“As long as they live for long enough, they will always become large cosmological beasts,” says Ricardo Ferreira, a cosmologist at the University of Coimbra in Portugal. He’s not talking about actual beasts but rather about hypothetical humongous sheets of spacetime that could divide one region of the universe from another. Such so-called domain walls are the natural outcome of theories that try to solve some of the deepest mysteries in physics, such as the origins of gravity. As Ferreira says, however, had they formed after the big bang, by today they’d be the dominant source of energy in our universe, and there’s no evidence that’s the case. So any theory invoking their existence has been considered suspect—until now, perhaps.
In a theoretical study that was recently posted on the preprint server arXiv.org, Ferreira and his colleagues have shown that if these domain walls formed, grew and then mostly annihilated in fractions of a second after the big bang—thus accounting for their absence in today’s universe—they would have created a random, cosmos-suffusing background of ripples in spacetime. And given tentative claims from multiple collaborating groups of astronomers, such a background “hum” of stochastic gravitational waves may have already been detected. According to the theoretical study, the few domain walls that did not annihilate would now appear as black holes of about one solar mass. In some scenarios, these black holes would be even smaller and would be numerous enough to be dark matter—the unseen stuff that’s thought to make up about a quarter of the universe.
“The authors have done quite a thorough analysis,” says theoretical physicist Tanmay Vachaspati of Arizona State University, who was not involved in the study. “The idea is fascinating in that it simultaneously addresses the stochastic gravitational-wave background observations and raises the possibility that there is an abundance of small black holes.”
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