[Paper Review] 21cm absorption signal from charge sequestration

Adam Falkowski, Kalliopi Petraki|arXiv (Cornell University)|Mar 27, 2018

Particle Detector Development and Performance17 citations

TL;DR

This paper proposes that a stable, negatively charged particle with a cosmological abundance can suppress electron density during recombination, leading to earlier decoupling of baryon gas from the CMB and significantly lowering gas temperature. This results in a stronger 21cm absorption signal at z ~ 17, consistent with the EDGES observation, and identifies viable parameter regions for sub-eV millicharged particles, MeV-scale particles, or TeV-scale multi-charged particles that evade existing constraints.

ABSTRACT

The unexpectedly strong 21cm absorption signal detected by the EDGES experiment suggests that the baryonic gas was colder at redshift $z\sim 17$ than predicted in the standard scenario. We discuss a mechanism to lower the baryon temperature after recombination. We introduce a stable, negatively-charged particle with a non-negligible cosmological abundance, such that the universe remains charge-neutral but the electron and proton numbers are no longer equal. The deficit of electrons during recombination results in an earlier decoupling of the baryon gas temperature from that of the CMB. This implies a smaller ratio of the gas and CMB temperature at $z\sim 17$. The parameter space of the mechanism where the 21 cm absorption signal is significantly enhanced is probed by the CMB spectrum, cooling of stars and supernovae, and colliders. Nevertheless, we find viable regions corresponding to sub-eV or MeV-scale milli-charged particles, or to TeV-scale multi-charged particles.

Motivation & Objective

• To explain the anomalously strong 21cm absorption signal observed by EDGES at z ~ 17, which deviates from standard ΛCDM predictions.
• To explore alternative cooling mechanisms for baryonic gas during the dark ages, distinct from dark matter-baryon interactions.
• To identify viable particle physics models—specifically stable, negatively charged particles—that can suppress electron abundance during recombination.
• To assess the cosmological and astrophysical constraints on such particles, including CMB spectral distortions, stellar cooling, and collider bounds.
• To provide testable predictions for future observations, including 21cm surveys and direct detection experiments.

Proposed method

• Introduces a stable, negatively charged particle X with non-negligible abundance, modifying the charge neutrality condition to n_e ≠ n_p.
• Uses a 3-level atom model for hydrogen recombination, incorporating modified ionization fractions via the modified evolution equation (3) that accounts for reduced electron density.
• Solves the coupled evolution equations for ionization fraction x_e and gas temperature T_g (Eq. 4a–4b), showing earlier decoupling due to suppressed Compton scattering rate.
• Computes the 21cm signal amplitude using the standard T_21 formula, showing enhanced absorption when T_g is reduced relative to T_γ.
• Applies constraints from CMB spectral distortions, stellar cooling, supernova cooling, and collider searches to bound the parameter space of X.
• Considers production mechanisms for X, including late decays of heavier particles, and evaluates their relic abundance and thermal history.

Experimental results

Research questions

• RQ1 Can a stable, negatively charged particle with cosmological abundance suppress electron density during recombination and thereby cool the baryonic gas earlier than in standard ΛCDM?
• RQ2 What is the resulting enhancement in the 21cm absorption signal at z ~ 17, and can it explain the EDGES observation of T_21 ≈ -0.5 K?
• RQ3 Which particle parameters—charge, mass, abundance—remain viable after imposing constraints from CMB spectral distortions, stellar cooling, and collider searches?
• RQ4 Can such particles account for dark matter while remaining consistent with the effective number of relativistic species (ΔN_eff)?
• RQ5 What are the detectable signatures of these particles in future 21cm surveys, direct detection, or X-ray observations?

Key findings

• A deficit of electrons during recombination, induced by a stable negatively charged particle X, leads to earlier decoupling of baryon gas from the CMB, reducing T_g at z ~ 17.
• The mechanism enhances the 21cm absorption signal by increasing the T_γ / T_g ratio, with the strongest enhancement occurring when ε_X r_X ≳ 10^{-4}.
• Viable parameter regions exist for sub-eV millicharged particles with ε_X ≲ 10^{-14}, MeV-scale particles that may constitute all or part of dark matter, and TeV-scale multi-charged particles.
• The model remains consistent with CMB spectral distortions, stellar cooling, and supernova cooling constraints, particularly when the X particle is non-relativistic by matter-radiation equality.
• The model predicts a stronger 21cm absorption signal during the dark ages (z ~ 200), which could be tested by upcoming low-frequency radio surveys.
• The X particles may be detectable via direct detection (especially in the millicharge regime), cosmic ray searches, or X-ray emission from p-X atomic bound states in halos for ε_X ≳ 1.