Smoothed quantum fluctuations and CMB observations

Abstract

In this paper we investigate power spectrum of a smoothed scalar field. The smoothing leads to the regularisation of the UV divergences and can be related with the internal structure of the considered field or the space itself. We apply procedure of smoothing to the quantum fluctuations generated during the phase of cosmic inflation. We study whether this effect can be probed observationally and conclude that the modifications of the power spectrum due to the smoothing on the Planck scale are negligible and far beyond the observational abilities. Subsequently we investigate whether smoothing in any other form can be probed observationally. We introduce phenomenological smoothing factor $e^{-k^2\sigma^2}$ to the inflationary spectrum and investigate its effects on the spectrum of CMB anisotropies and polarisation. We show that smoothing can lead to suppression of high multipoles in the spectrum of the CMB. Based on five years observations of WMAP satellite we indicate that the present scale of high multipoles suppression is constrained by $\sigma < 2.86$ Mpc (95% CL). This corresponds to the constraint $\sigma < 100 \mu$m at the end of inflation. Despite this value is far above the Planck scale, the other processes of smoothing can be possibly studied with this constraint, e.g. diffusion or decoherence of primordial perturbations.