The Noisy Universe

Abstract

We present observational constraints on large-scale white noise (LSWN) in the cosmic density field, a phenomenon predicted to arise from non-linear mode coupling during cosmological evolution. Building on the theoretical framework of Paper I, where we demonstrated that non-linearities inevitably redistribute power from small to large scales through mode mixing, we confront these predictions with current cosmological data. We modify the CLASS Boltzmann code to incorporate a white noise component $k_{\mathrm{BH}}/k$ in the primordial power spectrum and perform parameter estimation using current cosmological data. The non-detection of excess power on the largest observable scales places stringent upper bounds: $k_{\mathrm{BH}}\le 1.80\times 10^{-13}{\mathrm{Mpc}}^{-1}$ at 99\% confidence. These constraints imply the primordial power spectrum must deviate from perfect scale invariance on small scales, either through a cutoff at $k_{\mathrm{cut}}\lesssim 3{\mathrm{pc}}^{-1}$ or through running of the spectral index with $\hat{I}{\pm }_s\lesssim -0.015$. Our results demonstrate that LSWN provides a powerful probe of the primordial spectrum at scales orders of magnitude smaller than directly observable, offering unique constraints on early-universe physics.

All sorts of typos corrected. Now 11 pages, but much clearer ones. Still one figure