just a one-night stand, and repeatedly sent radio waves into space. This is not the behavior you'd expect if the bursts were caused by singular events such as a collision between two black holes.
The unusual behavior of FRB 121102 prompted astronomers to study it using the Very Large Array in New Mexico, an ensemble of 27 antennas that boasts a much sharper "eye on the sky" than Arecibo. The researchers spent six months hoping for the FRB to do its thing. And it did. Many times.
When the signal reappeared, the VLA — and eventually other radio arrays — were able to finger the FRB's position on the sky. In a chest-thumping demonstration of astronomical teamwork, observers swung the Gemini North telescope in Hawaii to this position, made a photo, and pinpointed the source of the burst in a bantam-sized galaxy 3 billion light-years away.
That's incredible. Something is happening (or rather, happened!) that was energetic enough to be picked up with a radio receiver from a distance equivalent to 30 thousand Milky Way's laid end-to-end, if that's something you can picture.
The best guess as to what's causing the FRBs is that some sort of massive object is acting up, or perhaps swallowing something else. Maybe a black hole, maybe a neutron star. No one knows for sure, but now that at least one FRB has been tracked down, astronomers can ratchet up their observations to learn more. If past is prologue, you can expect the mystery of the FRBs to be solved within a decade.
But given the strange radio signature of FRBs, it's tempting to wonder if they could be screeches transmitted by intelligent beings. That's not impossible: The raw ingredients for life, including habitable planets, were certainly in place many billions of years ago.
On the other hand, why would anyone make such a brief broadcast? It couldn't convey much information. In a few thousandths of a second, even a fast internet connection won't send many bytes. Of course, one could argue that the FRBs are more like alien lighthouses, not designed to tell anyone very much — merely mark a location. You can imagine them as a "We're here!" signal.
Unfortunately, that's the problem with postulating that these intriguing radio sources are the result of someone, rather than something. There's no limit to your imagination. You can cook up an alien explanation for just about any sort of signal you discover. Extraterrestrials have been given credit for pulsars, quasars, and lots of other odd celestial behavior. But while extraterrestrials are easy to profile, they are — and should be — hard to convict. There's not been a single case in which aliens were responsible for any new cosmic mystery.
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But that hardly diminishes the excitement that should accompany these oddball objects. There are currently 18 known FRBs, but working the statistics, estimates suggest that if you could search the whole sky at once you'd find thousands going off every day.
Yes, astronomy may be the oldest science, but it's far from spent. In the past few years, we've learned that planets are as common as weeds, and that space is being endlessly roiled by gravitational waves. Now there's a new member of the cosmic bestiary, something that's mysterious and certainly dramatic: FRBs.
And every minute, a few more pop off above your head, the silent voices of cataclysmic events that occurred a long time ago in galaxies far, far away.
Seth Shostak is the Senior Astronomer at the SETI Institute, in Mountain View, California. He writes frequently on astronomy and other topics, and hosts the SETI Institute's weekly radio show, "Big Picture Science."
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