Signatures of Short Distance Physics in the Cosmic Microwave Background
Nemanja Kaloper, Matthew Kleban, Albion Lawrence, Stephen Shenker
TL;DR
This paper investigates whether short-distance, high-energy physics can leave observable imprints on inflationary CMB fluctuations. It argues, within an EFT framework, that heavy physics modifies inflaton fluctuations by terms of order $H^2/M^2$, and then evaluates these corrections in various string/M-theory contexts. The findings indicate that in typical weakly coupled theories the effects are far below detectability, while certain M-theory constructions with lowered fundamental scales can push signals toward observational relevance, albeit requiring near-cosmological-variance-limited data on both scalar and tensor modes. The work highlights the stringent experimental demands and identifies specific scenarios (notably some G2/M-theory compactifications) where the signatures could be within reach, offering a potential window into Planck-scale physics via the CMB.
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.