Evidence has been growing that the Universe should be full of the waves from the black holes that were detected in 2015.
Every big event should have sent ripples across space-time.
The combined effect of all these waves would create a hum that is pervasive in the Universe. The background is weak and hard to detect. Scientists with the international NanoGRAV collaboration said a year ago that they may have done that.
There is new evidence that we may have detected the hum. This will be a big deal if it is confirmed.
"This is a very exciting signal!" says Siyuan Chen, an astronomer at the Observatory of Paris.
We may be starting to detect a background of the waves.
The signal comes from observations of a dead star called a pulsar. These are stars that are oriented in such a way that they emit radio waves from their poles as they spin at speeds comparable to a kitchen blender.
The flashes are incredibly timed, which means that pulsars are the most useful stars in the Universe.
Variations in their timing can be used for navigation, probing the interstellar medium, and studying gravity. Astronomers have been using the waves to look for other waves.
The timing of the radio waves given out by pulsars as space-time between us and them stretches and contracts should change because of the warp space-time caused by the waves.
If these timing variations are observed in a number of pulsars, that could be a sign of the existence of the waves. This is a timing array.
The team's dataset is based on observations of 65 millisecond pulsars, which showed characteristics consistent with what we'd expect from a gravitational wave background.
It's not solid evidence yet. It is a step closer to it.
The strength of the signal in pairs of pulsars depends on their spatial distance in the sky. We haven't seen that yet because the signal is too weak, but we have seen what we'd expect to see first.
The first hint of a background would be a signal seen in the International Pulsar Timing Array Data Release 2.
"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 looking forward to contributing several years of new data to the IPTA in order to help detect a wave.
Other causes of the signal need to be ruled out.
Boris Goncharov is an astronomer at the Gran Sasso Science Institute in Italy.
"For example, it could be the result of noise that is present in individual pulsars' data that may have been modeled in our analyses."
There's a lot of science to be done before we can say that the background of the wave has been detected. It's reasonable to start getting a little bit excited.
If we have detected the background of the wave, the most likely source is from the collision of some of the most massive objects in the Universe.
This means that the signal could help resolve the final parsec problem, which is a question of whether supermassive black holes will be able to merge.
The detection of waves from a black hole will give us new insights into how the universe formed and how it continues to grow.
The next few years could bring us a golden age for these explorations of the Universe, because of a major international effort of the scale of IPTA.
The team's research was published in the monthly Notices of the Royal Astronomical Society.
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