The nature of intrinsic fluctuations in cosmic diffuse radiation

TL;DR

This paper investigates the intrinsic fluctuations in cosmic diffuse radiation, especially the CMB, revealing two sources of noise and discussing how wave components influence the fluctuation spectrum.

Contribution

It provides a novel analysis of the wave noise origin in cosmic radiation fluctuations and offers generic formulas applicable to various incoherent light spectra.

Findings

Identifies two sources of fluctuation variance: spontaneous and stimulated emission.

Shows wave superposition contributes to noise, previously not well understood.

Suggests wave components influence the large-scale fluctuation spectrum.

Abstract

The spatial and temporal noise properties of diffuse radiation is investigated in the context of the cosmic microwave background (CMB), although generic formulae that enable application to any other forms of incoherent light of a prescribed energy spectrum are also provided. It is shown that the variance of fluctuations in the density and flux consists of two parts. First is a term from the spontaneous emission coefficient which is the contribution from a random gas of classical particles representing the corpuscular (photon) nature of light. Second is a term from the stimulated emission coefficient which leads to a `wave noise', or more precisely the noise arising from the superposition of many plane waves of arbitrary phase - the normal modes of the radiation. The origin of this second term has never been elucidated before. We discussed one application. In the spatially homogeneous post-inflationary epochs when the universe was reheated to GUT temperatures, the thermal CMB density fluctuations on the horizon scale is of order the WMAP measured value of 10$^{-5}$. Beyond (larger than) the horizon, the power spectrum of perturbations could moreover assume the observed form of $P(k) \sim k$ if thermal diffusion to equilibrium does not merely involve classical particles, but also non-localized wave components. The outcome of our study clearly demonstrates that such components do indeed exist.