This summer, an international team of astronomers, including many faculty and students here at West Virginia University, announced that they had detected evidence for the gravitational wave background, made of ripples in spacetime. WVU belongs to a part of the international collaboration, called NANOGrav, or the North American Nanohertz Observatory for Gravitational Waves.
Massive objects, like black holes, curve spacetime, and the movement of those objects cause gravitational waves. These waves were first theorized in the early 1900s after Albert Einstein published his theories of relativity. However, they were not detected until 2015, when the Laser Interferometer Gravitational- Wave Observatory, LIGO, made its first detection.
Image Credit: NANOGrav
The gravitational waves detected by LIGO are not the gravitational wave background, though. They are much higher in frequency than the waves in the new detection by NANOGrav and their international partners. The new detection was made using a pulsar timing array, or PTA.
Pulsars are the remains of dead stars. These stars were much more massive than the Sun, and the pulsars they’ve left are about the mass of the Sun, but compressed into a much smaller volume. The average pulsar is about as big across as the continental United States. Pulsars have beams of radiation coming from their magnetic poles. As they spin, these beams pass over Earth. Radio telescopes pick up these signals, which are incredibly regular. In fact, pulsars are better clocks than anything on Earth.
Because of the extreme regularity of pulsars, their signals are incredibly sensitive to gravitational waves. When a gravitational wave passes through the space between Earth and a pulsar, that signal is delayed by a very small amount. Astronomers can measure and classify this signal. By timing the pulses of dozens of pulsars, collaborations like NANOGrav can see the effects of gravitational waves across space.
However, these signals are relatively faint and difficult to detect. NANOGrav and their international partners have been monitoring the pulsars in their arrays for over fifteen years. It took that much data before the signal was clear enough to see.
The new dataset shows clear evidence of a background of gravitational waves that comes from massive objects in other galaxies moving through spacetime. In time, astronomers should be able to locate some of those objects, but no single source is strong enough to rise above the background yet. Stay tuned!
For more information: https://nanograv.org/, https://gwac.wvu.edu/, Announcement