Never mind the rumours. A New Scientist investigation finds that the most-sensitive-ever search for gravitational waves may have caught a whiff of its quarry before it even started listening officially.
Gravitational waves are ripples in space-time predicted by Einstein’s theory of general relativity. It tells us that massive objects like black holes and neutron stars warp space-time around themselves, and when two such behemoths collide, the distortions ripple outward at the speed of light. Although we are pretty confident this happens, the waves themselves have never been directly measured.
That may be about to change. For months, rumours of the detection of a gravitational wave signal by LIGO, the Laser Interferometer Gravitational-Wave Observatory, have spread on Twitter. With a press conference planned for Thursday, 11 February, they are building to a crescendo.
Beyond the gossip, we have reason to believe this is the real deal. By analysing public observation logs, New Scientist has found that the LIGO team has followed up on at least three sources since they started listening to the sky last September.
Cheshire cat grin
The LIGO detectors, located 3000 kilometres apart in Hanford, Washington and Livingston, Louisiana, can pick up passing gravitational waves by measuring how space-time stretches and contracts – by as little as one ten-thousandth the diameter of a proton.
Models show that the signal from the merger of two neutron stars or black holes resembles a chirp, its frequency rising until a final crash. Studying the signal gives an idea of how far away the event happened and the masses of the two bodies.
If we had a third detector, we could triangulate the source of the signal. That will be possible later this year, when the VIRGO experiment in Cascina, Italy, comes online. But with only the twin detectors, the best we can do is gesture vaguely toward a region of the sky.
So the LIGO team reaches out to astronomers at 75 observatories around the world, who scan that region for more traditional signals: visible light, gamma rays, even neutrinos. If a gravitational wave event is like the elusive Cheshire cat, then finding these counterparts is like spotting the grin.
Although the results of these searches are not public, whether or not they have taken place is – and so are the coordinates.
New Scientist focused on searches from the European Southern Observatory, a set of telescopes in Chile that will hunt for the bright flash of light that may accompany a gravitational wave signal. By plotting the locations of these searches, we can infer where and when LIGO’s ears may have been pricked.
We found that the first search began on 17 September, starting in the constellation Dorado and veering across other southern sky constellations (see blue spots on map). LIGO’s observing run officially began the very next morning, but the experiment had spent the previous few weeks collecting data in preparation.
The first rumour of a signal appeared just a week later, to the consternation of team members. If borne out, then the first signal ever seen fell into LIGO’s lap before the experiment officially started.
Two other searches – one around the constellation Aries, the other in the rough vicinity of Hydra – both began on 28 December and continued until the end of the run on 14 January (see red spots on map). The three searches together suggest LIGO may have been unbelievably lucky.
Textbook chirp, or fake?
The last time something like this happened, it ended in a let-down – because the LIGO team creates false signals to test their analysis.
In 2010, before LIGO had been upgraded to its present sensitivity, a textbook chirp that looked like two black holes colliding came through. The team drafted a paper and sent maps of where the signal may have come from to astronomers, who searched for a counterpart with other telescopes.
There was just one problem: the signal was a fake deliberately injected into the data stream to make sure the team would be able to spot a real one. The dramatic opening of a sealed envelope revealed that fact to 300 team members in the room, with 100 more watching via a video link.
It’s not clear whether any of the new signals are false, but given the team’s plans for introducing fakes, it’s unlikely that they all are.
We’ll know on Thursday if any of these new signals were more than just a quality-control exercise.