According to Einstein’s theory of general relativity, gravitational waves warp spacetime. In the process, they could imprint permanent changes, or “gravitational memory,” on their surroundings.
Researchers now think that gravitational memory could be written on photons all over the cosmic microwave background—the oldest radiation in the universe.
Though gravitational memory is thought to be too subtle to be detected even by our most sensitive equipment, upcoming instruments might finally be able to pick up on a signal.
If two black holes crashed into each other billions of years ago, even though we weren’t around to observe, could we find out?
Well, as it turns out, maybe we could. Einstein’s theory of general relativity describe how the gravity of massive objects and extreme phenomena—such as black hole mergers and core-collapse supernovae (which sometimes end up as black holes)—causes ripples that permanently warp spacetime and traverse the void at the speed of light. These ripples are known as gravitational waves, and the effects of these waves thought to be permanently imprinted on their surroundings as “gravitational memory.”
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Evidence of gravitational memory, however, continues to elude telescopes. Even the Laser Interferometer Gravitational Wave Observatory (LIGO)—which first detected gravitational waves in 2015—has not been able to pick up a memory signal. Fast-forward a decade, and a team of researchers from the Niels Bohr Institute in Denmark and the University of Valencia in Spain now have an idea of where we could search to finally find these mysterious gravitational memory imprints, which they described in a study uploaded to the preprint server arXiv.
Cataclysmic events leave their mark on the cosmic microwave background (CMB)—the remnant of the immense shockwave sent through space by the Big Bang and the oldest detectable radiation in the universe. The researchers behind this new paper think that the CMB is probably embedded with “memories” of black hole mergers as a result of the gravitational waves that resulted from those mergers leaving behind temperature changes in CMB radiation.
“Photons traveling through space may experience permanent distortions and deflection caused by the gravitational wave memory from one such merger event,” the team said in the paper. “[There is] a resulting change in photon wavelength.”
Gravitational waves interacting with particles of light, or photons, can shift their direction, velocity, or angular momentum. As a result, photons affected by those permanent changes are essentially taking gravitational memory with them as they travel. If we were to somehow detect the changes made to the photons, we could analyze these effects and find out what kinds of events caused them.
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Gravitational memory could reveal such things as distances to merging black holes, masses, and the forces of collisions. It could also illuminate more about how the early universe evolved. In the case of a core-collapse supernova, the death of a massive star that has burned all its energy and collapses in a violent explosion, gravitational memory could give us insight into properties no telescope or spacecraft can observe.
Because oscillations of waves from gravitational memory are predicted to have much smaller amplitudes than the gravitational waves they come from—never mind the additional noise from human activity on Earth—they have not yet been detected. Even the most hypersensitive equipment is still not sensitive enough. NASA’s upcoming LISA (Laser Interferometer Space Antenna) observatory might be our best shot at finding evidence.
“Though hidden below a myriad of other signals,” the researchers wrote, “the entire merger history of black holes is marked on [the CMB, which is] the oldest image of our universe.”
Lettermark
Elizabeth Rayne is a creature who writes. Her work has appeared in Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.