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Physics Letters B 784 (2018) 130–136

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

The EDGES signal: An imprint from the mirror world?

D. Aristizabal Sierra

a,b

, Chee Sheng Fong

c,d,∗

Universidad Técnica Federico Santa María - Departamento de Física, Casilla 110-V, Avda. España 1680, Valparaíso, Chile

IFPA, Dep. AGO, Université de Liège, Bat B5, Sart Tilman B-4000 Liège 1, Belgium

Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, Rio de Janeiro, Brazil

Instituto de Física, Universidade de São Paulo, São Paulo, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:

Received

17 May 2018

Received

in revised form 20 July 2018

Accepted

25 July 2018

Available

online 27 July 2018

Editor: M.

Trodden

Keywords:

Cosmic

dark ages

Reionization

epoch

Neutrino

transition moments

Dark

photons

Mirror

neutrinos

Twin

Higgs models

Recent results from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) show

an anomalous spectral feature at redshifts z ∼ 15–20 in its 21-cm absorption signal. This deviation from

cosmological predictions can be understood as a consequence of physics that either lower the hydrogen

spin temperature or increases the radiation temperature through the injection of soft photons in the

bath. In the latter case, standard model neutrino decays ν

→ ν

γ induced by effective magnetic and

electric transition moments (μ

eff

) are precluded by the tight astrophysical constraints on μ

eff

. We show

that if mirror neutrinos are present in the bath at early times, an analogous mechanism in the mirror

sector can lead to a population of mirror photons that are then “processed” into visible photons through

resonant conversion, thus accounting for the EDGES signal. We point out that the mechanism can work

for mirror neutrinos which are either heavier than or degenerate with the standard model (SM) neutrinos,

a scenario naturally realized in mirror twin Higgs models.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

After recombination the universe was ﬁlled with radiation, dark

matter (DM) particles and primordial gas (mainly hydrogen). This

cosmic stage, known as the “dark ages”, lasted until the formation

of the ﬁrst structures, an event that started when Compton scat-

tering

processes could not maintain the gas and the radiation in

equilibrium. The gas, being cooled faster than the radiation ﬁeld,

got gravitationally trapped in DM haloes and eventually ended up

collapsing and fragmenting, giving rise to the appearance of stars,

quasars and galaxies. At lower redshifts the Lyman-alpha pho-

tons

emitted by these ﬁrst structures led to a re-ionization period,

known as the re-ionization era.

The

only known observable with which the dark ages can be

observationally accessed is the 21-cm line of the ground-state hy-

perﬁne

transition of atomic hydrogen [1]. This probe provides as

well a way to test the re-ionization epoch, thus allowing the study

of the cosmic time when astrophysical objects became the dom-

inant

source of the intergalactic medium. The cosmic microwave

Corresponding author.

E-mail

addresses: daristizabal@ulg.ac.be (D. Aristizabal Sierra),

cheesheng.fong@gmail.com (C.S. Fong).

background (CMB) photons are resonantly absorbed by the hy-

drogen

atoms, thus producing a change in the CMB brightness

temperature T

, which at present time depends upon cosmolog-

ical

parameters, redshift and the radiation and spin temperatures

CMB

and T

(see discussion in sec. 2) the latter characterizing the

relative population of the hyperﬁne energy levels of neutral hy-

drogen.

is determined by the coupling with T

CMB

through the

absorption of CMB photons as well as by its coupling with T

gas

that happens through either collisions among the hydrogen atoms

or the absorption of Lyman-alpha photons. In the absence of non-

standard

physics, both T

CMB

and T

are well determined and so is

the brightness temperature. Observation of any deviation on this

prediction can therefore be interpreted in terms of new physics

effects, for instance non-Gaussianities [2,3]or baryon-DM interac-

tions

[4,5].

Recently

the Experiment to Detect the Global Epoch of Reion-

ization

Signature (EDGES) has reported on the measurement of the

CMB brightness temperature. The signal is the result of the recou-

pling

of T

gas

and T

due to Lyman-alpha photons from early stars.

The observed absorption proﬁle is centered at around z  17 and

covers redshifts in the range 15–20 [6]. To a large extent, the pro-

ﬁle

is consistent with cosmological predictions, but the observed

amplitude indicates more absorption than expected. The, arguably,

most simple explanation would be an earlier T

CMB

− T

gas

decou-

https://doi.org/10.1016/j.physletb.2018.07.047

0370-2693/

SCOAP

D. Aristizabal Sierra, C.S. Fong / Physics Letters B 784 (2018) 130–136 131

pling (at z  250 rather than z  150), which would produce an

earlier cooling of the gas. This, however, does not work since it re-

quires

the ionization fraction to be less than the expected fraction

by about an order of magnitude, something strongly disfavored by

Planck data [6].

explanation of the observed spectral proﬁle requires either

decreasing T

gas

(gas cooling) or increasing T

CMB

(radiation heat-

ing).

And indeed since the release of the EDGES result both al-

ternatives

have been studied in the literature. Ref. [7] considered

DM-baryon scatterings determined by a velocity-dependent cross

section resulting from a Coulomb-like interaction. After discarding

the possibility of a light mediator due to ﬁfth force constraints,

refs. [8,9]showed that subdominant millicharged DM can explain

the 21-cm spectral feature, despite in a constrained region in pa-

rameter

space that must be endowed with an additional depletion

mechanism to prevent overproduction. It has been pointed out

that these constraints can be relaxed provided the millicharged

DM is produced after recombination [10]. Ref. [11] discussed var-

ious

mechanisms, among which those based on the emission of

soft photons that can heat up the radiation temperature [12]. Us-

ing

dipole DM as a benchmark model [13], it ruled out this kind

scenarios. Other mechanisms put forward include black hole

remnants from Pop-III stars [14], interacting dark energy models

[15], charge sequestration models [16] and more relevantly for our

study dark-photon to photon resonant conversion [17]. This lat-

ter

relies on a non-thermal population of dark photons, resulting

from the decay of an unstable relic, which are then resonantly

converted into photons at redshifts z  17. Additional references

include the 21-cm ﬂuctuations induced by millicharged DM, con-

straints

on DM and primordial black holes derived from the EDGES

signal and axion–photon resonant conversion [18–22].

Neutrinos

can couple to electromagnetic radiation through elec-

tric

charge (milli-charged), electric/magnetic dipole (transition)

moments and/or anapole moments. Of these couplings those bet-

ter

understood and probably with best experimental prospects are

magnetic dipole/transition moments μ

eff

(see e.g. ref. [23]for a re-

view).

They enable neutrino decay processes ν

→ ν

+ γ and so

—in principle— could contribute to the radiation temperature at

early times. However, they contribute as well to astrophysical pro-

cesses

of which stellar cooling places pretty stringent constraints

on their values μ

eff

 3.0 × 10

−12

(with μ

the Bohr magne-

ton)

[24]. In this paper, we start by checking whether despite these

bounds one could moderately raise the radiation temperature by

injecting photons though neutrino decays. After showing that the

bounds on μ

eff

always lead to a suppressed photon ﬂux, we then

entertain the possibility that mirror neutrinos endowed with the

same type of couplings can inject a suﬃciently high photon ﬂux

so to enable addressing the EDGES anomalous spectral feature. We

study in detail mirror neutrino decays to mirror (dark) photons,



→ ν



+ γ



, occurring at high redshift and then getting reso-

nantly

converted into visible photons γ



→ γ .

For that aim we

consider two scenarios inspired in mirror twin Higgs models de-

ﬁned

by degenerate and non-degenerate SM and mirror neutrino

masses with T



< T (where T



and T refer to the mirror and SM

temperatures respectively), as required by cosmological constraints

on additional dark radiation N

eff

[27].

The

rest of the paper is organized as follows. In sec. 2 we dis-

cuss

generalities on the 21-cm absorption signal and settle the

conditions required for addressing the EDGES signal. In sec. 3 we

consider the case of SM neutrino decays during the redshifts rel-

evant

for EDGES, we discuss in more detail current bounds on

For pioneer works on neutrino decay and mirror neutrino cosmology/astro-

physics

see e.g. refs. [25,26].

neutrino transition moments and calculate the photon ﬂux assum-

ing

eff

is a free parameter. In sec. 4 we consider mirror neutrino

decays and resonant conversion of dark photons into visible ones.

We then provide a theoretical motivation in sec. 5, based on mirror

twin Higgs models, for the mirror neutrino scenarios we consider.

In sec. 6 we summarize and present our conclusions.

2. Generalities

During the recombination era (z ∼ 1100) electrons and protons

recombined to form neutral hydrogen. As shown by the high de-

gree

of isotropy of the CMB, the universe was highly uniform at

that time thus suggesting that few, if any, luminous objects could

have formed. Adiabatic expansion thus led to a stage in which

the universe consisted mainly of a neutral gas, CMB photons and

DM particles, a cosmic stage known as the dark age. The uni-

verse

evolved adiabatically and the radiation temperature, T

CMB

decreased with redshift according to T

CMB

= 2.7(1 + z) K. The re-

maining

small ionization fraction, X

= n

/n, enabled the injection

of energy from the CMB to the gas through Compton scattering

processes, thus keeping both baryons and radiation at the same

temperature until z ∼ 150.

The

virial temperature of a DM halo (T

vir

) determines the bind-

ing

energy of the material within the halo. Accordingly, only gas

for which T

gas

< T

vir

can be trapped by the halo gravitational pull.

For z  150, Compton scattering effects became less effective and

so the temperature of the gas decreased faster than the radiation

temperature. The gas then was trapped by the DM halo, but the

shocks induced by the gravitational collapse heated up the gas

to T

vir

, thus driving the system to hydrostatic equilibrium. After

departing from this state, the gas contracted within the halo and

became gravitationally stable, at some point it fragmented and led

to the formation of the ﬁrst stars, quasars and galaxies. The high-

energy

radiation emitted from these ﬁrst objects reionized the hy-

drogen

in the intergalactic medium, leading to the re-ionization

epoch.

The

only known observable with which the dark age period

can be studied is the redshifted hydrogen hyperﬁne transition

spectral line. It enables as well detailed studies of the epoch of

re-ionization such as structure formation and the formation of

the ﬁrst galaxies. The ground state of neutral hydrogen is split

into two hyperﬁne states due to proton-electron spin-spin cou-

pling:

a singlet, corresponding to the anti-alignment of the two

spins and a degenerate triplet state corresponding to the align-

ment

of both spins. The energy splitting between these states is

E = E

− E

 5.9 μeV, which corresponds to a ∼ 21 cm photon

wavelength and a rest-frame frequency ν

= 1420 MHz, redshifted

as ν(z) = 1420/(1 + z) MHz. Some of the CMB photons propagating

in the medium can be absorbed by hydrogen resulting in a singlet-

triplet

transition which modiﬁes the brightness temperature of the

CMB according to [28]

(z) = T

CMB

(z)e

−τ (z)



1 − e

−τ (z)



(z). (1)

Here T

CMB

(z) is the brightness temperature of the CMB without

absorption, T

(z) is the spin temperature which characterizes the

relative population of the triplet to the singlet states. The optical

depth reads

τ (z) =

(z)

32πν

(z)H(z)

(2)

with A

 2.9 × 10

−15

−1

the spontaneous decay rate for the

excited to the ground hyperﬁne states, n

the density of neu-

tral

hydrogen, c the speed of light, h

the Planck constant, k

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