Every massive event – every black hole or neutron star merger, every supernova – should have sent gravitational waves ringing out across space-time.
Now, from the International Pulsar Timing Array (IPTA) consortium, tentative new evidence that we may have detected the hum has emerged. If confirmed, this will be a very big deal indeed.
“Although we do not have definitive evidence yet, we may be beginning to detect a background of gravitational waves.”
“This is a very exciting signal!” says astrophysicist Siyuan Chen of the Observatory of Paris and CNRS in France.
The combined effect of all these waves would be to create a faint, background hum that permeates the entire Universe. This gravitational wave background is predicted to be weak and very hard to detect. Nevertheless, a year ago, scientists with the international NanoGRAV collaboration said they may have done just that.
As we explained last year, the signal comes from observations of a type of dead star called a pulsar. These are neutron stars that are oriented in such a way that they flash beams of radio waves from their poles as they rotate at millisecond speeds comparable to a kitchen blender.
These flashes are incredibly precisely timed, which means that pulsars are possibly the most useful stars in the Universe. Variations in their timing can be utilized for navigation, for probing the interstellar medium and studying gravity. Since the discovery of gravitational waves, astronomers have been using them to look for those, too.
That’s because gravitational waves warp space-time as they ripple through, which theoretically should change – ever-so-slightly – the timing of the radio pulses given out by pulsars as space-time between us and them stretches and contracts. A single pulsar wouldn’t be able to tell us much, but if these timing variations are observed in a number of pulsars, that could indicate the presence of gravitational waves. This is called a pulsar timing array.
The team’s dataset is based on observations of 65 millisecond pulsars, the timing of which displayed characteristics consistent with what we’d expect from a gravitational wave background. It’s not solid evidence, at least not yet. But it’s a step closer to it.
“The first hint of a gravitational wave background would be a signal like that seen in the International Pulsar Timing Array Data Release 2,” says astrophysicist Bhal Chandra Joshi of the National Centre for Radio Astrophysics in India. “Then, with more data, the signal will become more significant and will show spatial correlations, at which point we will know it is a gravitational wave background. We are very much looking forward to contributing several years of new data to the IPTA for the first time, to help achieve a gravitational wave background detection.”
What scientists really need to see is a particular signal in pairs of pulsars, the strength of which depends on their spatial distance in the sky. We haven’t seen that yet, because the signal is too weak, but the signal we have seen is what we’d expect to see first.