Insights on Dark Matter from Hydrogen During Cosmic Dawn

Insights on Dark Matter from Hydrogen during Cosmic Dawn

Julian B. Mu˜noz∗ Department of Physics, Harvard University, 17 Oxford St., Cambridge, MA 02138

Abraham Loeb Astronomy Department, Harvard University, 60 Garden St., Cambridge, MA 02138 (Dated: March 28, 2018) 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 ∼ 10−6—in units of the electron charge—and masses mχ ∼ 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.

The dynamics of our Universe is strongly influenced by be most prominent [26–28]. We will use this insight to the pervasive, albeit elusive, dark matter (DM), known explore the possibility that the DM interacts with the to outweigh regular baryonic matter roughly five to standard model (SM) through a small “minicharge” un- one [1, 2]. Despite this, little is known about its nature der the usual electromagnetic force. and composition [3, 4]. All the evidence for DM relies on We adopt as a benchmark the measurement of its gravitational pull on baryons, and thus does not re- the EDGES collaboration [29], which—if confirmed— quire any nongravitational interactions between the two indicates that baryons have a temperature of T ≈ 4 fluids. Nonetheless, some level of nongravitational in- b K at z ≈ 17, roughly half of its expected value. This teractions can naturally explain their comparable cosmic poses a problem to the standard cosmological model, as abundances [5–7], as well as perhaps solve the well-known astrophysical processes would produce baryonic heating, small-scale problems in the pure cold-DM models [8]. As rather than cooling [28]. However, elastic DM-baryon of now, a vast array of direct- and indirect-detection ex- scattering can produce thermalization between the cold periments [9–12], cosmological observations [13–17], and DM and the baryons, thus explaining this “cold-baryon” accelerator-based probes [18, 19], have not been able to problem1. In this work we will explore the region of the find conclusive evidence for nongravitational interactions DM charge-mass plane that can be probed by 21-cm ob- between DM and baryons, placing stringent constraints servations, and compare with current constraints. We on the origin of the DM, and on its interactions with will argue that the minicharges required to cool down the standard-model particles. baryons would be too large to allow the entirety of the A novel arena on the search for DM-baryon interac- DM to be charged, as it would not behave as a cold col- tions can be found at cosmic dawn. During this era, lisionless fluid. Nonetheless, we will show that if a frac- −2 the first stars were formed [20, 21], which coupled the tion fdm . 10 of the DM is composed of particles with −6 −2 spin temperature of neutral hydrogen to its much-lower charge ∼ 10 , and mass mχ . 60 MeV × (fdm/10 ), kinetic temperature [22, 23], causing cosmic-microwave- the baryonic temperature can be reduced by a factor of background (CMB) photons with a local wavelength of two, while being consistent with all other constraints. We arXiv:1802.10094v2 [astro-ph.CO] 26 Mar 2018 21 cm to be resonantly absorbed by the intervening neu- will conclude with some remarks about the 21-cm fluctu- tral hydrogen. Eventually, however, the baryonic gas was ations induced by these interactions. heated by stellar remnants (such as X-ray binaries), and Endowing the dark matter with a small electric charge the hydrogen spin temperature increased above that of has unique phenomenological consequences, as charged the CMB, causing 21-cm emission [24, 25]. The absorp- particles respond to background magnetic fields, which tion era provides one of the lowest-velocity environments are rather common in astrophysical environments. In in our Universe, where DM interactions with the visible sector mediated by a massless field are expected to

1 After this manuscript was completed a diﬀerent explanation was put forward in Ref. [30], where it was suggested that an exotic ∗Electronic address: [email protected] radio excess at high redshift could potentially bias 21-cm results. 2

Ref. [31] it was argued that supernova shocks would eject the momentum-transfer integral [14, 34]. This factor is all minicharged particles from the Galactic disk, while the roughly constant during the era of interest, so we will set Galactic magnetic field, known to extend beyond Galac- it to tic heights of 3 kpc [32], would prevent their reentry. 9T 3 ξ = log b ≈ 68 − 2 log , (2) Given that the dark-matter density within 1.5 kpc of the 4π2α3x n 10−6 disk is in agreement with predictions [33], we can con- e H clude that not all DM can be evacuated from the disk, where we adopt a fiducial baryonic temperature Tb = 10 and thus, minicharged particles with charges larger than K, and we evaluate the free-electron fraction xe, and the −16 −1 number density n of hydrogen nuclei, at redshift z = 20. /mχ & 5 · 10 MeV [34] (barring some fraction near H the edge of that constraint, which could diffuse back to The velocity behavior of the cross section in Eq. (1) is the disk [35]), are precluded from being the entirety of indicative of where these interactions will be most promi- the cosmological DM. Here, and throughout, we define nent: wherever the DM-baryon (DM-b) fluid is slowest. The relative velocity between the DM and baryons is not the minicharge ≡ eχ/e in units of the electron charge only determined by their thermal motion. Baryons are e, where eχ is the DM charge, and we work in natural units, with ~ = c = 1. impeded to collapse until recombination, whereas the DM Minicharged particles can, however, avoid disk ejection is not, causing a velocity difference between them [43]. if they cool efficiently. Moreover, we estimate the Galac- Assuming that the DM is not strongly coupled enough tic magnetic-field energy density to be at least three or- to dissipate this velocity prior to recombination, this ve- ders of magnitude smaller than the DM kinetic energy locity shows acoustic oscillations on Mpc scales [43], and density in the Solar vicinity. Thus, DM could in prin- has a root-mean-square (rms) value of vrms = 29 km −1 ciple be able to breach through magnetic-field lines and s ×[(1+z)/(1+zkin)] after kinetic decoupling, starting reenter the disk (albeit altering the magnetic-field struc- at zkin ≈ 1010. In Ref. [26] it was shown that DM-b in- ture of our Galaxy). We note, however, that indepen- teractions cause a drag, D(vχ,b) ≡ dvχ,b/dt, on the DM-b dent constraints can be achieved by requiring the DM relative velocity vχ,b, which here we recast as to not be trapped in coherent regions of magnetic field X mχnχ + ρb ρt F (rt) D(v ) = σ¯ , (3) in galaxy clusters, which have typical correlation lengths χ,b t m + m ρ v2 t=e,p χ t b χ,b rcorr ∼ 10 kpc and field strengths B ∼ 5 µG [36]. This means that charged particles with charges larger than where fHe ≡ nHe/nH ≈ 0.08 is the Helium fraction, mt −17 −1 /mχ & 3 · 10 MeV would not be distributed as is the target mass, ρb is the energy density of baryons, cold dark matter, but instead clump wherever magnetic and ρt = xemtnH is the energy density of target parti- fields are coherent (or, if the DM breached through the cles. This drag depends on the minicharged-DM number field lines, these regions should lose coherence). Addi- density, given by nχ = fdmρd/mχ, where ρd is the (total) 3 tional constraints can be derived through plasma effects DM energy density (ρd = Ωc(1 + z) ρcrit). Here we have in cluster collisions, such as the bullet cluster [37], as well defined the function as by requiring the minicharged particles to not diffuse r rt 2 −r2/2 within clusters [38], although simulations might be re- F (rt) = Erf √ − rte t , (4) quired to isolate these effects from nonlinear gravity [39]. 2 π The constraints discussed thus far would not apply if where rt ≡ vχ,b/uth,t, and the (iso)thermal sound speed only a fraction of the DM is charged, as the majority of of the DM-t fluid is given by DM would behave as expected. Given that the local DM s measurements are accurate within tens of percent, we Tb Tχ uth,t = + , (5) will focus on the possibility that the minicharged parti- mt mχ cles compose a small fraction fdm ≤ 0.1 of the dark matter, while the rest of it is neutral. This can be naturally where Tχ is the minicharged-DM temperature. By com- achieved if DM forms “dark atoms” [40, 41], and there is paring this sound speed with the relative velocity, we can a free charged-DM fraction after its recombination [42]. see that, immediately after recombination (and prior to Nonetheless, we will posit no assumptions about the ori- the X-ray heating of the baryons), the baryonic sound gin of the minicharged particles, and parametrize them speed falls below the DM-proton relative velocity, making the DM-proton (albeit not the DM-electron) fluid through their mass mχ and charge . In that case, the momentum-transfer cross section between a minicharged “supersonic”. particle and a target t (electron or proton) is [14, 34] In addition to this drag, interactions between DM and baryons will tend to bring both fluids into thermal equi- 2πα22ξ librium. This gives rise to a baryonic heating [26] σ¯t = 2 4 , (1) µχ,tv ˙ xe X mχmt σ¯t Qb =nχ 2 × 1 + fHe (mχ + mt) uth,t where µχ,t is the DM-target reduced mass, α is the fine- t=e,p structure constant, and v is the relative velocity be- "r −r2/2 # 2 e t F (rt) tween the two particles. We have defined the Debye × (Tχ − Tb) + mχ . (6) π u2 r logarithm ξ, which regulates the forward divergence of th,t t 3

Here we have included DM interactions with both pro- We solve the Eqs. (7a-7d), starting at zkin, for different tons and electrons, as the latter can dominate if the (i) (i) initial velocities vχ,b, and find Tb(z, vχ,b). In order to minicharged-DM fluid is not cold. The DM heating can remove dependencies on the coupling between the spin be found by symmetry, through nχ → nH × (1 + fHe), and kinetic temperatures, we define the average baryonic mχ ↔ mt, and Tχ ↔ Tb. Notice that Q˙ b can in principle temperature as change signs depending on r , as for r → 0 (correspond- t t Z ing to vχ,t uth,t) only the temperature-dependent (i) (i) (i) hTb(z)i = dv P(v )Tb(z, v ), (10) term survives, corresponding to the usual thermalization; χ,b χ,b χ,b whereas for r 1 (which implies v u ), the heat- t χ,b th,t where the initial-velocity PDF is given by a Maxwell- ing term proportional to F (rt) can dominate, converting the mechanical energy of the relative velocity into posi- Boltzmann distribution tive heat for both fluids. 2 −3 v2/(2v2 ) 4πv e rms Equipped with Eqs. (3) and (6), we can now obtain P(v) = , (11) (2πv2 /3)3/2 the thermodynamical evolution of these two systems by rms following Ref. [26] and solving −1 with an rms velocity vrms = 29 km s at decoupling [43]. We show, in Fig. 1, the line in the − mχ plane that T˙b = −2HTb + 2Q˙ b/3 + ΓC (Tγ − Tb) (7a) would give a baryonic temperature of hTbi (zcentral) = 4 T˙χ = −2HTχ + 2Q˙ χ/3 (7b) K, for different values of fdm. We find that, given fdm, h i the minicharge required always satisfies ∝ mχ for x˙ = −C n A x2 − 4(1 − x )B e3E0/(4Tγ ) (7c) e H B e e B mχ < 6 GeV × fdm. However, there is no simple an- alytic solution for the slope of this line as a function of v˙χ,b = −Hvχ,b − D(vχ,b), (7d) fdm. This is because for small energy transfers the baryon ˙ 2 2 where H is the Hubble parameter at time t, C is the Pee- heating is Qb ∝ fdm /mχ, whereas for large energy bles factor [44–46], E is the ground-level energy of the ˙ 5/2 2 2 0 transfers (and assuming fdm < 0.2), Qb ∝ fdm /mχ, hydrogen atom, and AB and BB are the effective recombi- given the uχ scaling in Eq. (9). We have empirically nation/reionization coefficients, obtained from Ref. [47]. found that We have ignored photoheating and recombination cool- −3/4 ing [48], as well as possible baryonic heating due to DM −7 mχ fdm (mχ, fdm) ≈ 6 · 10 , (12) annihilations, if these were present [49]. Here Tγ is the MeV 10−2 temperature of the CMB photons, and the Compton thermalization rate is is sufficient to reduce the baryonic temperature by a factor of two, although we emphasize that the 21-cm results 4 8σT arTγ xe of Fig. 1 have been calculated numerically for each value ΓC = , (8) 3(1 + fHe)me of fdm. We will now briefly review constraints on minicharged where σT is the Thomson cross section, and ar is the dark matter on the literature, to find how the 21-cm pre- Stefan-Boltzmann constant [45, 50]. ferred regions in Fig. 1 compare. The EDGES measurement [29] requires the baryon Minicharged particles lighter than ∼ 100 keV could be temperature to be Tb ≈ 4 K at a central redshift zcentral = produced in stellar objects, such as white dwarfs and red 17, a factor of two smaller than the usual result. In order giants, cooling these objects too rapidly [55]. Similarly, to halve the baryonic temperature with DM-b interac- minicharged-particle production during the supernova tions, we require mχ < µ¯b fdmΩc/Ωb, whereµ ¯b ≈ 1.2 mp 1987A would have altered its neutrino luminosity, which is the mean molecular weight of baryons, due to sim- places constraints in the range 2 × 10−6 < < 10−9, ple equipartition. Thus, we will only show results for where the upper limit is due to self-absorption [51, 56]. mχ ≤ 6.2 GeV×fdm. Moreover, for illustration purposes, We label this limit as SN1987A in Fig. 1. we note that transferring half of the baryonic thermal en- Current accelerators only limit minicharges & ergy to the DM at z = zcentral would induce a DM sound 10−2 [57], with the exception of the SLAC millicharge ex- speed of periment, which constraints charges larger than ∼ 10−4 for mχ < 100 MeV [53]. We show this constraint in −12 2 Tχ Tb Ωb 0.1 km s Fig. 1 labeled as SLAC mQ. We note that a proposed uχ(zcentral) = ≈ ≈ , (9) mχ µ¯b fdmΩc fdm extension to the LHC would allow it to probe the range ∼ 10−3 [58], for the entire mass range we consider. which redshifts as (1 + z). Interestingly, for fdm . 0.2, Measurements of the matter power spectrum, from this would give the minicharged component of the DM the CMB and the Lyman-α forest, can constrain the −9 a sound speed larger than that of protons, which then DM minicharge to be < 2 · 10 × (mχ/MeV), for determines the value of uth,p in Eq. (5). Moreover, we fdm = 1 and DM masses between an MeV and a GeV [14] −2 estimate that for fdm . 10 the DM-e interactions can (see Ref. [59] for an updated constraint). However, if dominate over the DM-p, due to this velocity. minicharged particles do not comprise all the DM these 4

bang nucleosynthesis [52]. We label the constrained re- FIG. 1: Regions of the minicharged-particle parame- gion as BBN in Fig. 1. Moreover, if a dark photon (γ0) ter space that can be explored by 21-cm observations, is present—as is expected to obtain minicharged parti- and current constraints. Each solid line represents the cles [61], although not necessary [62]—this particle can minicharge required to reduce the baryonic tempera- also appear as light degrees of freedom during both BBN ture by a factor of two, if a fraction fdm of the DM is and the CMB [54, 63]. We can estimate for what value minicharged, where the color scales from black to red as of the minicharge dark photons would be produced, by fdm decreases. Each line ends at mχ = 6 GeV × fdm, as requiring that the timescale for two minicharged particles heavier particles would not be able to produce enough to annihilate into dark photons is longer than a Hubble cooling. We note that, if fdm = 1, all charges in the plot time. For minicharged particles in thermal equilibrium are ruled out, as explained in the main text. In hatched with the SM in the early universe, their rate of annihila- we show the regions excluded by supernova cooling from tion into dark photons2 is [54] 1987A (blue) [51] (updated from Ref. [56], shown for ref- −3 04 erence in blue short-dashed line), from a change in BBN Γχχ¯→γ0γ0 = nχσv ≈ 10 g Tγ , (13) (orange) [52], and constraints from the SLAC millicharge experiment (brown) [53]. The purple hatched region rep- where g0 is the coupling constant between χ and γ0. By 2 resents the region above which DM would have efficiently requiring this rate to be smaller than H ≈ Tγ /Mpl, where annihilated into dark photons (if present), and caused a Mpl is the reduced Planck mass, we can obtain a con- 0 change in Neff & 1 [54]. Finally, in green long-dashed straint on g , so that DM does not annihilate to dark line we show the minicharge required to obtain the right photons before Tγ = mχ. Since the DM minicharge, is DM abundance, for fdm = 1, computed with Eq. (15). the product of the dark-photon mixing κ times the dark coupling g0, and we require κ < 1, this translates into the approximate constraint 10-3 1/4 −5 mχ SLAC mQ Th. Rel. . 2 × 10 , (14) 10-4 Neff MeV BBN -2 -1 for mχ ≤ ΛQCD ≈ 200 MeV, where ΛQCD is the QCD -5 -3 10 =10 10 fdm scale, which we label as Neff in Fig. 1. Here we have ϵ -4 10 10 assumed that χ are spin-1/2 particles, and we note that 10-6 this constraint can, of course, be tightened if κ 1, extending to χ masses as high as a GeV [54]. In the standard freeze-out scenario, the DM production 10-7 SN1987A is halted when the baryonic temperature drops below its mass, and its annihilation rate determines the relic abun- 10-8 0.3 1 3 10 30 100 300 dance left in the dark sector. To compute the minicharge required to produce the right DM abundance, we use the m [MeV] χ approximate formula

x 10−26cm3s−1 Ω h2 ≈ 0.1 × f , (15) limits are less strict, and in particular, even particles with c 10 (σv) −6 0.3 minicharges as large as & 10 × (mχ/MeV) , which would be in thermal contact with baryons at the CMB where xf = mχ/Tf , Tf is the freeze-out temperature, epoch, are allowed to compose up to a percent of the and for minicharged dark matter the annihilation cross DM [60]. This closes the apparent gap for mχ & 200 MeV section to fermions is [34] in Fig. 1, as the 21-cm data would require a fraction of s ! the DM with charges above this threshold that is above 2 2 m2 m2 −2 πα f f a few percent. Thus, we will focus on the fdm < 10 (σv) = 2 1 − 2 1 + 2 . (16) case for the rest of this Letter. mχ mχ 2mχ So far it has been suﬃcient for us to assume that We will ignore any dark-sector interactions, and for sim- minicharged particles compose a fraction fdm of the dark plicity, we will only consider annihilation into electron- matter, regardless of their origin. However, the cos- positron pairs. To obtain a simple estimate for this quan- mology of minicharged particles can place constraints tity we further approximate x to be a constant, as it only on their charge. Particles with minicharges larger than f −8 1/2 & 10 (mχ/MeV) , which encompasses the region of interest, would reach equilibrium with the visible sector in the early Universe. This severely limits minicharged 2 Note that χ annihilations into γγ0, or Compton-like processes particles lighter than electrons, as they would appear (χγ → χγ0) would be suppressed by a κ2 factor, and at most as additional light degrees of freedom (Neﬀ ) during big alter the constraints by a O(1) factor. 5

−6 depends logarithmically on the DM mass and charge, and minicharges ∼ 10 (mχ/MeV), if the minicharged- find the region of the − mχ plane that produces the particle density on the Earth’s surface was one percent of right DM relic abundance, which we show3 in Fig. 1. A the Solar-vicinity DM density. This result is comparable more detailed discussion can be found, for instance, in to that required for baryonic cooling during cosmic dawn, Refs. [34, 54]. From Fig. 1 it is clear that—barring a although the specific number is controlled by the frac- −6 small region for mχ ∼ few × MeV, and & 10 —most tion of the minicharged-DM that would diffuse to Earth. of the parameter space we are considering is below this More importantly, the cross section of these minicharged thermal-relic line, implying that minicharged DM could particles would be similar to the atmospheric column not annihilate efficiently into SM particles, and would density, so any constraints would depend strongly on the overclose the Universe. Particles heavier than ∼ 200 MeV DM momentum loss during atmospheric entry. could annihilate to light dark-sector fields, effectively pro- Let us now study how a change in baryonic temper- ducing a small ∆Neff ∼ 0.1 [64, 65]. This small change ature translates into an observable 21-cm temperature. of Neff is below the sensitivity of current CMB probes, The brightness temperature of the 21-cm line can be writ- although within the reach of next-generation CMB ex- ten as [78] periments [66]. However, for DM lighter than ∼ 200 2 1/2 MeV, the SM bath has been heated by the QCD phase 21 Ts − Tγ xHIΩbh 0.15 1 + z T = 27 mK 2 , transition, and any populated light degrees of freedom Ts 0.023 Ωmh 10 in the dark sector will leave a trace on the CMB, as (17) they cause ∆Neff & 1 (depending on the nature of the where xHI is the neutral-hydrogen fraction (approxi- light particles), strongly disfavored by Planck [1]. In mately unity during the region of interest), and Ts is that case, other mechanisms may be invoked to set the the spin temperature of the hydrogen gas. We will use right DM abundance [67–69], such as elastically decou- the solutions for Tb of Eqs. (7a-7d), assuming full Ly-α pling relics [70], cannibal dark matter [71], or having 3- coupling (so Ts = Tb) [79], to obtain the sky-averaged to-2 annihilations dominate [72]. It is beyond the scope 21-cm temperature [80, 81] of this work to build a dark-sector model producing the Z right relic abundance, so we leave this question for future T 21 ≡ T 21 = dv P(v )T 21 [T (v )] , (18) work. avg χ,b χ,b b χ,b Interestingly, minicharged particles can remain in which we show in Fig. 2 for a specific choice of /m , the Galactic disk if they cool efficiently, for which χ and f . This Figure shows that the T 21 data from minicharges > 10−5(m /MeV)1/2 are required [31]. dm avg χ EDGES [29] is in tension with the maximum absorption This might, nonetheless, not lead to direct-detection sig- possible without DM-b interactions, whereas this tension nals in, e.g., the Xenon [73], CRESST [74], or LUX [75] is alleviated when introducing minicharged particles. experiments; as the gyroradius of these particles is r ∼ g Moreover, DM-b interactions do not homogeneously 10 km (m /MeV) (/10−5)−1, on the terrestrial mag- χ cool down the baryons. The DM-b relative velocity, with netic field of B ∼ 0.1 G. They might, however, be ⊕ fluctuations over Mpc scales [43], modulates the overall detectable through atmospheric ionizations, acting as a cooling/heating, thus sourcing additional 21-cm fluctu- nox borealis, similar to the regular aurora borealis pro- ations [26]. We can estimate the size of these fluctua- duced by solar-wind particles. We point out, however, tions by finding the root mean square (rms) of the 21-cm that Earth-based experiments could be sensitive to even brightness temperature, defined as a minuscule trace of minicharged particles, as these can interact rather strongly. Given that neither disk ejec- q 21 21 2 21 2 tion, due to the complex astrophysics of the interstellar Trms ≡ h(T ) i − hT i . (19) medium; nor the terrestrial magnetic field would have We show this function in Fig. 2, where we can read that perfect efficiency shielding the Earth from these parti- the same interactions that cause a lower baryonic tem- cles, there might be hope for direct detection, especially perature also cause additional fluctuations, of amplitude through surface- and space-based experiments (as op- T 21 ≈ 10−2 T 21 for f = 10−2. These are compara- posed to underground facilities). An example are the rms avg dm ble to the Mpc-scale adiabatic fluctuations at z ∼ 17, limits from the X-ray-calorimeter [76], which, however, of order δ (k = 0.1 Mpc−1, z = 17) ∼ 0.03, thus per- do not constrain these particles if their masses are be- b haps making them detectable with the upcoming HERA low ∼ 100 MeV. Additionally, we estimate that torsion- experiment [82], or the more-futuristic SKA [83]. No- balance experiments [77], which can constrain acceler- tice, however, that even in the absence of interactions ations as small as 10−13 cm s−2, could be sensitive to the DM-b relative velocity can affect the formation of the first structures in our Universe, and thus the 21-cm intensity [84–87].

3 Our results apply for minicharged particles interact- We have assumed fdm = 1 to obtain the thermal-relic line. To obtain a smaller fraction of DM with minicharges, the rest of ing through any massive dark photon, as long it is it would have to form dark atoms, or otherwise have a larger lighter than the typical momentum transfer, which is minicharge. ∼ eV − keV for DM masses in the MeV−GeV range. 6

ator would give rise to an anomalous ﬁfth force [89, 90]. 0 So far we have conservatively assumed that the small T21 (ϵ=0) fraction of DM that is charged does not thermalize with -200 avg ] 21 the rest of the DM. If it did, one could simply rescale our T avg −1/2 -400 results for from the fdm = 1 case by (fdm) . More-

mK over, as a check, we have estimated the size of the interac- [ -2 21 -600 21 f = 10 tions of minicharged particles with the neutral baryonic -50 xT rms dm

T medium through Linhard’s formula [34, 91], and found ϵ 5 * 10-7 -800 = that they are always subdominant, by at least four or- m MeV -1000 χ ders of magnitude. 15 16 17 18 19 20 In this Letter we have explored the possibility that part of the DM is charged under the usual electromag- z netic force, albeit with a small minicharge . In that case, its momentum transfer with baryons is largest when their FIG. 2: We show the 21-cm brightness temperature, relative velocity is smallest, which occurs prior to reion- obtained with Eq. (18), assuming full Lyman-α coupling ization, during cosmic dawn. We have shown that, while and no X-ray heating, both in the case with (in red-solid minicharged particles cannot comprise the majority of line) and without (in green-dashed line) DM-b interac- the DM, if a sub-percent fraction of the DM has a charge −6 tions, assuming that minicharged particles compose 1% ∼ 10 , and mass mχ ∼ 1−60 MeV, it can cause signifi- −7 of the DM and have a charge = 5 · 10 × (mχ/MeV), cant baryonic cooling, while being consistent with all cur- with a mass mχ . 60 MeV. The red data point represents rent observations. This cooling would lead to deeper 21- (21) +200 the data from EDGES [29], of T = −500−500 mK, at cm absorption, as recently reported by the EDGES col- 3-σ. We also plot, in blue long-dashed line, the rms of laboration [29], perhaps hinting at a dark-matter origin the 21-cm temperature due to velocity fluctuations for for the discrepancy. We conclude that, through their ac- this case, multiplied by a factor of −50. cess to the coldest epochs of our Universe, 21-cm experiments, such as EDGES [29], LEDA [92], and SARAS [93], provide us with a unique window into the dark sector, Additionally, we can easily translate our results for a which may furnish the first nongravitational detection of DM minicharge to a new hadro/lepto-phillic DM inter- the cosmological dark matter. action mediated by a light scalar φ. For fdm = 1 we −8 found that a DM minicharge of ≈ 10 × (mχ/MeV) is sufficient to decrease the baryonic temperature by a factor of two (although we remind the reader that this Acknowledgments case is ruled out for minicharges). Even ignoring DM self interactions [88], and setting the φ-DM coupling gχ We wish to thank Prateek Agrawal, Yacine Ali- to unity, the φ-nucleon coupling required would be gN = Haimoud, Cora Dvorkin, Marc Kamionkowski, Ely −1/2 −11 −4 8πα(¯xe) ≈ 2·10 ×(mχ/MeV), wherex ¯e ≈ 2·10 Kovetz, David Pinner, Christopher Stubbs, and Shawn during the era of interest. Similarly, the φ-electron cou- Westerdale for enlightening discussions, and Hongwan −1/2 pling required would be ge = 8πα(¯xe me/mp) ≈ Liu and Tracy Slatyer for finding a mistake in an ear- −9 10 × (mχ/MeV). For DM in the MeV-GeV range (and lier version of Eq. (3). This research is supported in part thus mediators with mφ . keV), this is constrained by by the Black Hole Initiative, which is funded by a John stellar cooling, whereas for lighter dark matter the medi- Templeton Foundation grant.

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