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Signs of Dark Matter at 21-cm?

Rennan Barkana, Nadav Joseph Outmezguine, Diego Redigolo, Tomer Volansky

TL;DR

The paper investigates whether a velocity-dependent, Rutherford-like DM–baryon scattering with a light mediator can explain the EDGES 21-cm absorption signal at $z\approx17$. It systematically analyzes two mediator paradigms (unscreened long-range forces and screened portals via hidden photons or millicharged DM) and applies a suite of constraints from 5th-force tests, stellar cooling, DM self-interactions, SN1987A, and direct detection. The main finding is that the dominant DM component cannot account for the EDGES absorption, as the required cross sections are excluded across the viable mediator mass ranges; however, a subcomponent of millicharged DM at the percent level may still provide an explanation. This work tightly constrains early-universe DM–baryon interactions and informs model-building by showing that simple, fully DM-driven cooling scenarios are unlikely, guiding future explorations of partial DM solutions and other cooling mechanisms.

Abstract

Recently the EDGES collaboration reported an anomalous absorption signal in the sky-averaged 21-cm spectrum around $z=17$. Such a signal may be understood as an indication for an unexpected cooling of the hydrogen gas during or prior to the so called Cosmic Dawn era. Here we explore the possibility that dark matter cooled the gas through velocity-dependent, Rutherford-like interactions. We argue that such interactions require a light mediator that is highly constrained by 5th force experiments and limits from stellar cooling. Consequently, only a hidden or the visible photon can in principle mediate such a force. Neutral hydrogen thus plays a sub-leading role and the cooling occurs via the residual free electrons and protons. We find that these two scenarios are strongly constrained by the predicted dark matter self-interactions and by limits on millicharged dark matter respectively. We conclude that the 21-cm absorption line is unlikely to be the result of gas cooling via the scattering with a dominant component of the dark matter. An order 1\% subcomponent of millicharged dark matter remains a viable explanation.

Signs of Dark Matter at 21-cm?

TL;DR

The paper investigates whether a velocity-dependent, Rutherford-like DM–baryon scattering with a light mediator can explain the EDGES 21-cm absorption signal at . It systematically analyzes two mediator paradigms (unscreened long-range forces and screened portals via hidden photons or millicharged DM) and applies a suite of constraints from 5th-force tests, stellar cooling, DM self-interactions, SN1987A, and direct detection. The main finding is that the dominant DM component cannot account for the EDGES absorption, as the required cross sections are excluded across the viable mediator mass ranges; however, a subcomponent of millicharged DM at the percent level may still provide an explanation. This work tightly constrains early-universe DM–baryon interactions and informs model-building by showing that simple, fully DM-driven cooling scenarios are unlikely, guiding future explorations of partial DM solutions and other cooling mechanisms.

Abstract

Recently the EDGES collaboration reported an anomalous absorption signal in the sky-averaged 21-cm spectrum around . Such a signal may be understood as an indication for an unexpected cooling of the hydrogen gas during or prior to the so called Cosmic Dawn era. Here we explore the possibility that dark matter cooled the gas through velocity-dependent, Rutherford-like interactions. We argue that such interactions require a light mediator that is highly constrained by 5th force experiments and limits from stellar cooling. Consequently, only a hidden or the visible photon can in principle mediate such a force. Neutral hydrogen thus plays a sub-leading role and the cooling occurs via the residual free electrons and protons. We find that these two scenarios are strongly constrained by the predicted dark matter self-interactions and by limits on millicharged dark matter respectively. We conclude that the 21-cm absorption line is unlikely to be the result of gas cooling via the scattering with a dominant component of the dark matter. An order 1\% subcomponent of millicharged dark matter remains a viable explanation.

Paper Structure

This paper contains 13 sections, 28 equations, 4 figures.

Table of Contents

  1. Introduction
  2. The 21 cm global spectrum
  3. The Standard Model
  4. Dark Cooling
  5. The 21-cm Signal
  6. Models of Dark Matter and Constraints
  7. Unscreened Long Range Forces
  8. Hidden Photons
  9. Millicharged Dark Matter
  10. Conclusions
  11. Acknowledgements
  12. Heating Formalisem
  13. Cross Sections

Figures (4)

  • Figure 1: The cross section defined in Eq. \ref{['eq:xsec']} required to fit the EDGES signal for DM-hydrogen interactions ( red), DM-helium interactions ( blue) and interactions with the ionized fraction assuming the interacting particle constitutes all of the DM ( green) or only 1% of the DM density ( brown). The solid (dashed) lines correspond to the minimal cross section needed to obtain a brightness temperature $T_{21}=-300\textrm{\,mK}\ (-500\textrm{\,mK})$ assuming infinite Ly-$\alpha$ radiation rate which couples the spin temperature to that of the gas, and assuming no heating of the gas due to X-ray radiation.
  • Figure 2: Constraints on the effective couplings of light mediators to the SM as a function of the mediator mass. The gray shaded region is excluded for theories that mediate a long range Rutherford-like force which cannot be fully screened. For the astrophisical bounds we assume democratic mediator couplings between electrons and protons. The purple-shaded region holds also for a light hidden photon under which SM charges are proportional to their electric charge and can therefore be screened. The solid ( dashed) lines indicated the minimal$\alpha_{\text{eff}}$ needed to fit the EDGES signal (see Fig \ref{['fig:signal']}) when cooling via the scattering of a keV (GeV) DM with hydrogen ( red), helium ( blue) and free electrons and protons ( green) is assumed. In order to show the severeness of the constraints on $\alpha_{\rm eff}$, the cross sections are obtained for the best case scenario where the coupling of the DM to the mediator, $\alpha_D=1$, ignoring any possible limits. The gray shaded region is excluded by various 5th force experiments Adelberger:2003zxSalumbides:2013dua while the purple-shaded region shows various limits including those from stellar cooling constraints Hardy:2016kmeAn:2014twa.
  • Figure 3: Constraints on the hidden photon parameter space for the case when the interacting particle constitutes all of the measured DM density ($\Omega_\chi=\Omega_{\rm CDM}$) and for the choice of very light mediator $m_{A'}=10^{-18} ~\mathrm{eV}$. The same parameter space is plotted on the $\epsilon-m_\chi$ plane ( left) and $\bar{\sigma}_e - m_\chi$ plane ( right). In both plots we choose $\alpha_D=10^{-10}(m_\chi/\mathrm{MeV})^{3/2}$ which is consistent with the self-interaction limits Tulin:2013teo. The green line represent the minimal cross section needed to explain the EDGES measurement, while in dashed-gray lines we show contours of constant $\hat{\sigma}$ for better orientation. Constraints from cooling of the supernova (SN) 1987A Chang:2018rso ( purple), direct detection limits from XENON10 Essig:2012yxEssig:2017kqs ( green), effective number of relativistic particles at CMB and at BBN Vogel:2013raa ( blue), SLAC millicharge experiment Prinz:1998ua ( gray), cooling of white-dwarfs (WD), horizontal-branch (HB) stars and red-giants (RG) Vogel:2013raa ( pink and brown), limits on DM-SM coupling at the time of CMB McDermott:2010pa ( light green) and DM-SM momentum transfer Vera_future ( light purple) are shown in the shaded regions.
  • Figure 4: Constraints on the charge, Q, of a millicharged particle as a function of the DM mass. The red line indicates the minimal cross section needed to explain the EDGES measurement, assuming the millicharged particle constitutes only $1\%$ of the DM density. The dashed-gray lines show contours of constant $\hat{\sigma}$. Constraints from cooling of the supernova (SN) 1987A Chang:2018rso ( purple), direct detection limits from XENON10 Essig:2012yxEssig:2017kqs ( green) and SENSEI Crisler:2018gci, SLAC millicharge experiment Prinz:1998ua ( gray), BBN Davidson:2000hf (light blue) and cooling of white-dwarfs (WD), horizontal-branch (HB) stars and red-giants (RG) Vogel:2013raa ( pink and brown) are shown in the shaded regions. We also add constraints from heating due to DM annihilation derived in Liu:2018uzy ( blue). This bound only applies to fermionic DM for which the annihilation is $s$-wave. The shaded yellow band indicates where millicharge DM might be evacuated from the galactic disk Chuzhoy:2008zyMcDermott:2010pa.