Tighter tests for Einstein with gravitational waves

May 26. (EUROPE PRESS) –

Two new Caltech-led studies, published in Physical Review X and Physical Review Letters, describe new methods for putting general relativity to even more stringent tests.

By taking a closer look at the structures of black holes and the ripples in space-time they produce, these investigations look for signs of small deviations from general relativity. that hint at the presence of quantum gravity.

Albert Einstein’s general theory of relativity describes how the fabric of space and time, or spacetime, curves in response to mass. As fundamental as this theory is to the very nature of the space around us, physicists say it might not be the end of the story. Instead, they argue that theories of quantum gravity, which attempt to unify general relativity with quantum physics, they hold secrets about how our universe works at the deepest levels.

One place to look for quantum gravity signatures is in powerful collisions between black holes, where gravity is most extreme. Black holes are the densest objects in the universe: their gravity is so strong that they squeeze objects that fall into them into spaghetti-shaped noodles. When two black holes collide and merge into a larger body, they shake up space-time around them, sending gravitational waves in all directions.

The LIGO experiment, run by Caltech and MIT, has been routinely detecting gravitational waves generated by black hole mergers since 2015 (its partner observatories Virgo and KAGRA joined the search in 2017 and 2020, respectively). Yet so far, the general theory of relativity has passed test after test with no sign of failure.

“When two black holes merge to produce a larger black hole, the final black hole rings like a bell,” he explains. it’s a statement Yanbei Chen, a professor of physics at Caltech and a co-author of both studies. “The quality of the timbre, or its timbre, can be different from the predictions of general relativity if certain theories of quantum gravity are correct. Our methods are designed to look for differences in the quality of this timbre phase, such as harmonics and nuances, for example”.

The first paper, led by Caltech graduate student Dongjun Li, reports a unique new equation to describe what black holes would sound like in the framework of certain theories of quantum gravity, or in what scientists call the regime beyond general relativity.

The work is based on a groundbreaking equation developed 50 years ago by Saul Teukolsky, professor of theoretical astrophysics at Caltech. Teukolsky had developed a complex equation to better understand how space-time geometry waves propagate around black holes. In contrast to the methods of numerical relativity, in which supercomputers are required to simultaneously solve many differential equations pertaining to general relativity, Teukolsky’s equation is much simpler to use and, as Li explains, provides a direct physical view of the problem.

“If you want to solve all of Einstein’s equations for a black hole merger to accurately simulate it, you have to go to supercomputers,” says Li. “Numerical relativity methods are incredibly important for accurately simulating black hole mergers and provide a crucial foundation for interpreting LIGO data. But it is extremely difficult for physicists to draw insights directly from numerical results. Teukolsky’s equation gives us an intuitive view of what is happening in the call phase.”

Li was able to take Teukolsky’s equation and fit it to black holes in the regime beyond general relativity for the first time. “Our new equation allows us to model and understand gravitational waves propagating around black holes that are more exotic than Einstein predicted,” he says.

The second article, published in Physical Review Letters, led by Caltech graduate student Sizheng Ma, describes a new way to apply Li’s equation to real data acquired by LIGO and its partners in their next observing run. This data analysis approach uses a series of filters to remove the characteristics of a black hole sound predicted by general relativity, so that potentially subtle signatures beyond general relativity can be revealed.

“We can look for the features described by Dongjun’s equation in the data that will be collected by LIGO, Virgo, and KAGRA,” says Ma. “Dongjun has found a way to translate a large set of complex equations into a single equation, and this is tremendously useful. . This equation is more efficient and easier to use than the methods we used before.”

The two studies complement each other well, Li says. “I was initially concerned that the signatures my equation predicts would be buried under the multiple overtones and harmonics; luckily, Sizheng’s filters can remove all of these known features, allowing us to focus only on the differences,” he says.