摘要：After decades of searching, astronomers have found a distinctive pattern of light, from spinning stars called pulsars, that suggests huge gravitational waves are creating gentle ripples in space-time across the universe
Though this explanation makes the most sense, when Blecha and her colleagues modelled a gravitational wave background caused by merging supermassive black holes across the universe, they found a slightly different signal to that of NANOGrav, suggesting that these cosmic behemoths are either more massive or more common in the universe than previously thought. If true, this could change our understanding of both galaxy formation and how the universe is structured on large scales.
One way to shore up the supermassive black hole explanation would be to see a gravitational wave background signal growing in strength in a specific portion of the sky, which might be caused by a nearby merger. Australia’s PPTA is seeing hints of this in its analysis, but it is still too early to tell.
There is enough uncertainty in the NANOGrav signal that the door is open for alternative explanations, says Nelson Christensen at Carleton College in Minnesota. “We’re going to have hundreds of papers from theorists in the coming days where they’re going to be presenting other models.”
One possibility is that the background waves come from defects in the very early universe as it changed phases. The idea is that this left an imprint in space-time, like the cracks that form when water freezes into ice. Another is that the background in fact comprises long-theorised primordial gravitational waves, produced by the universe rapidly expanding shortly after the big bang during a period known as cosmic inflation.
Nothing ruled out
However, the data isn’t currently anywhere near precise enough to rule out one scenario or the other, says Pedro Ferreira at the University of Oxford. “The problem with this topic is, yes, it could be any number of types of new physics, but you can’t really distinguish between them.”
To solve that, we need more data. Recently built telescopes like FAST in China and MeerKAT in South Africa, as well as the Square Kilometre Array, the world’s largest telescope that is under construction in Australia and South Africa, will allow us to measure the pulsars more often and with much greater precision. Discovering new and more regular pulsars will also help, says McKee.
Combining the datasets of all the various PTAs in a global collaboration, too, will allow for a more detailed analysis. There are some pulsars that only the Australian telescopes can see, and vice versa for the European ones. An analysis combining all of the results is already under way, says DeCesar, and should be released in the coming years.
“This is a golden era for gravitational waves,” says Christensen. “Within about eight years, not only have we detected gravitational waves on the ground, but now we’ve detected them with a completely other method at a very different frequency — this is just super exciting.”