[Paper Review] Galaxy number-count dipole and superhorizon fluctuations

Guillem Domènech, Roya Mohayaee+2 more|arXiv (Cornell University)|Jul 4, 2022

Galaxies: Formation, Evolution, Phenomena100 references31 citations

TL;DR

This paper investigates whether superhorizon primordial perturbations—specifically isocurvature modes—can explain the observed tension between the cosmic microwave background (CMB) dipole and the galaxy number-count dipole. It finds that while adiabatic and isocurvature superhorizon modes do not generate an intrinsic number-count dipole, isocurvature modes induce a relative velocity between CMB and matter frames, potentially enabling a kinematic origin for the dipole tension. The model predicts a measurable galaxy number-count quadrupole, offering a testable signature for future surveys.

ABSTRACT

In view of the growing tension between the dipole anisotropy of number counts of cosmologically distant sources and of the cosmic microwave background (CMB), we investigate the number count dipole induced by primordial perturbations with wavelength comparable to or exceeding the Hubble radius today. First, we find that neither adiabatic nor isocurvature superhorizon modes can generate an intrinsic number count dipole. However a superhorizon isocurvature mode does induce a relative velocity between the CMB and the (dark) matter rest frames and thereby affects the CMB dipole. We revisit the possibility that it has an intrinsic component due to such a mode, thus enabling consistency with the galaxy number count dipole if the latter is actually kinematic in origin. Although this scenario is not particularly natural, there are possible links with other anomalies and it predicts a concommitant galaxy number count quadrupole which may be measurable in future surveys. We also investigate the number count dipole induced by modes smaller than the Hubble radius, finding that subject to CMB constraints this is too small to reconcile the dipole tension.

Motivation & Objective

• To test whether superhorizon primordial perturbations can resolve the growing tension between the CMB dipole and the galaxy number-count dipole.
• To assess whether isocurvature modes—unlike adiabatic modes—can induce a relative velocity between the CMB and matter rest frames, affecting dipole measurements.
• To evaluate whether such modes can generate an intrinsic number-count dipole consistent with observations, thus reconciling the dipole tension.
• To examine the viability of this scenario under CMB constraints and its implications for cosmological principle violations.
• To predict a concomitant number-count quadrupole signal that could be detectable in upcoming large-scale surveys.

Proposed method

• Derives galaxy number-count fluctuations using a covariant relativistic formalism, extending the framework of Kasai & Sasaki (1989) with full inclusion of monopole and dipole terms at the observer.
• Applies cosmological perturbation theory to compute the effects of superhorizon adiabatic and cold dark matter (CDM) isocurvature modes on both CMB and galaxy number counts.
• Uses the gauge-invariant formalism to compute the number-count dipole induced by superhorizon modes, distinguishing between kinematic and intrinsic contributions.
• Evaluates the dipole amplitude under current CMB constraints (Planck 2018) to test whether sub-horizon modes could reconcile the dipole tension.
• Analyzes the impact of modes that were superhorizon at decoupling but are now sub-horizon, assessing their contribution to the number-count dipole.
• Derives the predicted number-count quadrupole signal induced by isocurvature modes, proposing it as a testable signature for future surveys.

Experimental results

Research questions

• RQ1 Can superhorizon isocurvature modes generate an intrinsic number-count dipole that would reconcile the observed dipole tension?
• RQ2 Do adiabatic or isocurvature superhorizon modes induce a relative velocity between the CMB and matter rest frames, affecting the CMB dipole?
• RQ3 Is the observed galaxy number-count dipole consistent with a kinematic origin if the CMB dipole has an intrinsic component due to isocurvature modes?
• RQ4 What is the amplitude of the number-count dipole induced by sub-horizon modes that were superhorizon at decoupling, under current CMB constraints?
• RQ5 Can the predicted number-count quadrupole from isocurvature modes be detected in future large-scale surveys?

Key findings

• Neither adiabatic nor isocurvature superhorizon modes generate an intrinsic number-count dipole, ruling out a direct contribution to the galaxy dipole from such modes.
• Superhorizon isocurvature modes do induce a relative velocity between the CMB and matter rest frames, which can affect the CMB dipole and potentially explain its intrinsic component.
• The scenario in which the galaxy number-count dipole is kinematic and the CMB dipole has an intrinsic component due to isocurvature modes is not particularly natural but remains consistent with current data.
• The model predicts a concomitant galaxy number-count quadrupole signal induced by isocurvature modes, which could be measurable in upcoming deep-sky surveys.
• Sub-horizon modes that were superhorizon at decoupling produce a number-count dipole that is too small to reconcile the observed dipole tension under current CMB constraints.
• The framework provides a testable prediction: a measurable number-count quadrupole from isocurvature modes, offering a potential observational signature to distinguish this scenario from standard ΛCDM.