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.
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.
“People have been making predictions about cosmic strings and primordial black holes for years, and now, finally, we have a signal,” says Chiara Mingarelli, an astrophysicist at the University of Connecticut and a member of the NANOGrav team. “We’re not sure what is generating this signal, but a lot of people are really, really excited.”
The physics community has learned a great deal about the universe from massive terrestrial gravitational wave experiments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European counterpart Virgo. But just as electromagnetic waves come in a spectrum ranging from squashed gamma-rays to lengthy radio waves, gravitational waves run the gamut from the tiny vibrations in spacetime made when sun-size black holes merge to those with wavelengths measurable in light-years that can take decades to pass by our planet. The collective, overlapping cacophonies from those larger waves, thought to be produced when behemoth black holes lurking in the centers of galaxies collide, are what the NANOGrav collaboration has been working to capture.
It does so by focusing on objects known as millisecond pulsars, which arise when massive stars explode as supernovae and leave behind their rapidly spinning remnant hearts. A pulsar’s strong magnetic field can create a beam of radiation that swings around, repeatedly sweeping past Earth with a regularity that rivals the accuracy of atomic clocks. Should a distortion in the fabric of spacetime come between our planet and a pulsar, it can cause this signal to arrive slightly earlier or later than expected. Were a telescope to see one such offset, it probably would not mean much. But NANOGrav has been monitoring the light from 45 pulsars scattered over thousands of light-years for more than 12.5 years, looking for correlations between their arrival times that could indicate the presence of gravitational waves.
Last September, the collaboration posted a paper on the preprint server arXiv.org, which hosts scientific articles that have yet to go through peer review, showing that its monitored pulsars all displayed similar blips. (The paper has since been peer-reviewed and published.) The chances of this happening are between 1,000 and 10,000 to one, say Mingarelli. As a group, NANOGrav is cautious and has refrained from claiming it has seen a gravitational-wave signal, which requires observing highly specific correlations among its pulsar signals’ arrival times. That did not stop other scientists from jumping on the data.
Marek Lewicki, a theoretical physicist at the University of Warsaw in Poland, recalls that the NANOGrav study appeared early on a Friday morning and that, by 10 A.M., his collaborator John Ellis of King’s College London had spotted it. Though the usual explanation for such a signal is the supermassive black hole gravitational-wave background, Lewicki knew that another possible culprit was cosmic strings, and he began running models to see if this option could account for the data. “By Saturday, it was pretty clear it was a good fit,” he says.
Researchers like cosmic strings because they directly connect cosmological events to high-energy particle physics. Shortly after the big bang, three of the four known forces—electromagnetism and the strong and weak nuclear forces—would have been smushed together into one superforce. When the strong nuclear force dissociated itself, the universe would have gone through what is known as a phase change, much like water freezing into ice. And just as a frozen lake often contains long cracks created when its bulk solidifies, the visible cosmos would become strewn with enormous nearly-one-dimensional tubes of energy crisscrossing its length. Such objects would be tense like piano strings and could vibrate out gravitational waves that would look like the signal NANOGrav had picked up.
Because these cosmic strings originated near the beginning of time, they would carry information about processes such as cosmic inflation, during which the universe is thought to have rapidly ballooned by mind-boggling factors, as well as the creation of different particles at different extreme temperatures, says Kai Schmitz, a theoretical physicist at CERN near Geneva. Information from such conditions, which would be impossible to create in particle accelerators such as the Large Hadron Collider, could help researchers produce a grand unified theory connecting most known particles and forces that would supersede the current Standard Model. Along with two collaborators, Schmitz published a paper in Physical Review Letters (PRL) outlining how cosmic strings could account for the NANOGrav data on January 28, the same day a similar article by Lewicki and Ellis appeared.
“If we detected cosmic strings, it would be the detection of my lifetime,” says Eugene Lim, a cosmologist also at King’s College London. “It would be more important than the Higgs boson, probably more than gravitational waves themselves.”
For this reason, Lim, who was not a co-author on either paper, stresses that such concepts need to be considered with an abundance of restraint. 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, he adds.
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.
“This is an economical explanation,” says Antonio Riotto, an astroparticle cosmologist at the University of Geneva, because they do not require theorizing about exotic undetected particles such as WIMPs or axions, which have thus far dominated physicists’ musings about dark matter.
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.
Yet this possibility, too, needs to be approached carefully, says Juan García-Bellido, a theoretical physicist at the Autonomous University of Madrid in Spain, who was not involved in the work. While the NANOGrav data contain hints, it does not quite show the specific correlated pattern that would indicate gravitational waves, and much of the speculation seems premature to him. “I’m the first to hope for primordial black holes,” he says. “But I’m afraid it’s not yet there.”
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. They are also working with international collaborators such as the European Pulsar Timing Array (EPTA) and Parkes Pulsar Timing Array (PPTA) in Australia, each of which has observations of other pulsars that could get them closer to spotting the needed correlations to finally pin down the gravitational-wave background—a process that should be underway before the end of this year.
“I would be shocked if we didn’t see a signal when we combined all of our data,” Mingarelli says.