Clues to missing components of the universe from ripples in space-time

Space time wrinkle concept

University of Chicago scientist explains how LIGO gravity waves can be scratched, yielding information.

There is something small to our theory about the universe. Almost everything fits, but there is a fly in the cosmic ointment, a particle of sand in the infinite sandwich. Some scientists think that the culprit may be gravity – and that subtle wrinkles in the fabric of space-time can help us find the missing piece.

A new paper compiled by a University of Chicago scientist outlines how it can work. The method, published December 21 in Physical Review D, relies on finding wrinkles bent by traveling through supermassive black holes or large galaxies en route to Earth.

The problem is that something causes the universe to not only expand faster and faster over time – and no one knows what it is. (The search for the exact course is an ongoing debate in cosmology).

Scientists have proposed all sorts of theories for the missing piece. “Many of these rely on changing the way gravity works on a large scale,” said co-author of the paper, Jose María Ezquiaga, a NASA Einstein postdoctoral fellow at the Kavli Institute of Cosmological Physics at UChicago. “Gravity waves are therefore the perfect messenger to see these possible adjustments of gravity, if they exist.”

“Gravity waves are the perfect messenger to see these possible adjustments of gravity, if they exist.”

Astrophysicist Jose María Ezquiaga

Gravity waves are wrinkles in the fabric of space-time itself; since 2015, mankind has been able to pick up these ripples with the help of the LIGO observatories. When two massive objects collide elsewhere in the universe, they create a ripple that moves through space and bears the signature of everything that made it – perhaps two black holes or two neutron stars that collide.

Merging of black holes gravity waves

A supercomputer simulation of fusion of black holes that emit gravitational waves. Scientists believe that there is a way to use these waves to find missing pieces in our understanding of the universe. Credit: Illustration by Chris Henze / NASA

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

One theory for the missing piece of the universe, for example, is the existence of an extra particle. Such a particle will, among other things, generate a kind of background or ‘medium’ around large objects. If a moving gravitational wave hits a supermassive black hole, it will generate waves that are mixed by the gravitational wave itself. Depending on what it encountered, the gravitational wave signature may carry an ‘echo’ or appear cluttered.

“This is a new way of exploring scenarios that have not been tested before,” Ezquiaga said.

Wave mixing animation

An illustration of waves mixing and creating a clear new signature. Credit: Ezquiaga and Zumalácarregui

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

‘In our last observation run with LIGO, we saw a new gravitational wave read every six days, which is incredible. But in the whole universe, we think they actually happen once every five minutes, ‘Ezquiaga said. “With the next upgrade, we could see so much of it – hundreds of opportunities a year.”

According to the increasing numbers, it is more likely that one or more waves will move through a massive object, and that scientists will be able to analyze it for clues to the missing components.

Reference: “Gravitational wave lens beyond general relativity: birefringence, echoes and shadows” by Jose María Ezquiaga and Miguel Zumalacárregui, December 21, 2020, Physical overview D.
DOI: 10.1103 / PhysRevD.102.124048

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

Funding: NASA, Kavli Foundation.