Superhorizon fluctuations and the cosmic dipole problem

TL;DR

This paper proposes that superhorizon isocurvature perturbations in dark matter could explain the observed discrepancy between the cosmic dipole measurements from galaxy counts and the CMB, offering a potential solution to the dipole tension.

Contribution

It introduces a novel mechanism involving superhorizon isocurvature modes to account for the cosmic dipole discrepancy, including specific models with axion-induced perturbations.

Findings

Superhorizon isocurvature modes can generate an intrinsic CMB dipole without affecting galaxy counts.

Axion models with evolving radial modes can match the required dipole amplitude while satisfying CMB constraints.

Constraints on axion potential self-coupling are derived, ranging from 10^{-9} to 4×10^{-9}.

Abstract

Recent observations have identified a significant 4.9$\sigma$ tension between the cosmic dipole inferred from galaxy number counts and that derived from the Cosmic Microwave Background (CMB), suggesting a potential deviation from the cosmological principle. This work investigates whether superhorizon isocurvature perturbations in cold dark matter (CDM) can account for this discrepancy. We demonstrate that, unlike adiabatic modes which cancel at leading order, superhorizon isocurvature modes can generate an intrinsic CMB dipole without significantly affecting galaxy number counts, thereby explaining the observed mismatch. We explore both single-mode and continuous-spectrum cases, focusing on two concrete models: a nearly scale-invariant power-law spectrum with a UV cutoff and axion-induced isocurvature perturbations. For the axion scenario, we show that if the radial mode evolves during inflation, the resulting perturbations can match the required amplitude while evading current CMB constraints. Our analysis constrains the self-coupling of associated potential for the axion to the range $10^{-9} < \lambda < 4 \times 10^{-9}$. These findings offer a viable solution to the dipole tension and may serve as indirect evidence for axion dark matter.