While the billion-dollar Laser Interferometer Gravitational-Wave Observatory (LIGO) detector watches 24/7 for gravitational waves to pass through the Earth, new research shows those waves leave behind “memories” –a permanent displacement of spacetime that comes from strong-field, general relativistic effects–that could help detect them even after they’ve passed, creating the potential to tell us about everything from the time following the Big Bang and the creation of cosmic strings–to more recent events in galaxy centers.
“That gravitational waves can leave permanent changes to a detector after the gravitational waves have passed is one of the rather unusual predictions of general relativity,” said Alexander Grant, lead author of Persistent Gravitational Wave Observables: General Framework.
Physicists have long known that gravitational waves leave a memory on the particles along their path, and have identified five such memories. Researchers have now found three more aftereffects of the passing of a gravitational wave, “persistent gravitational wave observables” that could someday help identify waves passing through the universe.
“The recent discovery of gravitational waves opens up a new opportunity to look back further to a time, as the Universe is transparent to gravity all the way back to the beginning. When the Universe might have been a trillion to a quadrillion times hotter than the hottest place in the Universe today, neutrinos are likely to have behaved in just the way we require to ensure our survival. We demonstrated that they probably also left behind a background of detectable gravitational ripples to let us know,” says Graham White, a postdoctoral fellow at TRIUMF, about a recent paper suggesting gravitational waves could contain evidence to prove the theory that life survived the Big Bang because of a phase transition that allowed neutrino particles to reshuffle matter and anti-matter,
Extract Information from the Cosmic Microwave Background
Each new observable, Grant said, provides different ways of confirming the theory of general relativity and offers insight into the intrinsic properties of gravitational waves. Those properties, the researchers said, could help extract information from the Cosmic Microwave Background—the radiation left over from the Big Bang to cosmic strings –a theoretical, as-yet undetected objects that are long, extremely thin objects that carry mass and electric currents. Previously, theorists had predicted that cosmic strings, if they exist, would migrate to the centers of galaxies. If the string moves close enough to the central black hole it might be captured once a portion of the string crosses the event horizon.
“Gravitational wave from cosmic strings has a spectrum very different from astrophysical sources such as merger of black holes. It is quite plausible that we will be completely convinced the source is indeed cosmic strings,” says Kazunori Kohri, Associate Professor at the High Energy Accelerator Research Organization Theory Center in Japan.
“We didn’t anticipate the richness and diversity of what could be observed,” said Éanna Flanagan, the Edward L. Nichols Professor and chair of physics and professor of astronomy.
This computer simulation shows the collision of two black holes, a tremendously powerful event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, which detected gravitational waves as the black holes spiraled toward each other, collided and merged. This simulation shows what the merger event would look if humanity could somehow travel for a closer look. It was created by the Cornell-founded SXS (Simulating eXtreme Spacetimes) project.
“What was surprising for me about this research is how different ideas were sometimes unexpectedly related,” said Grant. “We considered a large variety of different observables, and found that often to know about one, you needed to have an understanding of the other.”
The Three Observables
The researchers identified three observables that show the effects of gravitational waves in a flat region in spacetime that experiences a burst of gravitational waves, after which it returns again to being a flat region. The first observable, “curve deviation,” is how much two accelerating observers separate from one another, compared to how observers with the same accelerations would separate from one another in a flat space undisturbed by a gravitational wave.
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 looks at 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 could be measured by a detector. The detection procedures for curve deviation and the spinning particles are “relatively straightforward to perform,” wrote the researchers, requiring only “a means of measuring separation and for the observers to keep track of their respective accelerations.”
Detecting the holonomy observable would be more difficult, they wrote, “requiring two observers to measure the local curvature of spacetime (potentially by carrying around small gravitational wave detectors themselves).” Given the size needed for LIGO to detect even one gravitational wave, the ability to detect holonomy observables is beyond the reach of current science, researchers say.
“But we’ve seen a lot of exciting things already 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 different sources than LIGO,” Flanagan said.
Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via Cornell University. and UC Berkeley. Avi was formerly 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 one new, larger black hole. LIGO/A. Simonnet