Signatures of short distance physics in the cosmic microwave background

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 Hořava-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.

Stringy effects during inflation and reheating

“…Comparing this rate to the decay rate into massless 4D gravitons imposes strong restrictions on constructing a viable model -see archive version of [56]. It may be that considering the tunneling process with a more sophisticated model reduces this suppression (22) and eases these restrictions. For example, the potential barrier of a cascading geometry is smaller [135] than in the simple AdS model of [128,129].…”

“…The value of n has been a matter of some debate in the literature. Standard EFT arguments give n 2 [22] unless boundary terms are added [23][24][25][26][27][28][29][30]. The latter may yield n 1 and seem to reproduce the effects of an unconventional initial state or nonadiabatic evolution [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47].…”

Cosmological perturbations from a new-physics hypersurface

“…There is instead much debate [2,3,4,5,6] about the possibility that present largescale experiments may test physics at even shorter scales, down to the string/Planck-length scales or even below. A particularly important issue is that of finding out the order of these "transplanckian" corrections in the (supposedly small) parameter H/Λ, where H is a typical Hubble parameter during inflation and Λ is the scale of some unknown new physics.…”

Cosmic 21 cm fluctuations as a probe of fundamental physics

“…In addition to allowing a direct probe of the physics of inflation (see, e.g., [1][2][3][4]), this observation has lead to the suggestion that the spectrum of temperature and polarization anisotropies in the Cosmic Microwave Background (CMB) could be used as a probe of unconventional physics at scales at or above H i (see [5][6][7][8][9][10][11][12][13][14], and references therein). This approach is limited by several factors: the inflationary scale H i , while high, is at best 1% of the fiducial scale of M ∼ 10 16 GeV, and cosmic variance, coupled with the Silk damping of anisotropies due to photon diffusion, limits the theoretical precision of data taken from the CMB to approximately that same * Although its potential as a cosmological probe has long been known [15][16][17][18], there has recently been renewed interest in using the 21-cm hyperfine "spin-flip" transition of neutral hydrogen as a probe of the cosmic dark ages [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35].…”

“…Another example involves the direct effect of high-scale physics on inflaton dynamics. Such effects occur when integrating out massive fields coupled to the inflaton gives rise to wave-function renormalizations [12]. Because these features can occur at any time during inflation, the CMB alone provides a limited observational window, and is hindered by the constraints of cosmic variance.…”