The Noisy Universe

TL;DR

This paper constrains large-scale white noise in the cosmic density field using cosmological data, revealing limits on primordial power spectrum deviations and demonstrating the potential of LSWN as a probe of early-universe physics.

Contribution

It introduces observational constraints on large-scale white noise in the cosmic density field, extending the theoretical framework with data-driven bounds.

Findings

Upper bound on white noise amplitude: $k_{BH} \, \leq 1.80 \times 10^{-13}$ Mpc$^{-1}$

Constraints on primordial power spectrum deviations: cutoff $k_{\mathrm{cut}} \lesssim 3$ pc$^{-1}$

Limits on spectral index running: $\alpha_s \lesssim -0.015$

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: $k_\mathrm{BH} \leq 1.80 \times 10^{-13}~\mathrm{Mpc}^{-1}$ 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 $\alpha_s \lesssim -0.015$. 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.