Over the last few years, gravitational wave observatories have been used to detect collisions between black hole pairs and even two neutron stars. Observing these very faint signals has been possible thanks to a combination of sophisticated technology and strong theoretical predictions of what we should see. Now physicists are considering what else might be out there.
A team of European researchers worked out what the gravitational wave signal would be like for a pair of interacting wormholes. Wormholes are hypothetical bridges between two different regions of space-time or even different universes. In this case, the researchers just considered them to be exotic compact objects, not worrying too much about their other properties.
If they exist, the researchers argue, they will have one important difference from black holes. The wormholes would be lacking an event horizon, or edge, the surface beyond which nothing – not even light – can escape the gravitational pull of the black hole. According to the work, published in Physical Review D, this difference is key.
"The final part of the gravitational signal detected by these two detectors – what is known as ringdown – corresponds to the last stage of the collision of two black holes, and has the property of completely extinguishing after a short period of time due to the presence of the event horizon," explained authors Pablo Bueno and Pablo A. Cano, from KU Leuven University in Belgium, in a statement.
"However, if there were no horizon, those oscillations would not disappear completely; instead, after a certain time, they would produce a series of 'echoes,' similar to what happens with sound in a well. Interestingly, if instead of black holes, we had an ECO, the ringdown could be similar, so we need to determine the presence or absence of the echoes to distinguish the two types of objects."
The study shows a remarkable similarity between the signal from black hole mergers and the theoretical wormhole merger, up to the ringdown point. There’s no evidence to conclude that previous detection simply missed the gravitational echoes, but the new research tells observers that if these objects exist and collide with each other, there is a way to study them.
"Time will tell if these echoes exist or not. If the result were positive, it would be one of the greatest discoveries in the history of physics," Bueno added.
Three gravitational wave observatories, the two LIGO facilities in the US and Virgo in Italy, will be back online observing the universe later this year.