It's as if someone in a galaxy far, far away rang a loud bell. Could it be a signal from aliens trying to get in touch? Or is it something hitherto unknown, but still completely natural?
The ring came in the form of a fast radio burst (FRB), a phenomenon that's been causing astronomers to scratch their scalps for nearly a decade. This week, an international team of researchers announced some significant progress in understanding FRBs.
Here's the back story. In 2006, Duncan Lorimer, an astronomer at West Virginia University, was sifting through reams of cosmic static that had been collected by the Parkes radio telescope, a 210-foot diameter antenna located in the sheep country of Australia. Unexpectedly, Lorimer found an intriguing signal that lasted less than an eyeblink - a radio pulse. It was quickly dubbed a fast radio burst for obvious reasons, but no one had a clue as to its cause. Was it really the radio signature of something out there - perhaps even a deliberate signal from intelligent beings - or was it merely an instrumental glitch?
That's a hard question to answer for something that seems to be a cosmic one-off. How can you study it if you no longer can see it?
Since there's no good reply to that, astronomers looked for other FRBs - a daunting task that's somewhat akin to studying giant squids. Yes, someone may claim to have seen one, but you won't be sure they exist until you've spotted them again. And the next sighting could happen anywhere.
The search for more FRBs was daunting - months of sifting through data looking for something that might not even be there. But additional bursts were found. They are, as noted, very brief: only milliseconds in length. They are also musical, in the sense that they rapidly run down the radio dial, similar to the sound you'd get from a slide whistle. This is exactly the type of behavior you'd expect from a radio pulse that's coming from far away, because hot interstellar gas between the source and your radio telescope will slightly delay the lower frequencies of the signal.
But how far is "far away"? No one knew.
Now they do. When an FRB was found in April of last year, the astronomy community was quickly put on alert. Within hours of the burst, a phalanx of six antennas known as the Australia Telescope Compact Array was swung in its direction. With its ability to make highly detailed radio images of the sky, the Array pinpointed small areas that could include the source of the burst. These patches were then examined by the Subaru telescope in Hawaii - an instrument with an 8-meter diameter mirror that could see light coming from the burst's afterglow.
Subaru fingered a suspect - an elliptical galaxy 6 billion light-years away. Elliptical galaxies are football-shaped, often massive stellar assemblages that frequently have giant black holes at their centers. The suspected source of the FRB is not exactly in our corner of the cosmos. It's three thousand times farther than the Andromeda galaxy - the familiar cousin of the Milky Way that's popular wallpaper on laptops.
So now we know from where this particular FRB comes. But we still don't know for sure what produced it. However, consider the big picture: Astronomers make two kinds of discoveries. In some cases, they find something that's been predicted to exist, such as planets in the outer solar system or gravitational waves. And in those cases, at least they know what to look for, if not where.
But in many other circumstances, they find something by accident - something that no one had predicted - such as quasars, pulsars, or strange star systems like KIC 8462852. These things begin as head-scratchers, and in every one of these examples, at least some folks have suggested that they're proof of alien activity.
That's certainly intriguing, and frankly also possible. But what usually happens is that researchers turn up additional examples of these phenomena, and eventually clever theorists figure out what they are. And so far they've all been due to nature, not little gray guys.
Mind you, the discoveries are not always unambiguous - some early FRBs were even traced to a microwave oven in the Parkes telescope used to warm up lunches for the astronomers. But there are almost 20 known cases of FRBs, and they are definitely cosmic, rather than culinary, in nature. And when you find so many examples of a celestial phenomenon, you can pretty much rule out the possibility that they're the result of a collaborative project by widely separated groups of aliens, all intent on whistling in the dark.
Now that we know more about FRBs - that at least one of them is produced in a massive galaxy far, far away - we can perhaps determine their cause. Whatever it is, it's got to be something enormously energetic, and that usually stimulates astronomer brains to imagine collisions between collapsed stars or black holes.
Maybe so. As John Lennon said, "reality leaves a lot to the imagination."