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It is theorized that during inflation gravitational waves are produced. See this article by Guzzetti, Bartolo, Liguori, and Matarrese (Cornell University). Or this one by Liu, Guo, Cai, and Shiu (Cornell University). Primordial gravitational waves are produced and proven to exist

Recent observation of B mode polarizations encoded in CMB anisotropy has indicated the presence of primordial gravity waves.

I'm not sure if this is caused by inflation or if they were already present before inflation started (in an eternal inflation scenario these waves had to co-exist with inflation).

My question is:
What, in theory, determines the frequency of the produced waves. One has to know the mechanism for calculating this, I guess. But what's the key feature?

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  • $\begingroup$ Recent observation of B mode polarizations encoded in CMB anisotropy has indicated the presence of primordial gravity waves. Nope. See Wikipedia regarding this fiasco. It was just cosmic dust, not primordial gravitational waves. $\endgroup$ – G. Smith Sep 5 '20 at 4:49
  • $\begingroup$ It was too good to be true, I guess. I think it's pretty sure there was no GW hanging around at the time. Why should they? $\endgroup$ – Deschele Schilder Sep 5 '20 at 5:25
  • $\begingroup$ Early grav. waves could be produced by a number of different processes, not just inflation, or they could simply be an aspect of the early universe however it came about. $\endgroup$ – Andrew Steane Sep 5 '20 at 18:59
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The claim about discovery of gravitational wave (GW) signatures in CMB were retracted, as already mentioned in the comments.

Any pre-inflationary signals would be washed out (redshifted, diluted beyond observation) by inflation.

As for the actual question, the spectrum of primordial GW is expected to be almost scale invariant, i.e. nearly equal emissions for all frequencies. But small deviation from scale-invariance is expected due to inflationary dynamics (primordial GW originate as metric fluctuations coupled to the scalar fluctuations and also to the slowly changing background of the inflaton field). There are also other sources of GW background in the early universe, such as phase transitions and primordial black hole production. These may be discovered by the future spaceborne GW detectors: LISA, DECIGO, TianQin etc.

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