Could we have detected the index of a cosmological background of gravitational waves?

A signal has been spotted in a dataset of pulsar observations spanning almost 13 years. It could be the index of the existence of a “bottom” composed of gravitational waves.

A strange low-frequency signal has been detected which could be attributed to gravitational waves. Could this be a first indication of the existence of a cosmological background made up of gravitational waves? We can not yet say, but it is certain that the detection of such radiation would have a significant impact in cosmology and physics.

The signal was spotted as part of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a consortium of astronomers trying to detect gravitational waves from various observatories, such as Green Bank (West Virginia) and Arecibo (Puerto Rico). On January 11, 2021, the consortium presented its results, also published on December 24 in The Astrophysical Journal Letters (and pre-published on arXiv). The work looks at almost 13 years of observations (between 2004 and 2017) of distant pulsars, dense stars that spin quickly and emit periodic signals.

What is a gravitational wave?

The gravitational waves move in the same way as waves on the surface of water into which a stone would have been thrown. Their origin is a very intense cosmic event, like a collision of black holes, for example. As this event occurs very far from our planet, the energy transported to Earth is very low, which is part of the reason why it is so difficult to detect them. Gravitational waves propagate at the speed of light and distort space-time.

It is these tiny variations in the position of objects that gravitational wave observatories seek to measure. Hence this interest in pulsars (the most stable known), because the passage of gravitational waves can disrupt the regularity of the signals emitted by pulsars as they are detected on Earth.

A stochastic background of gravitational waves

Here, scientists are not interested in a precise source of gravitational waves, but in a “background” of gravitational waves: one could imagine a kind of background, formed by gravitational waves emitted by a source overlays (astrophysical or cosmological). Scientific collaboration LIGO speaks of ” stochastic gravitational waves ».

We could therefore consider that there is a cosmological background of gravitational waves, which would be the relics of the first evolution of the Universe. This stochastic background of gravitational waves of cosmological origin could have been produced in the first moments of the Universe, after the Big Bang. We can make an analogy with the cosmological diffuse background, also called fossil radiation (the oldest light still present in the Universe). As LIGO summarizes, the cosmological background of gravitational waves must be the result of a combination of random and independent events that emitted gravitational waves.

The Big Bang could have been the source of these events producing ” stochastic gravitational waves »: If this is the case, the waves have probably stretched due to the expansion of the Universe (the Universe expands), which would make them difficult to detect. These (primordial) waves would then be a formidable means of knowing more about the Universe just seconds after the Big Bang – the cosmological diffuse background is dated 300,000 years after the Big Bang.

It is certainly still too early to say that the signal that has been discovered does indeed correspond to a stochastic background of gravitational waves (and whether it is of astrophysical or cosmological origin): as scientists write, it will have to be verified with larger datasets, including more pulsars. It is also necessary to rule out any possibility that the signal could be produced by effects other than gravitational waves.

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