University of Chicago scientist explains how LIGO gravitational waves could be scrambled, giving information.
There is something wrong with our theory of the universe. Almost everything is fine, but there’s a fly in the cosmic ointment, a particle of sand in the infinite sandwich. Some scientists believe gravity may be the culprit – and that subtle ripples in the fabric of spacetime could help us find the missing piece.
A new article co-authored by a scientist at the University of Chicago explains how this might work. Published Dec. 21 in Physical Review D, the method depends on finding such ripples that were bent while traveling through supermassive black holes or large galaxies on their way to Earth.
The problem is, something not only expands the universe, but expands faster and faster over time – and no one knows what that is. (The search for the exact rate is an ongoing debate in cosmology).
Scientists have come up with all kinds of theories about what the missing piece might be. “A lot of them are based on changing the way gravity works at scale,” said article co-author Jose María Ezquiaga, a NASA Postdoctoral Fellow Einstein at the Kavli Institute for Cosmological Physics at UChicago. “Gravitational waves are therefore the perfect messenger to see these possible changes in gravity, if they exist.”
“Gravitational waves are the perfect messenger to see these possible changes in gravity, if they exist.”
– Astrophysicist Jose María Ezquiaga
Gravitational waves are ripples in the fabric of space-time itself; since 2015, humanity has been able to capture these ripples thanks to LIGO observatories. Whenever two massively heavy objects collide elsewhere in the universe, they create a ripple that travels through space, bearing the signature of everything that made it – maybe two black holes or two stars in neutrons colliding.
In the article, Ezquiaga and co-author Miguel Zumalácarregui argue that if such waves hit a supermassive black hole or clusters of galaxies en route to Earth, the signature of the ripple would change. If there was a difference in gravity from 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 generate, among other effects, a sort of background or “medium” around large objects. If a traveling gravitational wave hit a supermassive black hole, it would generate waves that would mix with the gravitational wave itself. Depending on what it encountered, the signature of the gravitational wave could carry an “echo” or appear scrambled.
“It’s a new way of probing scenarios that couldn’t be tested before,” Ezquiaga said.
Their article sets out the conditions for finding such effects in future data. LIGO’s next run is slated for 2022, with an upgrade to make the detectors even more sensitive than they already are.
“During our last observation run with LIGO, we saw a new gravitational wave playing every six days, which is amazing. But across the universe, we think they actually happen once every five minutes, ”Ezquiaga said. “The next time we upgrade, we could see so many of these events, hundreds of events a year.”
The increased number, he said, makes it more likely that one or more waves will have passed through a massive object and that scientists can analyze them for clues about the missing components.
Reference: “The lens of gravitational waves beyond general relativity: birefringence, echoes and shadows” by Jose María Ezquiaga and Miguel Zumalacárregui, December 21, 2020, Physical examination D.
DOI: 10.1103 / PhysRevD.102.124048
Zumalácarregui, the other author of the article, 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.