Theoretical physicists have leapt upon the recent discovery of a background hum of gravitational waves permeating the entire universe, with claims that the signal could be a sign of dark matter or even shed new light on the earliest moments of existence.
鈥淭here has been a lot of excitement in the high-energy physics community,鈥� says at the California Institute of Technology, who is part of the team that discovered the gravitational wave background. 鈥淭o be honest, I have had some trouble to follow all the papers that have appeared.鈥�
On 29 June, astronomers at the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) announced they had detected low-frequency ripples in space-time across the entire universe. The leading explanation for this background hum is that it comes from the merging of colossal black holes found at the centre of many galaxies.
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These ripples affect every massive object, including fast-rotating neutron stars called pulsars, which act like a near-perfectly timed galactic lighthouse. Astronomers can use these stars to find gravitational waves by looking for tiny timing differences they create in the pulsars鈥� light.
The pattern of timing differences seen by NANOGrav is a very close match to the one predicted by Albert Einstein鈥檚 general theory of relativity, which says the timing from pulsar pairs should become broadly less similar as the angle between them grows, but the strength of the overall signal also grew unexpectedly as they measured the background at higher frequencies.
鈥淲hat these pulsar timing array observations show is qualitatively consistent with what the models predicted, but not precisely with the original predictions,鈥� says at King鈥檚 College London. 鈥淭hat frequency dependence wasn鈥檛 quite what was expected.鈥�
In physical terms, it appears the supermassive black holes are producing more, or more powerful, gravitational waves as they move closer to merging, but it isn鈥檛 clear why this should happen. Ellis and his colleagues have may initially be having to plough through nearby gas and stars as they begin to merge, causing them to lose energy that would otherwise have been emitted as part of the gravitational waves. But as the black holes spiral ever closer, and the frequency of the gravitational waves increases, the gas and stars have less of an effect and more of the energy goes into the waves.
Other explanations are more exotic, like a shroud of dark matter around the merging black holes. at the University of the Basque Country and his colleagues simulated this idea using an earlier NANOGrav data set and found a close match, but they struggled to produce the high-frequency data seen in the most recent measurements. One solution might be to simulate merging black holes with highly eccentric orbits, but they haven鈥檛 done this work yet, says Broadhurst.
A further wrinkle in the data is what appears to be a small peak at the lower end of NANOGrav鈥檚 range, in what is otherwise a relatively smooth line. While it isn鈥檛 statistically significant enough for astronomers to be certain it is real, it is unexpected, says NANOGrav team member at Vanderbilt University in Tennessee.
This spike could be from an unknown nearby supermassive black hole merger emitting that particular frequency, but confirming this will require at least five more years of data-gathering, says Pol.
But if these explanations don鈥檛 bear out, theorists might need to turn towards very different sources. One possibility being looked at is cosmic strings, theorised objects that are light-years long but with a width less than that of a proton. These strings, if they exist, would have been produced when the early universe went through a dramatic phase transition, similar to the cracks that form when water freezes into ice.
Ellis and his colleagues , when these strings collide, they should produce a distinct gravitational wave signature detectable by groups like NANOGrav, but again the stronger signal at higher frequencies doesn鈥檛 quite fit the bill.
Primordial black holes, another theorised denizen of the early universe formed from clusters of subatomic matter so dense that they collapsed space-time, are also being considered as a possibility. These black holes would have produced their own unique gravitational wave signal, different from the proposed supermassive black hole mergers of today.
at the German Electron Synchrotron and his colleagues that might be produced from a population of these primordial black holes, finding that these can only match the observed NANOGrav signal if they are distributed in clumps through the early universe, rather than found uniformly across the cosmos.
鈥淚f that鈥檚 the case, then it actually does work, because they鈥檙e kind of closer together and you have a larger rate for merging,鈥� says Schmidt-Hoberg. However, it is still unclear why primordial black holes would clump in this way or exactly the kind of signal they would produce, he says.
There is also the possibility that several of these scenarios are true at once, but disentangling them will require new tools that have yet to begin operation. One such tool is the Laser Interferometer Space Antenna (LISA), a set of three spacecraft designed to detect higher frequencies than NANOGrav. At these frequencies, the predictions from cosmic strings and supermassive black holes are very different, says Ellis, but as LISA won鈥檛 launch until the late 2030s, we could be waiting for some time.
