Signatures of Short Distance Physics in the Cosmic Microwave Background

TL;DR

This paper explores how short distance physics might influence the cosmic microwave background spectrum, finding effects are generally too small to detect, but some models could produce observable signatures with future precise measurements.

Contribution

It provides a systematic analysis of short distance physics effects on CMB anisotropies across various string and M theory models, highlighting potential observability.

Findings

Effects are typically too small to observe in weakly coupled models.

Horava-Witten compactifications show larger but still unobservable effects.

Certain M theory models could produce effects near the detection threshold.

Abstract

We systematically investigate the effect of short distance physics on the spectrum of temperature anistropies in the Cosmic Microwave Background produced during inflation. We present a general argument-assuming only low energy locality-that the size of such effects are of order H^2/M^2, where H is the Hubble parameter during inflation, and M is the scale of the high energy physics. We evaluate the strength of such effects in a number of specific string and M theory models. In weakly coupled field theory and string theory models, the effects are far too small to be observed. In phenomenologically attractive Horava-Witten compactifications, the effects are much larger but still unobservable. In certain M theory models, for which the fundamental Planck scale is several orders of magnitude below the conventional scale of grand unification, the effects may be on the threshold of detectability. However, observations of both the scalar and tensor fluctuation contributions to the Cosmic Microwave Background power spectrum-with a precision near the cosmic variance limit-are necessary in order to unambiguously demonstrate the existence of these signatures of high energy physics. This is a formidable experimental challenge.