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"Flashbacks" – gravitational waves create "memories" of the universe

Published on May 10, 2019

  Black Hole Melting

While the billion dollar detector of the Laser Interferometer Gravitational Wave Observatory (LIGO) watches around the clock for gravitational waves passing through the earth, new investigations have emerged that these waves leave "memories" that could help them recognize them even after the Big Bang, and that they have the potential to inform us about everything from events after the Big Bang to more recent events in galaxy centers.

"The fact that gravitational waves can make permanent changes to a detector after passing the gravitational waves is one of the more unusual predictions of general relativity," said graduate student Alexander Grant, lead author of "Persistent Gravitational Wave Observables: General Framework". Published on April 26 in Physical Review D.

Physicists have long known that gravitational waves leave a memory on the particles, and have identified five such memories. The researchers have now discovered three more aftereffects of the passage of a gravitational wave: "Observable waves with a sustained gravitational wave" that could someday help identify waves that travel through the universe.

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According to Grant, each new observable offers various possibilities to confirm the theory of general relativity and insights into the intrinsic properties of gravitational waves. These properties could help extract information from the cosmic microwave background – the radiation left over from the Big Bang.

"We did not anticipate the richness and variety of what could be observed," said Éanna Flanagan. Edward L. Nichols Professor and Chair of Physics and Professor of Astronomy.

This computer simulation shows the collision of two black holes, a tremendous event first recorded by the Laser Interferometer Gravitational Wave Observatory. The black holes swirled towards each other, colliding and colliding merged. This simulation shows what the fusion event would look like if humanity could somehow move closer. It was developed by the Cornell-based project SXS (Simulating eXtreme Spacetimes).

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"What surprised me about this investigation is how different ideas were sometimes unexpectedly related," Grant said. "We looked at a variety of different observables and found that in order to know something about one, one had to understand the other."

The Three Observables

The researchers identified three observables Effects of gravitational waves in a flat region in space-time, in which an outbreak of gravitational waves occurs, after which it returns to a flat region. The first observable "curve deviation" is how much two accelerating observers separate from each other in comparison to how observers would separate with the same accelerations in a flat space that is not disturbed by a gravitational wave.

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The second observable "holonomy" is obtained by transporting information about the linear and angular momentum of a particle along two different curves through the gravitational waves and comparing the two different results.

The third part investigates how gravitational waves affect the relative displacement of two particles when one of the particles has an intrinsic spin.

Each of these observables is defined by the researchers in a way that can be measured with a detector. The detection methods for the curve deviation and the rotating particles are "relatively easy to perform," the researchers wrote. They only required "a means of measuring the distance and observing the respective accelerations by the observers."

It was more difficult, she wrote, "to oblige two observers to measure the local curvature of space-time (possibly by carrying small gravitational-wave detectors with them)." Observables are beyond the reach of current science, researchers say.

"But we have seen many exciting things with gravitational waves, and we will see much more. There are even plans to place a gravitational wave detector in space that is sensitive to sources other than LIGO.

The Daily Galaxy via Cornell University

Credit: LIGO

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