In 1916, Albert Einstein’s general theory of relativity predicted ripples in the fabric of space known as gravitational waves. Just as accelerating electrons create electromagnetic waves, some of which we can see as visible light, gravitational waves are created by accelerating concentrations of matter like black holes.
The existence of such ripples in space was first confirmed in 2016 by several large physics experiments, for which researchers were awarded a Nobel Prize the following year. In mid-January 2021, a team of U.S. researchers announced the first hints of a new detection of gravitational waves using the remnants of exploded stars that are sprinkled across the galaxy.
Stars that are much bigger than the sun end their lives in supernova explosions. For very massive stars, the only thing that remains after these spectacular events is a black hole, a concentration of matter so dense that not even light can escape its gravity. For slightly smaller stars, the remnant of the explosion can be a pulsar, an object with more mass than the sun packed into a volume comparable to Manhattan. Pulsars spin as fast as a kitchen blender, and emit radio waves in a beam that sweeps around the galaxy like a cosmic lighthouse. In many cases, the radio pulses received at Earth are as regular as the most stable atomic clocks, so pulsars can be used as a sort of GPS for the solar system.
When two black holes are orbiting each other, they slowly lose energy to gravitational waves and eventually merge. At the moment of the merger, they produce a burst of gravitational waves in a fraction of a second, sending ripples out into the fabric of space like a pebble tossed into a pond. Galaxies can also orbit each other, and if each of them has a monster black hole at their center they will also eventually merge. For merging galaxies, the process takes much longer to play out. The resulting gravitational waves are more like swells in the ocean than ripples on a pond, and these gravitational swells might take years to pass through our neighborhood.
“What is really happening is that space is getting stretched and squeezed along different axes. So the pulsars in one part of the sky have their pulses arrive a little bit sooner than we expect, while the pulsars in another part of the sky arrive a little bit later than we expect,” says Joseph Simon, a postdoctoral researcher at CU Boulder.
Simon grew up in rural Pennsylvania, and went to graduate school in Wisconsin before landing a postdoctoral position at the Jet Propulsion Laboratory (JPL) in California. He was hired to help interpret observations from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a large collaboration funded by the National Science Foundation. In 2005, the NANOGrav team started making regular measurements of pulsars all around the sky using some of the most sensitive radio telescopes in the world. The focus of Simon’s work at JPL was to try and identify the background of gravitational waves from monster black holes at the centers of merging galaxies, the swells that take years to pass by.
“I’ve been leading this project on behalf of the entire collaboration for the better part of the last three years, but this result is the culmination of over a decade of work by over a hundred scientists at all levels from undergrads to emeritus faculty,” he says.
Simon came to Boulder last August to start a postdoctoral position with the local experts in galaxy mergers. At a recent online meeting of the American Astronomical Society, he presented the first results from his analysis of NANOGrav pulsar measurements. The idea was to look for tiny changes in the arrival times of radio pulses from pulsars all around the sky. Due to the warping of space by a slowly passing gravitational wave, the pulses were expected to arrive ahead of schedule in some directions and behind schedule in others. From the first 12.5 years of observations, Simon and the NANOGrav team did find significant evidence of the expected changes in pulsar timing. However, it will take at least a few more years of measurements to be able to conclude definitively that the changes are caused by gravitational waves.
Unfortunately, the work of the NANOGrav team took an unexpected hit in early December when the Arecibo observatory in Puerto Rico collapsed. The giant radio dish, which had been built into a natural sinkhole in the 1960s, suffered major damage earlier in the year. It was already scheduled to be dismantled for safety, but before the job could be finished some cables snapped and the instrument platform crashed down onto the dish below.
“The science legacy of the Arecibo observatory is really great, and we are incredibly saddened by the collapse,” Simon says. “Luckily, we have been able to continue using the (West Virginia) Green Bank Telescope (GBT), and in the near future we hope to be able to increase the amount of time that we use on the GBT to at least partially compensate for Arecibo’s loss.”
It’s wild to think that the Earth is constantly drifting through ripples and swells in the fabric of space, created by merging black holes across the galaxy and throughout the universe. Several teams around the world have been making measurements similar to the NANOGrav team, so it may be just a few years before researchers can announce a new confirmation of Einstein’s predictions.
Travis Metcalfe, Ph.D., is a researcher and science communicator based in Boulder. The Lab Notes series is made possible in part by a research grant from the National Science Foundation.