Gravitational waves may leave behind detectable traces

Gravitational waves leave behind tell-tale signatures which new research suggests we may be able to trace in order to test the predictions of general relativity and learn more about the Universe.

Gravitational waves — first detected by LIGO in 2016 — offer a new window on the Universe with the potential to tell us about everything from the time following the Big Bang to more recent events in galaxy centres.

As 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 may leave behind plenty of ‘memories’ that could help detect them even after they’ve passed.

Alexander Grant, a doctoral candidate and lead author of the study, says: “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.”

Numerical simulation of two inspiralling black holes that merge to form a new black hole. Shown are the black hole horizons, the strong gravitational field surrounding the black holes, and the gravitational waves produced ( S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project, W. Benger (Airborne Hydro Mapping GmbH)).

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.

Grant points outs that each new observable provides different ways of confirming the theory of general relativity and offers insight into the intrinsic properties of gravitational waves.

Properties that could help extract information from the Cosmic Microwave Background (CMB) — the radiation left over from the Big Bang.

“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.

Grant says: “What was surprising for me about this research is how different ideas were sometimes unexpectedly related.

“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.”

Gravitational waves as emitted during black hole merger. ( S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project, W. Benger (Airborne Hydro Mapping GmbH)).

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 — defines 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, undisturbed space.

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 — then 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.

Gravitational waves as emitted during black hole merger. ( S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project, W. Benger (Airborne Hydro Mapping GmbH)).

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,” the researchers say — requiring only measuring separation whilst the observers keep track of their respective accelerations.

Detecting the holonomy observable may be more of a challenge — requiring two observers to measure the local curvature of spacetime. This could mean observers potentially by carrying around small gravitational wave detectors themselves. Impractical to say the least given the size of the laser arms needed for LIGO to detect even one gravitational wave. Thus, say the team, the ability to detect holonomy observables is well beyond the reach of current science.

This doesn’t mean the researchers aren’t hopeful though. As Flanagan points out the future is bright for gravitational wave detection: “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.”

Original research: “Persistent Gravitational Wave Observables: General Framework,” published April 26 in Physical Review D.