Abstract
Motivated by the old trans-Planckian (TP) problem of inflationary cosmology, it has been conjectured that any consistent effective field theory should keep TP modes ‘hidden’ behind the Hubble horizon, so as to prevent them from turning classical and thereby affecting macroscopic observations. In this paper we present two arguments against the Hubble horizon being a scale of singular significance as has been put forward in the TP Censorship Conjecture (TCC). First, refinements of TCC are presented that allow for the TP modes to grow beyond the horizon while still keeping the de-Sitter conjecture valid. Second, we show that TP modes can turn classical even well within the Hubble horizon, which, as such, negates this rationale behind keeping them from crossing it. The role of TP modes is known to be less of a problem in warm inflation, because fluctuations start out usually as classical. This allows warm inflation to be more resilient to the TP problem compared to cold inflation. To understand how robust this is, we identity limits where quantum modes can affect the primordial power spectrum in one specific case.
Highlights
JHEP08(2020)071 time-dependent Hilbert space, with the degrees of freedom increasing in time, resulting in complicated non-unitary evolution [12, 13]
If we look at a perturbation mode at late times and track its physical wavelength going backwards, one would find that linear perturbations grow to become larger than 1 before the physical wavelength becomes smaller than or equal to Pl
For the purposes of this paper, we would assume that a TP mode has a physical wavelength that is slightly larger than Pl such that one can follow its evolution on cosmological spacetimes from thereon
Summary
Since there does not seem to be any compelling argument for restricting TP modes from crossing the Hubble radius, it is pertinent to question the premise that any consistent EFT should not allow TP modes from turning classical. In order to critically examine this claim, let us take the following approach — we want to show that a TP mode can turn classical even without crossing the Hubble horizon If such a claim can be established, there should be no fundamental argument against refining the TCC with some O(1) number, as has been done for all the other swampland conjectures in the past. That does not necessarily check the boxes for other criteria, and one can even get away with not defining an environment or any interaction with it, which is why it is sometimes referred to as ‘decoherence without decoherence’ [66] For these reasons, at first sight it might seem enough to prevent the classicalization of TP modes by keeping them hidden within the Hubble radius, as stated in the TCC. To get an intuitive understanding of the situation, we will present a few concrete examples of how much a TP mode is stretched, and how in every case classical physics is dominant, showing from this standpoint that TP modes come as a part and parcel of an expanding Universe
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