In 2007 astronomers detected a new class of radiation signal from deep space – what are called fast radio bursts. Fast radio bursts are highly energetic but very short-lived bursts of radio energy, typically lasting less than a hundredth of a second. Fewer than twelve of these bursts have been detected. For years, all of the detections came from a single telescope in Australia, but now the Arecibo Observatory in Puerto Rico has also detected such a fast radio burst.
Where are the signals coming from? Based on something called dispersion measures, an astronomer named Dan Thornton estimates that the signals come from between five and ten billion light years away. But other astronomers disagree. One team of astronomers estimates that the signals are coming from nearby stars inside our galaxy. But their reasoning is based on a “5% coincidence” argument that isn't very convincing (considering that 5% coincidences are not very improbable).
Based on the number of fast radio bursts that have been detected, astronomers have estimated that our planet could be receiving as many as 10,000 of these radio bursts per day. What could be causing the signals? Astronomers don't know. Some astronomers speculate that the fast radio bursts could be caused by various exotic types of stellar events, such as unusual solar flares or two neutron stars colliding with each other.
There is, however, a general problem with such explanations. A scientific paper on the fast radio bursts says this (which I'll “translate” in a moment):
Let me clarify this rather opaque comment. The highly energetic freak events imagined as the source of the fast radio bursts would probably have produced other types of radiation such as gamma ray radiation, x-rays, or visible light. But no one has detected a flash of any of these types of radiation with a position in space (and time of origin) matching any of the fast radio bursts. To give an analogy, it's kind of as if you felt the ground shaking, and assumed it was something heavy falling to the ground, but you didn't hear any noise at the same time. That would throw doubt on your explanation.
The same paper discusses various theories to explain the fast radio bursts. The paper mentions the possibility of neutron star mergers (two nearby neutron stars interacting with each other), but it notes that this extremely rare phenomenon would not occur often enough to explain the estimated occurrence rate of the fast radio bursts. The paper also notes the hypothesis of black hole evaporation being the source of the fast radio bursts, but notes that the expected energy from such an event would be much less than the energy coming from a fast radio burst. The paper notes that there is no way to get a fast radio burst merely from a core-collapse supernova event, but says that conceivably if a supernova was next to a neutron star, it might produce a fast radio burst. But a supernova occurs only about once every 50 years in our galaxy, and a supernova very close to a neutron star is very, very rare – probably too rare to explain the phenomenon.
In short, we seem to have no really good astrophysical explanations for the fast radio bursts. Given the fact that short radio bursts have been postulated as one means by which extraterrestrial civilizations could announce their existence, there would seem to be a very real possibility that some or many of these short radio bursts are coming from extraterrestrial civilizations.
Hypothetical extraterrestrial radio transmitter
I may note that even if the signals are coming as far away as five billion light years, that does not rule out the possibility that they are artificial signals from extraterrestrial civilizations. The universe is believed to be about 13.7 billion years old. If a radio signal came from five billion light-years away, it would have come from a time when the universe was about 8.7 billion years old. Was there enough time for intelligence to appear by that date? There might well have been. Recent telescopic observations show that when the universe was only a few billion years old, it already had surprisingly mature galaxies. We know of no reason why intelligence could not have arisen in other galaxies between five and seven billion years ago.
I may note the astonishing difference between the way astronomers have reacted to two different cases of unexplained new radiation observations: the fast radio bursts and the b-mode polarization signals detected recently by the BICEP2 team. The two cases are quite similar in some respects. In both cases we have a type of signal observation that might be explained through a relatively mundane explanation, and which might also be explained by imagining something monumental. In the case of the fast radio bursts, the mundane explanations are things like solar flares and star collisions, and the monumental explanation is to imagine deliberate signals from extraterrestrial civilizations. In the case of the b-mode polarization observations, the quite plausible mundane explanations are things like cosmic dust, synchrotron radiation, and gravitational lensing; the monumental explanation is to assume something coming from cosmic inflation in the universe's first second.
In the case of the fast radio bursts, astronomers have reacted with the greatest caution and circumspection. Their accounts merely report the observations, without speculating about any possible monumental explanation. In the case of the b-mode polarization signals reported by the BICEP2 team, astronomers and cosmologists instantly threw caution and circumspection out the window, and enthusiastically jumped the gun by calling the signals proof of cosmic inflation, seemingly before the public had even had time to scrutinize the scientific paper. I suspect that this case will be recorded as one of the great cases of over-enthusiastic gun-jumping hype, similar to the 1990's “life on Mars fossils” announcement that didn't pan out. A few weeks after the BICEP2 announcement, a scientific paper appeared reporting that a certain type of cosmic dust (not considered by the BICEP2 team) could be the source of their observations.
Can we imagine if astronomers had reacted to the fast radio bursts the way they reacted to the findings of the BICEP2 team? In that case they would have announced we had received the smoking gun of alien civilizations.
What is the proper way to consider both of these cases? An intelligent outlook is to say that some interesting signals have been discovered, and to cautiously note the fact that they could possibly be due to an epic, monumental explanation – while at the same time saying that the matter is very much undecided, because the universe has a thousand surprises up its sleeves, because there are almost always a dozen different ways to explain any very distant thing we see in our telescopes, and because our knowledge of the universe is shaky and fragmentary.