Space-time ripples can provide clues to missing components of the universe

There is something a little wrong with our theory of the universe. Most everything fits, but there is a fly in the cosmic ointment, a particle of sand in the endless sandwich. Some scientists believe that gravity may be the culprit – and that subtle ripples in the fabric of space-time can help us find the missing piece.

A new paper written by a scientist from the University of Chicago explains how this could work. Published December 21 in Physical assessment D, the method depends on finding such ripples bent by traveling through supermassive black holes or large galaxies on their way to Earth.

The problem is, something makes the universe not only expand but also expand faster and faster over time – and no one knows what it is. (The search for the exact rate is an ongoing debate in cosmology).

Scientists have proposed all kinds of theories as to what the missing piece might be. “A lot of these depend on changing the way gravity works on a large scale,” said study co-author Jose María Ezquiaga, a NASA Einstein postdoctoral researcher at the Kavli Institute of Cosmological Physics at UChicago. “So gravitational waves are the perfect messenger to see these possible changes in gravity, if they exist.”

Gravitational waves are ripples in the fabric of space-time itself; since 2015, humanity has been able to absorb these ripples with the help of the LIGO observatories. Every time two massively heavy objects collide elsewhere in the universe, they create a ripple that travels through space bearing the signature of whatever made it – maybe two black holes or two colliding neutron stars.

In the paper, Ezquiaga and co-author Miguel Zumalácarregui argue that if such waves hit a supermassive black hole or cluster of galaxies on their way to Earth, the ripple’s signature would change. If there was a difference in gravity compared to Einstein’s theory, the evidence would be embedded in that signature.

For example, one theory for the missing piece of the universe is the existence of an extra particle. Such a particle would, among other things, generate some kind of background or “medium” around large objects. If a traveling gravitational wave hits a supermassive black hole, it would generate waves that get confused with the gravitational wave itself. Depending on what it has encountered, the gravitational wave signature may contain an “echo” or appear distorted.

“This is a new way to explore scenarios that could not be tested before,” said Ezquiaga.

Their paper sets out the conditions for finding such effects in future data. The next LIGO run is scheduled for 2022, with an upgrade to make the detectors even more sensitive than they already are.

“During our last observational run with LIGO, we saw a new measurement of the gravitational waves every six days, which is amazing. But across the universe, we think they actually happen once every five minutes,” said Ezquiaga. “With the next upgrade, we could see so many of it – hundreds of events per year.”

The increased numbers, he said, make it more likely that one or more waves traveled through a massive object, and scientists can analyze them for clues to the missing components.

Zumalácarregui, the other author on the paper, is a scientist at the Max Planck Institute for Gravitational Physics in Germany and the Berkeley Center for Cosmological Physics at Lawrence Berkeley National Laboratory and the University of California, Berkeley.