[Paper Review] Post-recombination Dark Matter for the 21-cm Signal

Zhaofeng Kang|arXiv (Cornell University)|Mar 13, 2018

Dark Matter and Cosmic Phenomena1 references9 citations

TL;DR

This paper proposes that dark matter (DM) was produced after recombination but before cosmic dawn, thereby evading stringent cosmic microwave background (CMB) constraints on baryon-DM scattering. By decoupling from the thermal bath during recombination, minicharged DM with 100% relic density can explain the anomalous 21-cm absorption signal from EDGES without violating CMB bounds, offering a viable single-component DM scenario with velocity-enhanced scattering.

ABSTRACT

We have no certain knowledge of the early history of dark matter (DM). In this paper we propose a scenario where DM is produced post-recombination but prior to the cosmic dawn. It helps to relax the bounds on DM interactions, in particular with baryons, from the CMB. It may be of interest in some circumstances, for example, to understand the recent cosmic dawn 21-cm signal anomaly. We argue that the cosmic gas cooling mechanism via the minicharged DM-baryon scattering may be viable even if it takes up the total DM budget. We also investigate the possibility of a gluon-philic mediator of a few 10 keV, to find that the most reliable exclusion is from the neutron scattering.

Motivation & Objective

• To address the tension between the observed 21-cm absorption signal and standard models, which requires cooling of cosmic gas below the CMB temperature.
• To relax the CMB constraints on baryon-DM scattering by postulating that DM was produced after recombination, thus decoupling from early plasma interactions.
• To explore whether minicharged DM with 100% of the DM density can explain the EDGES anomaly without violating existing astrophysical bounds.
• To investigate the viability of a light gluon-philic mediator in the context of DM-baryon scattering, particularly under neutron scattering constraints.

Proposed method

• Assumes a late production mechanism for DM via decay of a long-lived mother particle Y with lifetime τY ∈ (tR, tcd), avoiding thermalization during recombination.
• Uses effective field theory to model DM-baryon scattering via a light mediator φ, with interactions governed by couplings gϕχχ and gϕNN.
• Applies velocity-dependent cross-section scaling σ̄(v) ∝ v−4 for minicharged DM to enhance low-velocity scattering relevant to cosmic dawn.
• Evaluates constraints from neutron scattering experiments, which provide the most reliable bounds on the mediator coupling gϕNN.
• Constructs effective Lagrangians for both CP-even and CP-odd gluonic operators, focusing on the CP-even case due to stronger scattering enhancement.
• Derives the momentum-transfer cross section using the effective theory: σ̄ = (gϕχχ² gϕNN² / π) (μ² / mϕ⁴), with μ as the reduced mass.

Experimental results

Research questions

• RQ1 Can a single-component minicharged dark matter model explain the EDGES 21-cm anomaly without violating CMB constraints?
• RQ2 What are the cosmological and astrophysical constraints on a light mediator (φ) in the 10 keV mass range for DM-baryon scattering?
• RQ3 How does post-recombination DM production affect the viability of velocity-enhanced DM-baryon scattering at cosmic dawn?
• RQ4 What are the strongest experimental bounds on the DM-baryon coupling gϕNN, and can they be evaded in a late-thermalized DM scenario?
• RQ5 Is a gluon-philic mediator with a few 10 keV mass compatible with neutron scattering and 21-cm observations?

Key findings

• The CMB constraint on baryon-DM scattering is evaded when DM is produced after recombination, allowing a single-component DM model to explain the 21-cm anomaly.
• Minicharged DM with 100% of the DM relic density can explain the EDGES signal if produced post-recombination, despite the usual requirement for only ~1% DM fraction.
• The neutron scattering bound gives gϕNN ≲ 10⁻⁶, which strongly suppresses the scattering cross-section and rules out the light mediator scenario for explaining the 21-cm anomaly.
• For the CP-even gluonic operator, the effective coupling is FϕNN = (2/9) fT_G^(N) mN / Λ, with fT_G^(N) ≈ 0.83 for nucleons, leading to a measurable interaction strength.
• The CP-odd case is less constrained but leads to suppressed v⁻⁴ velocity dependence, making it less effective for cooling the cosmic gas at low velocities.
• The most reliable exclusion for the light mediator comes from neutron scattering, not stellar cooling or fifth-force searches, due to stronger experimental sensitivity.