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February 6, 2021 10:00 am

Galaxy-Size Gravitational-Wave Detector Hints At Exotic Physics

The fabric of spacetime may be frothing with gigantic gravitational waves, and the possibility has sent physicists into a tizzy. A potential signal seen in the light from dead stellar cores known as pulsars has driven a flurry of theoretical papers speculating about exotic explanations. Scientific American reports: The most mundane, yet still quite sensational, possibility is that researchers working with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which uses the galaxy as a colossal gravitational-wave detector, have finally seen a long-sought background signature produced when supermassive black holes crash and merge throughout the universe. Another interpretation would have it originating from a vibrating network of high-energy cosmic strings that could provide scientists with extremely detailed information about the fundamental constituents of physical reality. A third possibility posits that the collaboration has spotted the creation of countless small black holes at the dawn of time, which could themselves account for the mysterious substance known as dark matter.[...]The NANOGrav collaboration still needs to confirm that it is in fact seeing gravitational waves. And the shape of those gravitational waves' spectrum has yet to be traced out and found to conform to the cosmic string interpretation, each of which is likely to take years. Meanwhile, another contingent of the physics community has suggested that the signal could originate from entities known as primordial black holes. Unlike regular black holes, which are born when gigantic stars die, these would form in the early universe, when matter and energy were nonuniformly scattered through the cosmos as a consequence of processes that occurred at the end of inflation. Certain overdense areas could collapse under their own weight, generating black holes in a variety of sizes. Observations from LIGO and Virgo that could indicate mergers between primordial black holes have already planted the idea in many researchers' minds that these strange objects are more than speculative fictions. Certain theorists like them because, as entities that give off no light, they could account for some or even all of the dark matter in the universe. Along with two co-authors, Riotto has written a third paper appearing in PRL showing how the NANOGrav signal could be accounted for by a multitude of black holes the size of asteroids being created shortly after the big bang, producing a gravitational wave relic that would travel to us in the modern day. According to the researchers' model, these miniature primordial black holes could comprise up to 100 percent of the dark matter in the universe. [...] Nevertheless, the burst of theoretical activity shows how seriously physicists are taking these results. NANOGrav researchers have another two and a half years of pulsar data they are combing through, which could help distinguish whether some or a combination of all these explanations might be viable.

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