Physics Letters B 785 (2018) 429–433
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Constraints on ALPs and excited dark matter from the EDGES 21 cm
absorption signal
Andi Hektor
a
, Gert Hütsi
a,b,∗
, Luca Marzola
a
, Ville Vaskonen
a
a
Laboratory of High Energy and Computational Physics, National Institute of Chemical Physics and Biophysics, Rävala pst. 10, 10143 Tallinn, Estonia
b
Tartu Observatory, University of Tartu, Observatooriumi 1, 61602 Tõravere, Estonia
a r t i c l e i n f o a b s t r a c t
Article history:
Received
4 June 2018
Received
in revised form 3 July 2018
Accepted
4 September 2018
Available
online 6 September 2018
Editor: A.
Ringwald
The recent observation of the 21 cm absorption signal by the EDGES experiment provides a new
observational window into the dynamics of the young Universe. Based on this result, we constrain the
properties of ALPs and excited dark matter via the energy injections into the gaseous medium that
these models produce. In particular, we derive bounds that outperform the present constraints for energy
injections in the 10.2 eV to 50 keV range by analysing the intensity of Lyman-α photons and the electron
emissivity produced by ionising radiation. Our results also show that once the standard temperature of
the soft radiation background is assumed, the explanations of the observed 3.5 keV photon signal within
the considered dark matter model are excluded at 95% confidence level.
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
The recent detection of the 21 cm spectrum by the EDGES
experiment [1]presents a first insight into the dynamics of the
Dark Ages of the Universe, close to the onset of star formation.
Within CDM, the observed absorption feature is explained by the
Wouthuysen–Field effect [2,3], after the first stars produced a sig-
nificant
population of Lyman-α photons at z ∼ 20. The effect of
this energetic radiation is then to decouple the hydrogen spin tem-
perature
from the soft radiation temperature that characterises the
redshifted CMB background, forcing it to converge to the lower ki-
netic
temperature of the gas.
Whereas
the redshift of the observed signal agrees with the
theoretical predictions, the measured magnitude of the absorption
feature greatly exceeds the CDM expectations, consequently rais-
ing
the problem of its origin. Explanations of the EDGES result
proposed in literature invoke either new mechanisms for cooling
the gaseous medium [4–16]or the presence of an additional soft
photon component [17–22]. However, the mechanism beyond the
anomaly remains to-date unclear. Furthermore, the significance of
the measurement and the treatment of the foregrounds is cur-
rently
under debate [23,24].
*
Corresponding author.
E-mail
addresses: andi.hektor@cern.ch (A. Hektor), gert.hutsi@to.ee (G. Hütsi),
luca.marzola@cern.ch (L. Marzola), ville.vaskonen@kbfi.ee (V. Vaskonen).
In spite of its possible origin, a confirmation of the EDGES
measurement would provide a new observable capable of probing
the dynamics of the young Universe. As an example, the mea-
sured
absorption profile strongly constrains the presence of addi-
tional
mechanisms resulting in Lyman-α radiation at z 20, which
would otherwise induce earlier absorption signals. At higher ener-
gies,
the 21 cm spectrum constrains instead possible energy injec-
tions
that would lead to ionizations and consequent heating of the
gaseous medium. These observations have been used in literature
to bound energy injections due to new physics processes [25–30],
the properties of the dark sector [31–33] and to investigate the
astrophysical consequences implied by the signal [34–37].
In
this Letter we adopt the same attitude and make use of
the EDGES results to investigate the properties of light dark mat-
ter
(DM) candidates that could result in energy injections in the
gaseous medium over the 10.2 eV to 50 keV range, a mass range
not considered by the above-cited papers. As concrete examples
we consider the case of axion-like particles (ALPs) [38] and of ex-
cited
DM [39], deriving the phenomenological consequences of the
EDGES measurement and comparing the resulting constraints to
the ones previously proposed in the literature [40,41]. Particular
attention is paid to the issue of the 3.5 keV line detected in galaxy
clusters [42,43], previously motivated both in excited DM [44] and
ALPs [45]models.
Our
analyses show that once a temperature of the soft radia-
tion
background in agreement with CDM is assumed, the con-
straints
resulting from the EDGES measurement outperform the
https://doi.org/10.1016/j.physletb.2018.09.009
0370-2693/
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.
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