[Paper Review] Insights on Dark Matter from Hydrogen during Cosmic Dawn

Julián B. Muñoz, Abraham Loeb|arXiv (Cornell University)|Feb 27, 2018

Dark Matter and Cosmic Phenomena47 references131 citations

TL;DR

The paper explores whether a subpercent fraction of dark matter with a small electric minicharge can cool baryons during cosmic dawn, potentially explaining the EDGES 21-cm absorption signal, and delineates constraints from other observations.

ABSTRACT

The origin and composition of the cosmological dark matter remain a mystery. However, upcoming 21-cm measurements during cosmic dawn, the period of the first stellar formation, can provide new clues on the nature of dark matter. During this era, the baryon-dark matter fluid is the slowest it will ever be, making it ideal to search for dark matter elastically scattering with baryons through massless mediators, such as the photon. Here we explore whether dark-matter particles with an electric "minicharge" can significantly alter the baryonic temperature and, thus, affect 21-cm observations. We find that the entirety of the dark matter cannot be minicharged at a significant level, lest it interferes with Galactic and extragalactic magnetic fields. However, if minicharged particles comprise a subpercent fraction of the dark matter, and have charges $ε\sim 10^{-6}$---in units of the electron charge---and masses $m_χ\sim 1-60$ MeV, they can significantly cool down the baryonic fluid, and be discovered in 21-cm experiments. We show how this scenario can explain the recent result by the EDGES collaboration, which requires a lower baryonic temperature than possible within the standard model, while remaining consistent with all current observations.

Motivation & Objective

• Motivate the search for nongravitational DM-baryon interactions using the cosmic-dawn 21-cm signal.
• Quantify how a minicharged DM fraction affects baryon temperature and 21-cm observables.
• Identify the parameter region (mass, charge, fractional abundance) compatible with EDGES and existing constraints.
• Assess accompanying 21-cm fluctuations and potential detectability by upcoming experiments.

Proposed method

• Model the DM-baryon elastic scattering with a massless mediator leading to a momentum-transfer cross section abardot_t = 2    \
•  (Equation 1) to describe DM-baryon interactions with Debye logarithm   (r_t) (Equation 4).
• Compute the baryon and minicharged DM temperature evolution via coupled differential equations for T_b, T_{ chi}, and v_{ chi,b} (Equations 7a–7d).
• Translate baryon cooling into 21-cm brightness temperature T^{21} (Equation 17) and its sky-averaged value (Equation 18).
• Incorporate observational constraints (SN1987A, SLAC millicharge, BBN, N_eff) and relic-abundance considerations (Equations 12–16, Fig. 1).
• Explore how a subpercent f_{dm} with b epsilon erences derived cooling region in the m_{hi}- plane and discuss potential 21-cm fluctuations (Figure 2).

Experimental results

Research questions

• RQ1 Can a subpercent fraction of dark matter with a small electromagnetic minicharge significantly cool the baryonic gas during cosmic dawn?
• RQ2 What regions of the minicharge epsilon and DM mass m_{ chi} allow sufficient cooling without violating external constraints?
• RQ3 How would DM-baryon interactions modify the 21-cm brightness temperature and its fluctuations during the EDGES-era epoch?
• RQ4 Are the required minicharged DM scenarios compatible with relic abundance and other astrophysical/cosmological bounds?

Key findings

• A subpercent fraction f_{dm} e 10^{-2} of minicharged DM with charge
  10^{-6} and mass m_{ chi}  10 MeV can noticeably cool the baryonic fluid.
• The needed minicharges to achieve baryon cooling scale roughly as
  1)  m_{ chi} for m_{ chi} < 6 GeV  f_{dm}; the paper provides an empirical relation b ractional result 6 epsilon(m_{ chi}, f_{dm})
  7 6  10^{-7} (m_{ chi}/MeV) (f_{dm}/10^{-2})^{-3/4}.
• The EDGES result, requiring T_b  4 K at z  17, can be reconciled with standard cosmology if minicharged DM is present in the stated regime, while remaining compatible with other bounds.
• Minicharged DM does not constitute all DM; however, even as a small fraction it can influence T^{21} and induce additional 21-cm fluctuations of order 1% of the sky-averaged signal, potentially detectable by HERA or SKA.
• Constraints from SN1987A, BBN, CMB N_eff, and relic abundance generally exclude large regions of parameter space for f_{dm}=1, but leave open viable regions for f_{dm} e 10^{-2} with 0^{-6} (m_{ chi}/{ m MeV})^{0.3} scaling.
• If the minicharged component cools the gas by a factor of two, the 21-cm signal could be deepened relative to the standard model prediction, consistent with current observations within the stated parameter window.