Whenever two compact objects (black holes or neutron stars) collide elsewhere in the Universe, they create gravitational waves that travel across space, carrying the signature of whatever made it. In a new paper published in the journal Physical Review D, physicists from the United States and Germany argue that if such waves hit a supermassive black hole or a cluster of galaxies on their way to Earth, their signature would change; if there were a difference in gravity compared to Einstein’s theory, the evidence would be embedded in that signature.

An artist’s impression of two merging neutron stars. Image credit: NASA / CXC / M.Weiss.

An artist’s impression of two merging neutron stars. Image credit: NASA / CXC / M.Weiss.

Something is making the Universe not only expand, but expand faster and faster over time — and no one knows what it is. Astrophysicists have proposed all kinds of theories for what the missing piece might be.

“Many of these rely on changing the way gravity works over large scales,” said Dr. Jose María Ezquiaga, a postdoctoral researcher in the Kavli Institute for Cosmological Physics and Enrico Fermi Institute at the University of Chicago.

“So gravitational waves are the perfect messenger to see these possible modifications of gravity, if they exist.”

One theory for the missing piece of the Universe is the existence of an extra particle.

Such a particle would, among other effects, generate a kind of background around large objects.

If a traveling gravitational wave hit a supermassive black hole, it would generate waves that would get mixed up with the gravitational wave itself.

Depending on what it encountered, the gravitational wave signature could carry an ‘echo,’ or show up scrambled.

“This is a new way to probe scenarios that couldn’t be tested before,” Dr. Ezquiaga said.

The team’s paper lays out the conditions for how to find such effects in future data.

“In our last observing run with LIGO (Laser Interferometer Gravitational-Wave Observatory), we were seeing a new gravitational wave reading every six days, which is amazing,” Dr. Ezquiaga said.

“But in the entire Universe, we think they’re actually happening once every 5 min.”

“In the next upgrade, we could see so many of those — hundreds of events per year.”

“The increased numbers make it more likely that one or more waves will have traveled through a massive object, and that scientists will be able to analyze them for clues to the missing components.”

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Jose María Ezquiaga & Miguel Zumalacárregui. 2020. Gravitational wave lensing beyond general relativity: Birefringence, echoes, and shadows. Phys. Rev. D 102 (12): 124048; doi: 10.1103/PhysRevD.102.124048