A West Virginia University professor and the Green Bank Telescope have played major roles in a 15-year effort producing evidence of low-frequency gravitational waves pushing and pulling on the universe.
Essentially, over many years the study focused on how gravitational forces in space affected pulsars. It’s a study of the wibbly wobbly, timey wimey stuff of the universe.
Einstein’s theory of general relativity predicts precisely how gravitational waves should affect pulsar signals. By stretching and squeezing the fabric of space, gravitational waves affect the timing of each pulse in a small but predictable way, delaying some while advancing others.
These shifts are correlated for all pairs of pulsars in a way that depends on how far apart the two stars appear in the sky.
One of the researchers testing that is Dr. Maura McLaughlin, Eberly Family distinguished professor of physics and astronomy and director of the WVU Center for Gravitational Waves and Cosmology.
She made clear that the long period of research has provided “significant evidence,” but is not definitive. Much more work is still to come.
“We’re not reporting a detection,” McLaughlin said during a press event this week. “We’re being very careful with our language, and we are calling this evidence for gravitational waves.”
McLaughlin has been a part of an international collaboration dedicated to exploring the low-frequency gravitational wave universe through radio pulsar timing.
“Pulsars are actually very faint radio sources, so we require thousands of hours a year on the world’s largest telescopes to carry out this experiment,” McLaughlin said.
The group’s findings are being published today in the Astrophysical Journal Letters.
The 15 years of observations came from through the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico.
The millisecond pulsars, the focus of the research, are remnants of extinguished massive stars. As they spin hundreds of times a second, their “lighthouse-like” radio beams are seen as highly regular pulses.
The gravitational waves stretch and squeeze space and time in a characteristic pattern, causing changes in the intervals between the pulses that are correlated across all the pulsars being observed.
The correlated changes are the specific signal being detected by the North American Nanohertz Observatory for Gravitational Waves, which was founded in 2007. NANOGrav’s most recent dataset offers compelling evidence for gravitational waves with oscillations of years to decades.
The waves being monitored by the observatory are thought to arise from orbiting pairs of the most massive black holes throughout the universe: billions of times more massive than the sun, with sizes larger than the distance between the sun and earth.
Future studies of the signals will allow researchers to view the gravitational-wave universe through a new window, providing insight into titanic black holes merging in the hearts of distant galaxies and potentially other exotic sources of low-frequency gravitational waves.
“We’re going to become even more sensitive if we can add more pulsars to the array and, most importantly, if we can gain access to even larger new radio telescopes,” McLaughlin said.
Additional research will “tell us an awful lot about how galaxies have merged and evolved over cosmic time. Like, we think you know, most galaxies are the products and mergers of smaller galaxies. what are the time scales? What is the astrophysics involved, you know it’ll be able to answer lots of these questions, which is really exciting.”
McLaughlin was one of the first researchers at the start of the collaborative effort, NANOGrav. “We were quite small,” she said, “about a dozen scientists. We’ve grown a lot since then. We now number 194 members at over 80 institutions throughout the United States, Canada, and 12 other countries.”
She said the volume of research has been building slowly, recently growing to the point that scientists could draw conclusions.
“In our last data set, we saw this source of common noise that we were pretty sure was gravitational waves, but we didn’t quite see the spatial correlations,” she said. “And then we predicted that we should see this in the next data set, you know, if the spatial correlations were there.”
About three years ago, that evidence became clearer in a preliminary version of the data.
“It was kind of a weird time, because it was during Covid,” she said. “So everyone was just sort of like home on zoom and I’m embarrassed to say I don’t remember like where I was sitting or what I was doing at the like exact moment. when I saw it, but I do remember being just completely awe-struck.
“Even though we knew that it should be in these data based on the statistics of the last detection and the additional sensitivity that we knew we had — we knew it should be there. Seeing those points like match that that curve for the first time was really a magical moment, honestly. It just it just was so special. And it’s the result of so much work on the part of so so many people. so it was just just really, really wonderful to see.”