# A composite solution to the EDGES anomaly

AA composite solution to the EDGES anomaly

Anubhav Mathur, Surjeet Rajendran, and Harikrishnan Ramani Department of Physics and Astronomy, Johns Hopkins University,3400 N. Charles St., Baltimore, MD 21218, USA Stanford Institute for Theoretical Physics, Stanford University, Stanford, CA 94305, USA

Subcomponent millicharged dark matter that cools baryons via Coulomb interactions has been in-voked to explain the EDGES anomaly. However, this model is in severe tension with constraints fromcosmology and stellar emissions. In this work, we consider the consequences of these millichargedparticles existing in composite blobs. The relevant degrees of freedom at high temperature areminuscule elementary charges, which fuse at low temperatures to make up blobs of larger charge.These blobs serve as the degrees of freedom relevant in cooling the baryons suﬃciently to accountfor the EDGES anomaly. In such a model, cosmology and stellar constraints (which involve high-temperature processes) apply only to the feebly-interacting elementary charges and not to the blobs.This salvages a large range of parameter space for millicharged blobs that can explain the EDGESanomaly. It also opens up new parameter space for direct detection, albeit at low momentumtransfers.

I. INTRODUCTION

The Experiment to Detect the Global Epoch of Re-ionization Signature (EDGES) experiment has reporteda dip in the 21 cm spectrum corresponding to strongabsorption around z = 17 [1]. This can be interpretedas a 3.8 σ deviation from the ΛCDM prediction for thebaryon temperature [2–4]. Dark matter (DM) coolingof the baryonic ﬂuid has been invoked as an explana-tion for this excess [2–4]. A DM model that maximizesthe cross-section around cosmic dawn is sub-componentmillicharged dark matter (mCDM), which has a largercross-section with Standard Model (SM) charges at thelowest relative velocities. However, the millicharge pa-rameter space is extremely constrained due to limits fromCMB and BBN, cooling of SN1987A and stars, and ter-restrial experiments [4]. It has subsequently been shownthat even this limited parameter space results in over-production of mCDM through freeze-in [5, 6].These diﬃculties have led to two other ways to solvethe EDGES anomaly. The ﬁrst involves heating theCMB relative to baryons [7–10], while the second in-volves mCDM which is tightly coupled to an additionalcold component that forms the dominant DM which doesthe bulk of the cooling [11]. In this paper, we pointto a third possibility. The mCDM explanations for theEDGES anomaly to date have treated the millichargedparticle (mCP) as elementary without internal structure.As a result, the same mCP is the physical particle at allenergies. In this work, we explore the consequence ofthis mCP being a composite state of elementary mCPswith much smaller mass and charge, glued together by aforce that conﬁnes at low temperatures. The elementarycharges are the relevant degrees of freedom at tempera-tures and energies much higher than cosmic dawn. As aresult, in our model, constraints from CMB, BBN, over-closure, stellar and SN cooling as well as colliders allapply only to the elementary charges. We demonstratehere that there is a drastic increase in new parameterspace for mCDM as long as it is in a composite state (blob). Furthermore, we explore the quirky thermal his-tory for the dark sector that involves conﬁnement when T D (the dark temperature) falls below Λ D (the dark con-ﬁning scale) and deconﬁnement if the dark temperatureincreases subsequently. We point out a novel dark phasewhere thermal contact with the SM results in a thermo-static dark bath, i.e. the dark bath staying at the sametemperature with the heat dump from baryons exactlycancelled by Hubble cooling. II. MODEL

We consider elementary mCP fermions that carry elec-tric charge (cid:15) f and mass m f with a conﬁning force thatconﬁnes at Λ D (cid:46) m f . We also assume that the mCPs arecharged under a dark U (1) with a dark charge g D . Thisdark U (1) accomplishes two goals: ﬁrst, it allows theelectric charge of the mCP to be generated via kineticmixing and second, it provides Coulomb repulsion thatprevents blobs from getting too big. The mass of thedark photon is unimportant as long as it is suﬃcientlylong ranged to allow the blobs to interact with baryonsvia Coulomb scattering. We can take this mass to bezero, or low enough to evade direct stellar constraints onthe dark photon.For simplicity we will assume that these elementarymCPs that conﬁne all have the same charge sign. Fornet neutrality, we envision an asymmetric dark “leptonic”component with the opposite charge that is not chargedunder this conﬁning gauge group, just like the leptonsin the SM which are singlets under strong interactions.We further assume that the elementary charges conﬁneto form composite blobs with “atomic number” A suchthat the composites χ carry charge (cid:15) χ = A(cid:15) f . The massof these blobs are roughly m χ = A ( m f − Λ D ) i.e. thediﬀerence between the total mass of the constituents mi-nus the binding energy. Since m f (cid:38) Λ D , we approximatethis to m χ = Am f . These mCP blobs also have non-zero a r X i v : . [ h e p - ph ] F e b size [12] R blob = A Λ D (1)We describe the highlights of the phenomenology of thissetup in sub-section II A. These aspects of the phe-nomenology are quantitatively shown in sections II B andII C. A. Synopsis of Blob Evolution

We start by assuming that the temperature T D of theblobs is lower than the conﬁning scale Λ D . Assumingwe are in the early universe where the baryon tempera-ture T b (cid:29) Λ D . We take the elementary mCPs to have alow enough charge that they are not in thermal equilib-rium in the early universe. The cold elementary mCPswill begin to fuse and form larger blobs. As the blobsbecome bigger, they will interact with the baryons andstart extracting energy from the standard model leadingto heating of the blob sector. But, the temperature ofthis sector cannot get larger than Λ D since the blobs arenot conﬁned above this temperature. This implies thatonly a small fraction of the mCPs are actually able tofuse and become blobs in the early universe. Further,the maximum size of the blobs during this phase is setby Coulomb repulsion—as the blob becomes larger, therepulsion from the dark U (1) grows and it inhibits theability of elementary mCPs to fuse with the blob.This scenario continues until T b drops suﬃciently sothat it is unable to transfer enough heat to inhibit blobgrowth. At this stage, there is rapid fusion of the mCPscreating blobs, resulting in most of the mCPs ending upin blobs. The maximum size of the blob in this case isalso set by Coulomb repulsion.Parameters are chosen so that this phase of blob forma-tion occurs around the redshifts of interest to the EDGESexperiment. At this stage, the blobs scatter with thebaryons, cooling the baryons and explaining the EDGESobservations. B. The Size of the Blob

It is necessary to limit the size of the blobs so thatthey can coherently scatter with the baryons, maximiz-ing the heat transfer between the two ﬂuids. The dark U (1) provides the Coulomb repulsion necessary to enforcethis limit. Since g D (cid:29) (cid:15) f , we will ignore the Coulombrepulsion from electromagnetism in this section.How do the blobs form? We follow the prescription de-veloped in [13], but modify it to account for the Coulombrepulsion due to the dark U (1) [14]. Initially, individualpartons merge to form states with atomic number 2. Wecall this the { , } stage. This process can be ineﬃcientsince it requires the emission of some other degree of freedom to make up for the reduced phase space for the2 → Unlike the case considered in [13], inour case as the blobs grow bigger, there is an increasedCoulomb barrier to fusion as treated in [14]. Considertwo cases: the fusion of two large blobs, which we refer toas { A, A } , or a large blob and a small blob which we referto as { A, } . The cross-section for fusion is proportionalto the geometric surface area of the blob. But, as theblobs become larger, this cross-section is suppressed bythe Coulomb barrier. Parametrically, this cross-sectioncan be expressed as: σ A,A (cid:48) = σ (Max( A, A (cid:48) )) P G ( T ) (2)where σ is the base fusion cross-section ∼ Λ − D and P G ( T ) is the Gamow factor [15] which is the temper-ature dependent factor that captures the eﬀects of theCoulomb barrier. This factor is: P G ( T ) = e − G E = e − (cid:114) µQ Q TD (3)Here Q and Q are the charges A , × g D of the twoblobs respectively and µ is their reduced mass. Thus fu-sion freeze-out depends critically on the Gamow factor.From the Gamow factor, it is clear that { A, } fusion willdominate over { A, A } type fusion due to weaker Coulombrepulsion. Moreover, as the blobs grow in size, the num-ber density of larger blobs is lower than that of smallerblobs. Further the cross-section for a smaller blob tomerge with a larger blob is set by the geometric size ofthe larger blob. All of these factors imply that the growthof the blobs in our case is dominated by the mergers ofsmall blobs with larger blobs i.e. processes of type { A, } .Let us now see how big these blobs can get i.e. esti-mate the freeze out of the fusion process. It can easily beveriﬁed that for the parameters of interest to this paper,in the absence of the exponentially suppressed Gamowfactor, the rate of the fusion process is very rapid com-pared to Hubble. The size of the blob is then restrictedpurely by the exponential suppression from the Gamowfactor which forces the process to freeze out.Taking T D ≈ µ ≈ Λ D , for { A, } fusion, the Gamowexponent is G E ≈ Ag D (4)This places a bound on fusion growth, A limGamow ≈ g − D (5) The ratio of fusion rate to Hubble rate for { A, A } fusion is nσvH = 10 f D z A (cid:18)

10 KΛ D (cid:19) (cid:115) T D

10 K (cid:18)

10 K m f (cid:19) (cid:29) This limit on the blob size arising from the inhibitionof their growth is stronger than the stability limit [12] A limstab = 1 /g D that can be placed on their size due toCoulomb repulsion. Blob freeze out occurs only due tothe exponential dependence on blob size in the Gamowfactor. Thus, blobs whose sizes are close to, but smallerthan the Gamow limit are rapidly formed. This impliesthat as the universe expands and the temperatures drop,blobs will continue to grow until the Gamow limit isreached.It is also important to consider the heat that is releasedby the fusion process as the blobs grow. Each fusionprocess that occurs in { A, } fusion releases energy ∼ Λ D .Thus in the roughly ∼ A fusion processes that occur toform a blob of size A , approximately A Λ D energy into ∼ A particles is released. Thus the heat released in thefusion cannot change the temperature by more than Λ D and thus does not hinder fusion.While we expect this mechanism to produce a range ofblob masses, the charge to mass ratio of all these blobs isthe same. As seen later in Sec. IV, for most of the rele-vant parameter space, results depend only on the chargeto mass ratio, so it is justiﬁed to make a simplifying as-sumption that all blobs are of the same mass m χ . Notethat this analysis of the blob size is independent of thebaryon temperature T b . As we show in the following sec-tion, T b is an important parameter in determining thenumber of mCPs that are fused into blobs but it doesnot determine the maximum size of a blob. C. Heat Transfer

In order to understand heat transfer with the SM bath,we start by deriving the transfer cross-section for blobsand elementary charges to scatter with baryons.The diﬀerential cross-section for a mCP with charge (cid:15) to scatter with protons/electrons is [16], dσd cos θ = 2 π(cid:15) α µ v (1 − cos θ ) . (6)with µ the reduced mass and v rel the relative velocity.The forward divergence is cut oﬀ by the Debye massof the mediator. For the SM photon, the Debye masssquared is given by Π A = e (cid:18) x e n b T b (cid:19) (7)where x e ≡ n e /n H is the free-electron fraction, deter-mined using [17, 18]. The Debye mass is approximately10 − eV at z = 1000 and 3 × − eV at z = 10. TheDebye mass square of the dark photon isΠ A (cid:48) = g D (cid:18) n D T D (cid:19) (8)For the parameter space we are interested in, g D n D (cid:28) e x E n b and T D ≤ T b , such that Π A (cid:48) (cid:28) Π A . Hence we take only the SM photon Debye mass to regulate thedivergence.Finally, for elementary charges, q max = 2 µv rel , suchthat the θ integral is taken between the limits θ = {− , (cid:15)α √ Π A T b } . For blobs, q max ∼ Min (cid:0) R − , µv rel (cid:1) ,such that θ min = 1 − q µ v . The thermal-averaged trans-fer cross-section in the q (cid:29) Π A limit is given by inte-grating Eqn. 6 over θ , giving, σ T = 2 π(cid:15) α ξµ v (9)with ξ = ln (cid:16) T b π(cid:15) α x e n b q µ v (cid:17) . In the region of interest,it is safe to ignore the factor q µ v since it is inside thelog.Next, we compare the rate of charges scattering oﬀbaryons to the Hubble rate, n b σ T v rel H ≈ − (cid:16) (cid:15) f − (cid:17) z (cid:18)

10 K T D (cid:19) (10)As a result, the elementary charges (cid:15) f which we take toobey stellar-cooling constraints discussed next in Eqn. 15,are never in thermal contact with the SM. We also seethat blobs with charge (cid:15) χ (cid:38) − can interact with theSM bath.At temperatures around T D ≈ Λ D , both elementarycharges and blobs can co-exist. Deﬁning F blob ( z ) as thefraction of millicharges that are in the blob phase, we get˙ T ref D ( F blob ) = − HT D + 23 m χ x e ρ b ( m χ + m b ) F blob σ u χ,b × (cid:40)(cid:114) π ( T b − T D ) (cid:41) (11)Here u χ,b = (cid:113) T D m χ + T b m b is the average relative velocitydue to thermal motion and σ = σ T v . We have veriﬁedthat the bulk relative velocity between the χ bath and SMﬂuids does not contribute substantially to the thermalevolution of either ﬂuid.When T D (cid:38) Λ D , the relevant degrees of freedom arethe elementary charges, which have no thermal contactwith the SM such that the dark ﬂuid cools due to Hubbleexpansion. When T D drops below Λ D there is rapid blobformation. These blobs can now interact with the SM andheat up, but the temperature cannot exceed T D ; after all,thermal contact with the SM would immediately be lost.Consequently if the second term in Eqn. 11 dominatesfor F blob →

1, then F blob adjusts to smaller values so asto keep ˙ T D = 0. Thus, we set˙ T D = (cid:40) Min (cid:16) , ˙ T ref D ( F blob = 1) (cid:17) T D ≥ Λ D ˙ T ref D ( F blob = 1) T D < Λ D In the regime where ˙ T D = 0, we can solve for the z dependent fraction in blobs F blob by setting Eqn. 11 to0. We ﬁnd for T D ≥ Λ D , F blob =Min (cid:32) , HT D × (cid:34) m χ x e ρ b ( m χ + m b ) σ u χ,b × (cid:40)(cid:114) π ( T b − T D ) (cid:41)(cid:35) − (12)We can see that in the limit where T b (cid:29) T D , and wheninteractions are strong enough, the quantity in squarebrackets is much larger than Hubble cooling and hence F blob →

0. This happens because in this limit, blobs thatform immediately break up into elementary charges. Asthe disparity between T b and T D shrinks, F blob → T b = − HT b + 23 m b x e ρ D ( m χ + m b ) F blob f D f He + x e σ u χ,b × (cid:40)(cid:114) π ( T D − T b ) (cid:41) + Γ C ( T CMB − T b ) (13)where f He ≡ n He /n H is the helium fraction and Γ C is theCompton scattering rate.The initial conditions we use are T b ( z = 1000) = T CMB ( z = 1000) ≈ T × T = 2 .

725 K T D ( z = 1000) = 0 K (14)Setting the initial dark temperature to 0 K is not phys-ical, but is accurate because the time evolution rapidlyadjusts the temperature to its correct value just below z = 1000. III. EXISTING LIMITS

As alluded to in the introduction, the constraints oncomposite mCPs can be quite diﬀerent from elementarymCPs of the same charge. We elucidate further below.

Stellar bounds:

For Λ D (cid:28) (cid:15) f < − for small enough m f . The blobs arenever produced in stellar environments. However thelimit on the elementary charges translates to a limit onblob charge: (cid:15) χ < − m χ Λ D . (15) BBN and CMB N eﬀ : As we have seen in theprevious section, when there is signiﬁcant thermal contact with baryons and T b (cid:29) Λ D , F blob → (cid:15) f (cid:38) − (cid:0) m χ

10 K (cid:1) .This is more restrictive than stellar constraints onlywhen m f ≈ Λ D ≤ µ eV. Dark photons arising frombremsstrahlung and mesons from dark fusion are pro-duced at the temperature of the dark bath and hence donot contribute appreciably to N eﬀ either. CMB power spectrum:

The eﬀect of mCP scatter-ing on protons was investigated in [4], and constraintsfrom Planck 2015 data eﬀectively ruled out mCPs as asolution to EDGES for f D > . (cid:15) χ m χ , they apply equally to blobs as well aselementary charges. However, it was found in [4] thatno limits exist for f D ≤ . IV. RESULTS

We now display results obtained by numerically solv-ing the coupled diﬀerential equations for time evolution.We consider a benchmark blob mass m χ = 1 MeV, andcharge (cid:15) χ = 4 × − and f D = 0 . F blob ( z ) for diﬀerent Λ D in Fig. 1. For large z , heatfrom the baryonic bath disintegrates the blobs rendering F blob (cid:28)

1. For lower z , cooling due to Hubble expansionbegins to dominate, resulting in larger values of F blob .For the same z , we see that as Λ D is reduced, a smallerfraction of the dark bath exists in blobs as it is easier tobreak them apart.

10 50 100 500 100010 - - - Λ � = � � � � Λ � = � � Λ � = �� � Λ � = ��� � Λ � = ���� � � χ = � ���� ϵ χ = � × �� - � � � � = ��� % FIG. 1. The evolution of the fraction of the dark millichargedbath in blobs as a function of redshift is shown for diﬀerentΛ D , the dark conﬁning scale. Smaller Λ D leads to smallerblob fractions. In Fig. 2, the time evolution of the baryonic temper-ature T b and the dark temperature T D are shown fordiﬀerent choices of Λ D , the dark conﬁning scale. TheCMB temperature T CMB and the baryon temperature T b in the absence of interacting DM are shown in black forreference. The colored solid lines correspond to diﬀerentchoices of Λ D . We see that models with smaller Λ D stayat the same temperature T D = Λ D for longer. If thesecharges were elementary, the dark temperature wouldbe higher than Λ D in this regime. Instead, for blobs,this is prevented by the rapid break up of blobs withthe resulting elementary charges losing thermal contactwith the SM, cooling rapidly, and forming blobs again.The dashed solid lines track the baryonic temperaturefor diﬀerent Λ D . The baryonic temperature at z = 17is roughly constant for diﬀerent Λ D . However, there isa small enough Λ D ≈ . F blob is toosmall even at z = 17, as seen in Fig. 1 and results inlower cooling of T b in Fig. 2.

10 50 100 500 10000.010.101101001000 ����� � � � � � � ( Λ � � � ) � � ( Λ � = � � � � � ) � � ( Λ � = � � � � ) � � ( Λ � = �� � ) � � ( Λ � = � � ) � � ( Λ � = ��� � ) � χ = � ���� ϵ χ = � × �� - � � � � = ��� % FIG. 2. Temperature evolution of the baryonic and DM bathare plotted as a function of redshift z . The CMB temperatureand the baryon temperature without DM are plotted in black.The solid lines track the evolution of the dark temperature T D for diﬀerent Λ D , the dark conﬁning temperature. Thedashed lines track the baryon temperature T b for diﬀerentΛ D with the same color code as T D . The error bar marks thebaryonic temperature at z = 17 as measured by the EDGEScollaboration. We next discuss the contours that explain EDGES inthe (cid:15) χ vs m χ plane and compare it to the parameterspace derived for elementary charges in [19]. Given aDM fraction, elementary charges that explain EDGESobey (cid:15) elem ∝ m elem as seen with the black curve. Thishappens due to the following reason. For a ﬁxed DMfraction, a drop in T b , ∆ T b is associated with an increasein dark temperature ∆ T D ∝ m elem × ∆ T b , i.e. larger el-ementary masses m elem undergo larger temperature gainbecause of equipartition. Another way to see this is thatthe total energy gained is equal to n elem × ∆ T D and thenumber density is inversely proportional to m elem , andhence T D is directly proportional to m elem . Starting withan initially-cold dark bath T D (cid:28) T b , the proportionalityfactor ensures that T D ∝ m elem throughout. This in turnimplies that the elementary charges’ thermal velocity isindependent of the mCP mass. Finally, the heat trans-fer is proportional to the transfer cross-section given inEqn. 9, which is dependent only on the charge to mass ratio since the velocity is mass-independent. Thus, thisbehavior applies to very small masses. It was also pointedout in [19] that for a choice of DM fraction, there is also amaximum mass due to the same equipartition arguments, m elem ≤ µ b f elem Ω c / Ω b . The elementary charge requiredto explain EDGES obeys [19], (cid:15) elem ≈ × − m elem MeV (cid:18) − f elem (cid:19) . (16)It is important to note that the entirety of the elementarycharge solution is ruled out [6].Next we discuss the contours for blobs for diﬀerent Λ D .In each case, we mark out the unphysical region wherethe elementary charges required to create blobs are ruledout by stellar constraints from Eqn. 15. For the samereason as explained for the elementary charge solution,we observe a linear relationship (cid:15) χ ∝ m χ for the blobs aswell. This linear regime obeys an approximate empiricalrelation (cid:15) χ ≈ − m χ MeV (cid:18) − f χ (cid:19) . . (17)Once again, there exists a cut-oﬀ mass, that is now Λ D dependent. For smaller Λ D , the mCP bath stays elemen-tary for longer, i.e. F blob (cid:28) m χ is required to increase heat capac-ity, so as to reach temperatures below Λ D suﬃcientlysoon. As a corollary, larger Λ D results in an enhancedrange in mass where the EDGES solution is viable. How-ever, larger Λ D translates to stricter stellar constraintsand for large enough Λ D , the charge required to explainEDGES, Eqn. 17 is ruled out by Eqn. 15.The blobs in most of the parameter space shown inFig. 3 do not survive galaxy formation. The parameterspace for which blobs do not break up in the galaxy isgiven in Eqn. 19 and can be recast as, m gal χ (cid:46)

86 eV Λ D D ≈

10 K, the blobs resize themselves tomasses below 1 keV, making prospects for direct detec-tion tricky.

V. CONCLUSION

Making mCDM inherently composite is a simple nu-ance with parallels in SM baryons. In this work, we haveconsidered this possibility and explored its myriad conse-quences with speciﬁc emphasis on explaining the EDGESanomaly.The DM degrees of freedom are blobs at temperaturesbelow the conﬁning scale and elementary charges at tem-peratures above it. For an appropriately chosen conﬁn-ing scale Λ D , the elementary charges are the degrees of - - - - - - - - - - ↑ ↑ � � � �� � � � � � �� � � � ↑ ↑↑ ↑ � � � �� � � � � � �� � � � ↑ ↑ Λ � = � � � � Λ � = � � � � � � � � � � � � � � � � � � � � � � � � � � [ � � � � � � � � � � ] � � = ���� % - - - - - - - - - - ↑ ↑ � � � �� � � � � � �� � � � ↑ ↑↑ ↑ � � � �� � � � � � �� � � � ↑ ↑↑ ↑ � � � �� � � � � � �� � � � ↑ ↑ Λ � = � � � � Λ � = � � Λ � = � � � � � � � � � � � � � � � � � � � � � � � � � � � [ � � � � � � � � � � ] � � = ��� % FIG. 3. The contours that explain the EDGES anomaly in the blob charge (cid:15) χ vs blob mass m χ plane are shown for diﬀerentchoices of the conﬁning scale Λ D for mCP bath fractions of f D = 0 .

04% (left) and f D = 0 .

4% (right). Also shown are stellarcooling constraints from Eqn. 15. The elementary charge solution from [19] is shown in black. freedom during BBN, CMB and in the interior of stars.The elementary charges are chosen to be feeble enoughto evade all these constraints. However, at temperaturesbelow the conﬁning scale, these rapidly fuse into blobsincreasing in size till they reach a size determined bystability considerations due to repulsion. These blobsnow have large enough charges that coherently scatterwith baryons at temperatures around z = 17, relevantfor physics during the dark ages, without suﬀering fromthe strict stellar and cosmology constraints that apply toelementary mCPs. Thus, we ﬁnd a large unconstrainedparameter space for mCP blobs for f D ≤ . v vir ≈ − . Self- interactions are large enough to break up the blobs oncemore if the kinetic energy exceeds the conﬁning scale.Thus the blobs stay intact till today only if m χ v (cid:46) Λ D . (19)Hence, for large enough blob masses m χ , there is sig-niﬁcant ﬁssion in galaxies, the blobs are resized intosmaller ones that obey Eqn. (19) which are present inthe galaxy today. These smaller blobs should neverthe-less be present in the galaxy today since the dark pho-ton sets the range for self-interactions [25] and cuts oﬀlong-range galactic processes such as evacuation from thegalactic disk [26, 27] and retention in galactic magneticﬁelds [28], and prevents the mCP from being blown awayby the solar wind [27, 29].This parameter space increases the scope of directdetection experiments sensitive to masses lower than1 MeV, albeit at momentum transfers smaller than R − to retain coherence. Experiments such as SENSEI [30],DAMIC [31], super-CDMS [32], and even future pro-posals [33–36] are not sensitive to momentum transfers q ≤ Λ D ≈ meV. Instead, manipulation with electric andmagnetic ﬁelds [37] is a promising detection strategy. Forlarge enough blob charge, terrestrial accumulation andsubsequent detection [38] might be a viable avenue. ACKNOWLEDGMENTS

We thank Asher Berlin, Diego Redigolo, Gordan Krn-jaic and Hongwan Liu for useful discussions. SR is sup-ported in part by the NSF under grant PHY-1818899.SR is also supported by the DoE under a QuantISEDgrant for MAGIS and the SQMS quantum center. HR issupported in part by NSF Grant PHY-1720397 and theGordon and Betty Moore Foundation Grant GBMF7946. [1] Judd D. Bowman, Alan E. E. Rogers, Raul A. Monsalve,Thomas J. Mozdzen, and Nivedita Mahesh, “An absorp-tion proﬁle centred at 78 megahertz in the sky-averagedspectrum,” Nature , 67–70 (2018), arXiv:1810.05912[astro-ph.CO].[2] Rennan Barkana, “Possible interaction between baryonsand dark-matter particles revealed by the ﬁrst stars,” Na-ture , 71–74 (2018), arXiv:1803.06698 [astro-ph.CO].[3] Rennan Barkana, Nadav Joseph Outmezguine, DiegoRedigolo, and Tomer Volansky, “Strong constraints onlight dark matter interpretation of the EDGES signal,”Phys. Rev. D , 103005 (2018), arXiv:1803.03091 [hep-ph].[4] Ely D. Kovetz, Vivian Poulin, Vera Gluscevic, Kim-berly K. Boddy, Rennan Barkana, and MarcKamionkowski, “Tighter limits on dark matter explana-tions of the anomalous EDGES 21 cm signal,” Phys. Rev.D , 103529 (2018), arXiv:1807.11482 [astro-ph.CO].[5] Asher Berlin, Dan Hooper, Gordan Krnjaic, andSamuel D. McDermott, “Severely Constraining DarkMatter Interpretations of the 21-cm Anomaly,” Phys.Rev. Lett. , 011102 (2018), arXiv:1803.02804 [hep-ph].[6] Cyril Creque-Sarbinowski, Lingyuan Ji, Ely D. Kovetz,and Marc Kamionkowski, “Direct millicharged dark mat-ter cannot explain the EDGES signal,” Phys. Rev. D , 023528 (2019), arXiv:1903.09154 [astro-ph.CO].[7] Chang Feng and Gilbert Holder, “Enhanced global sig-nal of neutral hydrogen due to excess radiation atcosmic dawn,” Astrophys. J. Lett. , L17 (2018),arXiv:1802.07432 [astro-ph.CO].[8] A. Ewall-Wice, T. C. Chang, J. Lazio, O. Dore, M. Seif-fert, and R. A. Monsalve, “Modeling the Radio Back-ground from the First Black Holes at Cosmic Dawn: Im-plications for the 21 cm Absorption Amplitude,” Astro-phys. J. , 63 (2018), arXiv:1803.01815 [astro-ph.CO].[9] Maxim Pospelov, Josef Pradler, Joshua T. Ruder-man, and Alfredo Urbano, “Room for New Physicsin the Rayleigh-Jeans Tail of the Cosmic MicrowaveBackground,” Phys. Rev. Lett. , 031103 (2018),arXiv:1803.07048 [hep-ph].[10] Anastasia Fialkov and Rennan Barkana, “Signature ofExcess Radio Background in the 21-cm Global Signaland Power Spectrum,” Mon. Not. Roy. Astron. Soc. ,1763–1773 (2019), arXiv:1902.02438 [astro-ph.CO].[11] Hongwan Liu, Nadav Joseph Outmezguine, DiegoRedigolo, and Tomer Volansky, “Reviving MillichargedDark Matter for 21-cm Cosmology,” Phys. Rev. D ,123011 (2019), arXiv:1908.06986 [hep-ph].[12] Dorota M. Grabowska, Tom Melia, and Surjeet Rajen-dran, “Detecting Dark Blobs,” Phys. Rev. D , 115020(2018), arXiv:1807.03788 [hep-ph].[13] Edward Hardy, Robert Lasenby, John March-Russell,and Stephen M. West, “Big Bang Synthesis of NuclearDark Matter,” JHEP , 011 (2020), arXiv:1411.3739[hep-ph].[14] Gordan Krnjaic and Kris Sigurdson, “Big Bang Dark-leosynthesis,” Phys. Lett. B , 464–468 (2015),arXiv:1406.1171 [hep-ph].[15] George Gamow, The Quantum Theory of the Atomic Nu-cleus (US Atomic Energy Commission, Division of Tech- nical Information Extension, 1963).[16] Cora Dvorkin, Kﬁr Blum, and Marc Kamionkowski,“Constraining Dark Matter-Baryon Scattering with Lin-ear Cosmology,” Phys. Rev. D , 023519 (2014),arXiv:1311.2937 [astro-ph.CO].[17] Yacine Ali-Ha¨ımoud and Christopher M. Hirata, “Hyrec:A fast and highly accurate primordial hydrogen andhelium recombination code,” Phys. Rev. D , 043513(2011), arXiv:1011.3758 [astro-ph.CO].[18] Nanoom Lee and Yacine Ali-Ha¨ımoud, “Hyrec-2: Ahighly accurate sub-millisecond recombination code,”Phys. Rev. D , 083517 (2020), arXiv:2007.14114[astro-ph.CO].[19] Julian B. Mu˜noz and Abraham Loeb, “A small amountof mini-charged dark matter could cool the baryonsin the early Universe,” Nature , 684 (2018),arXiv:1802.10094 [astro-ph.CO].[20] Saurabh Singh et al. , “SARAS 2 constraints on global 21-cm signals from the Epoch of Reionization,” Astrophys.J. , 54 (2018), arXiv:1711.11281 [astro-ph.CO].[21] DC Price, LJ Greenhill, Anastasia Fialkov, GIANNIBernardi, H Garsden, BR Barsdell, J Kocz, MM An-derson, SA Bourke, J Craig, et al. , “Design and charac-terization of the large-aperture experiment to detect thedark age (leda) radiometer systems,” Monthly Notices ofthe Royal Astronomical Society , 4193–4213 (2018).[22] Tabitha C. Voytek, Aravind Natarajan, Jos´e MiguelJ´auregui Garc´ıa, Jeﬀrey B. Peterson, and Omar L´opez-Cruz, “Probing the Dark Ages at z ∼

20: The SCI-HI 21cm All-sky Spectrum Experiment,” Astrophys. J. Lett. , L9 (2014), arXiv:1311.0014 [astro-ph.CO].[23] Morgan E. Presley, Adrian Liu, and Aaron R. Par-sons, “Measuring the Cosmological 21 cm Monopolewith an Interferometer,” Astrophys. J. , 18 (2015),arXiv:1501.01633 [astro-ph.CO].[24] Bang D. Nhan, Richard F. Bradley, and Jack O.Burns, “A polarimetric approach for constraining thedynamic foreground spectrum for cosmological global21-cm measurements,” Astrophys. J. , 90 (2017),arXiv:1611.06062 [astro-ph.IM].[25] Robert Lasenby, “Long range dark matter self-interactions and plasma instabilities,” JCAP , 034(2020), arXiv:2007.00667 [hep-ph].[26] Leonid Chuzhoy and Edward W. Kolb, “Reopening thewindow on charged dark matter,” JCAP , 014 (2009),arXiv:0809.0436 [astro-ph].[27] David Dunsky, Lawrence J. Hall, and Keisuke Hari-gaya, “CHAMP Cosmic Rays,” JCAP , 015 (2019),arXiv:1812.11116 [astro-ph.HE].[28] Roni Harnik, Ryan Plestid, Maxim Pospelov, andHarikrishnan Ramani, “Millicharged Cosmic Rays andLow Recoil Detectors,” (2020), arXiv:2010.11190 [hep-ph].[29] Timon Emken, Rouven Essig, Chris Kouvaris, andMukul Sholapurkar, “Direct Detection of Strongly In-teracting Sub-GeV Dark Matter via Electron Recoils,”JCAP , 070 (2019), arXiv:1905.06348 [hep-ph].[30] Liron Barak et al. (SENSEI), “SENSEI: Direct-Detection Results on sub-GeV Dark Matter from a NewSkipper-CCD,” Phys. Rev. Lett. , 171802 (2020),arXiv:2004.11378 [astro-ph.CO]. [31] A. Aguilar-Arevalo et al. (DAMIC), “Results on low-mass weakly interacting massive particles from a 11 kg-day target exposure of DAMIC at SNOLAB,” Phys.Rev. Lett. , 241803 (2020), arXiv:2007.15622 [astro-ph.CO].[32] R. Agnese et al. (SuperCDMS), “Results from the SuperCryogenic Dark Matter Search Experiment at Soudan,”Phys. Rev. Lett. , 061802 (2018), arXiv:1708.08869[hep-ex].[33] Sinead Griﬃn, Simon Knapen, Tongyan Lin, andKathryn M. Zurek, “Directional Detection of Light DarkMatter with Polar Materials,” Phys. Rev. D , 115034(2018), arXiv:1807.10291 [hep-ph].[34] Rouven Essig, Jes´us P´erez-R´ıos, Harikrishnan Ramani,and Oren Slone, “Direct Detection of Spin-(In)dependentNuclear Scattering of Sub-GeV Dark Matter UsingMolecular Excitations,” Phys. Rev. Research. , 033105 (2019), arXiv:1907.07682 [hep-ph].[35] Philip C. Bunting, Giorgio Gratta, Tom Melia, and Sur-jeet Rajendran, “Magnetic Bubble Chambers and Sub-GeV Dark Matter Direct Detection,” Phys. Rev. D ,095001 (2017), arXiv:1701.06566 [hep-ph].[36] Hao Chen, Rupak Mahapatra, Glenn Agnolet, MichaelNippe, Minjie Lu, Philip C. Bunting, Tom Melia, SurjeetRajendran, Giorgio Gratta, and Jeﬀrey Long, “QuantumDetection using Magnetic Avalanches in Single-MoleculeMagnets,” (2020), arXiv:2002.09409 [physics.ins-det].[37] Asher Berlin, Raﬀaele Tito D’Agnolo, Sebastian A. R. El-lis, Philip Schuster, and Natalia Toro, “Directly Deﬂect-ing Particle Dark Matter,” Phys. Rev. Lett.124