Unsqueezing of standing waves due to inflationary domain structure

TL;DR

This paper explores how inflationary domain structures could unsqueeze standing waves into traveling waves, affecting the cosmic microwave background and gravitational wave signals, with potential observational consequences at small scales.

Contribution

It introduces a model where domain-induced unsqueezing of waves alters the expected primordial signals, providing a new perspective on inflationary physics and potential observational signatures.

Findings

Unsqueezing erases Doppler peaks in CMB at small scales

Primordial gravitational waves become traveling waves due to unsqueezing

Potential detectability of unsqueezing effects in gravitational wave background

Abstract

The so-called "trans-Planckian" problem of inflation may be evaded by positing that modes come into existence only when they became "cis-Planckian" by virtue of expansion. However, this would imply that for any mode a new random realization would have to be drawn every $N$ wavelengths, with $N$ typically of order 1000 (but it could be larger or smaller). Such a re-drawing of realizations leads to a heteroskodastic distribution if the region under observation contains several such independent domains. This has no effect on the sampled power spectrum for a scale-invariant raw spectrum, but at very small scales it leads to a spectral index bias towards scale-invariance and smooths oscillations in the spectrum. The domain structure would also "unsqueeze" some of the propagating waves, i.e., dismantle their standing wave character. By describing standing waves as travelling waves of the same amplitude moving in opposite directions we determine the observational effects of unsqueezing. We find that it would erase the Doppler peaks in the CMB, but only on very small angular scales, where the primordial signal may not be readily accessible. The standing waves in a primordial gravitational wave background would also be turned into travelling waves. This unsqueezing of the gravitational wave background may constitute a detectable phenomenon.