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We have discovered the CMB, Cosmic Microwave Background, in 1965. It's the oldest light that we can see and it came from 3,80,000 years after the Big Bang. It has been proposed that there must be a Gravitational Wave Background (GWB) too.

But on what grounds it is proposed ? There were no massive binary mergers at that time. What could have caused this GWB ?

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It's actually the same mechanism that's ultimately responsible for the temperature fluctuations in the CMB.

Quantum fluctuations during inflation get stretched outside the horizon, and "freeze" into a classical state which we later observe as background radiation.

The CMB temperature fluctuations are thought to be fluctuations in density, which arise from over and under densities in the gravitational field and in matter from this process.

The GWB would come from gravitational waves produced in this process.

Some references

[1] https://arxiv.org/abs/0907.5424

[2] https://arxiv.org/abs/1801.04268

[3] https://arxiv.org/abs/1605.01615

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  • $\begingroup$ Ok, got it. Thanks $\endgroup$
    – Shriesh Kumar
    Commented Aug 30, 2021 at 7:17

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