Early in the morning on 28 August, the Laser Interferometer Gravitational Wave Observatory (LIGO) felt two ripples in spacetime wash over it. But due to a strange effect of general relativity, the two signals may actually be from the same event.
Gravitational waves, which stretch and contract spacetime itself, emanate from massive objects moving around and colliding with each other. LIGO detected its first gravitational wave from a pair of black holes merging in 2015, and since then has observed more than 30 cosmic smash-ups. It has also added the Virgo detector in Italy to its network, allowing it to find more signals and figure out their locations more precisely.
The two signals that came in on 28 August are unusual in a variety of ways. For one, they hit the detectors only 21 minutes apart, making this only the second time that two gravitational wave events have been spotted in the same day. The black holes that merged to make the signals are also in roughly the same place in the sky. And they are at similar distances from us, between five and six billion light years away.
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That has led some astronomers to suggest that they might be from the same event. This could be possible because of a phenomenon called gravitational lensing. This normally refers to how the gravity of some massive object warps the path of light. But in this case the suggestion is that gravity might have also have bent the path of the gravitational wave. 鈥淚t would look like two separate events, but it鈥檚 really one event that鈥檚 being split into two by a large object in the way,鈥� says Asantha Cooray at the University of California, Irvine.
That could account for the time delay between the two signals: if the gravitational waves were split and bent, the later signal might come from waves that had to travel further along a curved path to get to our detectors. Such a short delay means that the lensing object would have to be something small and compact, like a black hole, Cooray says. 鈥淚f a whole galaxy was lensing this the time delay would be much larger.鈥�
Some members of the LIGO collaboration say this is probably not the case. 鈥淚t is much more likely that the localisation and distance of the candidates coincide by chance than that the source aligned with a lensing object in its way,鈥� says LIGO team member Gabriela Gonz谩lez at Louisiana State University.
Also, while the areas the signals seem to have come from are close together, they . If they come from a single cosmic event, they must be in exactly the same spot in the sky.
鈥淚f the two gravitational wave events arise from lensing, then the properties of the two events, including final localizations, will have to be almost entirely consistent,鈥� says LIGO team member Derek Fox at Pennsylvania State University. 鈥淭he preliminary localisations are regularly seen to shift around upon final analysis.鈥� This means the fact that they do not appear to line up exactly at the moment, might not rule out lensing.
We will find out for sure whether the two events could really just be one when LIGO and Virgo analyse this data further over the coming months. If they are really a single merger of black holes, it could teach us more about not only binary black holes but also the sort of objects that can lens them. 鈥淚f they are lensed it鈥檚 a pretty big deal,鈥� says Fox.
