Colliding Stars, Black Holes And Gravitational Waves: How Astronomers Could Change Physics in 2018

Updated | 2017 was a big year for gravitational wave science, which won this year's Nobel Prize and caught four new detections, including the first-ever detection of two merging neutron stars. But there are still signals astrophysicists believe should exist but that the American Laser Interferometer Gravitational-Wave Observatory, or LIGO, detectors and their European counterpart Virgo have yet to detect.

"I think probably the most exciting thing would be a black hole-neutron star [collision]," Raffaella Margutti, an astrophysicist at Northwestern University, told Newsweek. That's the one type of stellar merger scientists have yet to spot with gravitational waves.

So far, LIGO has detected five black hole mergers, of the densest celestial bodies that exist and that are undetectable except through gravitational waves, and one neutron star merger, of smaller, slightly less dense stars, which produced an accompanying burst of light called a kilonova. But black holes and neutron stars should also be perfectly capable of colliding with each other. "They should be there in nature, so sooner or later we should find one," Margutti added.

Predicting what precisely that signal will look like is still tricky business. "It's going to be a chirp like the other two—so that is to say, something that sweeps up in frequency and gets stronger as it sweeps up," said Peter Saulson, a physicist at Syracuse University who works on the LIGO detectors. How fast the signal sweeps through LIGO's listening range will tell astrophysicists about the masses involved: LIGO registered the first black hole merger for a little more than a tenth of a second and the first neutron star for a little more than a minute, due to the 20-fold difference in masses involved.

Scientists' theories about a neutron star merger offer an optimistic evaluation of our predictions, Margutti said. "The predictions were actually very, very accurate in that case, and that's amazing considering the level of uncertainty that was in the models." Unfortunately, astrophysicists haven't spent as much effort on modeling what a mixed merger might look like, in part because they're predicted to be a rarer occurrence.

Read more: Timing Was Everything for Discovery of Gold-Producing Neutron Star Collision 130 Million Light-Years Away

Margutti adds that there should be some form of light signal associated with a mixed black hole-neutron star merger, just as there was for this year's binary neutron star detection. "That said, I would not trust any prediction that we have—it's really an open game," she said. "I think that we will learn a lot from the first that we will see."

There's no way to predict when that first observation will come, since that depends on just how many black hole-neutron star pairs there are dancing around each other throughout the universe. And understanding that tally is one of the reasons astrophysicists want to start spotting these mergers. "There's definitely guesses out there, and we just don't know whose estimate is right," Saulson said. "Now that we're in the business of finding things, we can say, well, soon we'll know."

An artist's rendering of the collision of two neutron stars. Carnegie Institution for Science

But whether or not that moment comes in 2018, there are a few other signals gravitational wave detectors could pick up next year as well. "Every single time you deform spacetime, you produce gravitational waves," Margutti said. In order for us to hear them here on Earth, that deformation needs to be asymmetrical, which limits our probability of catching any given signal with current technology.

Those signals would be very different from the chirps of mergers, however. One possibility is a steady tone from special type of neutron star called a pulsar, Saulson said, or a hiss that originated in the universe's very first moments. ("Do people know what static is anymore?," he joked to explain what that background hiss would sound like.) We could even eavesdrop on a supernova, a giant explosion as a distant star dies, assuming we get lucky about its precise characteristics. "That would be a really brief blip, even shorter than the chirps from black holes, but the details of what that signal would look like are very difficult to calculate," Saulson said. "The physics is a lot more complicated."

Right now, the LIGO detectors aren't hearing anything, turned off for a year of technical upgrades that astrophysicists hope will mean the instruments will be ready to pick up even more signals. They'll resume observations in the fall of 2018, with all eyes on what new phenomena they may spot.

This story has been updated to remove an inaccurate characterization of scientists' predictions of what black hole binary and mixed mergers would sound like.