Gravitational waves create “memories” of the universe (weekend feature)

LIGO Gravitational Waves

While the billion-dollar Laser Interferometer Gravitational-Wave Observatory (LIGO) detector monitors 24/7 the passage of gravitational waves through the Earth, recent research shows that these waves leave behind “memories” – a permanent shift in spacetime that comes from strong-field general relativism effects – which could help detect them even after they pass, creating the potential to tell us everything from the time after the Big Bang and the creation of cosmic strings – to more recent events in the centers of galaxies.

“The fact that gravitational waves can leave permanent changes to a detector after the gravitational waves pass is one of the rather unusual predictions of general relativity,” said Alexander Grant, lead author of Observables of persistent gravitational waves: general framework.

Physicists have long known that gravitational waves leave a memory on particles along their path and have identified five such memories. Researchers have now discovered three more aftermaths of a gravitational wave’s passage, “persistent gravitational wave observables” that could one day help identify waves traversing the universe.

“The recent discovery of gravitational waves opens up a new opportunity to look further back in time, as the Universe has been transparent to gravity from the beginning. While the Universe could have been a trillion to a quadrillion times hotter than the hottest place in the Universe today, neutrinos probably behaved exactly the way we need to ensure our survival.We demonstrated that they probably also left behind a background of ‘detectable gravitational ripples to let us know,’ says TRIUMF postdoctoral fellow Graham White of a recent post suggesting that gravitational waves may contain evidence to prove the theory that life survived the Big Bang due to a phase transition that allowed neutrino particles to reshuffle matter and antimatter,

Extract information from the cosmic microwave background

Each new observable, Grant said, provides different ways to confirm the theory of general relativity and offers insight into the intrinsic properties of gravitational waves. These properties, the researchers say, could help extract information from the cosmic microwave background — the radiation left by the Big Bang at the cosmic strings — of theoretical, as yet undetected objects, which are long, extremely thin objects that carry ground and electric currents. . Previously, theorists predicted that cosmic strings, if they exist, would migrate to the centers of galaxies. If the string gets close enough to the central black hole, it can be captured once part of the string crosses the event horizon.

Have astronomers detected a cosmic string since the dawn of the universe?

“Gravitational waves from cosmic strings have a very different spectrum from astrophysical sources such as black hole mergers. It is entirely plausible that we are completely convinced that the source is indeed cosmic strings,” says Kazunori Kohri, associate professor at the High Energy Accelerator Research Organization Theory Center in Japan.

“We had not anticipated the richness and diversity of what could be observed,” said Éanna Flanagan, professor and holder of the Edward L. Nichols Chair in Physics and Professor of Astronomy.

This computer simulation shows the collision of two black holes, an extremely powerful event first detected by the Laser Interferometer Gravitational-Wave Observatory, which detected gravitational waves as the black holes approached, collided and collided. have merged. This simulation shows what the meltdown event would look like if humanity could somehow travel to get a closer look. It was created by the Cornell-based SXS Project (Extreme Space-Time Simulation).

“What surprised me about this research was how different ideas sometimes came together in unexpected ways,” Grant said. “We looked at a wide variety of different observables and found that often to know one you had to understand the other.”

The three observables

The researchers identified three observables that show the effects of gravitational waves in a flat region of spacetime that experiences a burst of gravitational waves, after which it reverts to a flat region. The first observable, the “curve deviation”, is the distance between two accelerating observers, compared to how observers with the same accelerations would separate from each other in flat space undisturbed by a wave gravitational.

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

“First Second After” – Gravitational waves reveal the secrets of the Big Bang

The third examines how gravitational waves affect the relative motion of two particles when one of the particles has intrinsic spin.

Each of these observables is defined by the researchers in a way that could be measured by a detector. The procedures for detecting deviation from the curve and spinning particles are “relatively simple to perform”, the researchers wrote, requiring only “a means to measure the separation and for observers to keep track of their accelerations. respectively”.

Detecting the observable holonomy would be more difficult, they wrote, “requiring two observers to measure the local curvature of spacetime (potentially carrying small gravitational wave detectors themselves).” Given the size needed for LIGO to detect even a single gravitational wave, the ability to detect holonomy observables is beyond the reach of current science, the researchers say.

“But we have already seen a lot of exciting things with gravitational waves, and we will see a lot more. There are even plans to put a gravitational wave detector in space that would be sensitive to sources other than LIGO,” Flanagan said.

Avi Sporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via Cornell University. and UC Berkeley. Avi was previously a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL).

Image Credit: LIGO scientists detected a third gravitational wave after two black holes merged, forming a new, larger black hole. LIGO/A. Simonnet

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