If what little we know about gravitational waves is correct, there should be something known as a gravitational wave background, a kind of resonant “buzz” that pervades the entire universe.
A team of scientists has announced the detection of an intriguing low-frequency signal that could be attributable to gravitational waves. However, the researchers are not yet ready to say they have found evidence of a gravitational wave background.
The study, conducted by scientists at the North American Nanohertz Gravitational-Wave Observatory (NANOGrav), was published in The Astrophysical Journal Letters.
Gravitational waves were first theorized by Albert Einstein in 1916, but were not directly detected until nearly a century later. Einstein showed that instead of being a rigid backdrop for the universe, space is a flexible fabric that is warped and curved by massive objects and inextricably linked with time.
In 2015, a collaboration between the Laser Interferometric Gravitational-Wave Observatory (LIGO) and the Virgo interferometer in Europe announced the first direct detection of gravitational waves. These emanated from two black holes, each with a mass about 30 times greater than the Sun, which were circling each other and merging.
Clearly, it is not the only merger that has occurred in the entire universe. Imagine each pair of colliding black holes or neutron stars producing gravitational waves that spread throughout space-time.
After all these years, the waves produced should now be faint and hard to find, but they are thought to form a resonant “buzz” that permeates the entire universe. This is what we call the gravitational wave background and it is what astronomers think they have detected when studying a group of pulsars.
gravitational waves in pulsars
Pulsars are dense structures left behind after a star explodes as a supernova. From Earth, pulsars appear to flicker. The light actually comes from two fixed beams emanating from opposite sides of the pulsar as it spins, like a lighthouse.
These objects send pulses of radio waves at very precise intervals towards our planet. This regularity makes them especially useful in astronomical study. They are often referred to as the stopwatches of the universe.
“The (gravitational wave) background stretches and shrinks the space-time between the pulsars and Earth, causing the signals from the pulsars to arrive a little later (stretch) or earlier (shrink) than they would if there would be no gravitational waves,” astrophysicist Ryan Shannon of Swinburne University of Technology explained to ScienceAlert.
However, as exciting as the signal is, the researchers aren’t quite ready yet to say they’ve found evidence of a gravitational-wave background. Why? To confirm the direct detection of a gravitational wave signature, scientists will have to find a distinctive pattern in the signals between individual pulsars. At this point, the signal is too weak for such a pattern to be distinguishable, according to the researchers.
From now on, the researchers will have to expand their data set to include more pulsars studied over even longer periods. Combining the NANOGrav data with other experiments could help reveal an interesting pattern.
At the same time, astronomers are developing techniques to ensure that the detected signal cannot be from another source. They are producing computer simulations that help test whether the detected noise could be caused by effects other than gravitational waves, to avoid a false detection.
“However, because the gravitational-wave signal we’re looking for spans the entire duration of our observations, we need to carefully understand our noise. This leaves us in a very interesting place, where we can strongly rule out some known noise sources, but still can’t tell if the signal is actually gravitational waves. For that, we will need more data.”