PoS(NOW2018)088 https://pos.sissa.it/ ∗ CDM standard model of cosmology is in excellent agreement with data on large scales Λ CDM cosmology. It would be very attractive to identify the new light particle with a sterile [email protected] Λ Speaker. but has difficulty explaining all observationsmodel on involving small a scales. new I MeV-scale discuss gaugedark a matter boson simple interactions that particle with mediates physics a dark newof matter light fermion. self-interactions and This allows toneutrino, solve but it all appears small-scale that problems this option is disfavored. The ∗ Copyright owned by the author(s) under the terms of the Creative Commons c Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). Neutrino Oscillation Workshop (NOW2018) 9–16 September, 2018 Rosa Marina (Ostuni, Brindisi, Italy) Jörn Kersten University of Bergen, Norway E-mail: Small-Scale Crisis in Cosmology – Sterileto Neutrinos the Rescue? PoS(NOW2018)088 ]. 4 . In CDM X ) Λ 1 ( U Jörn Kersten MeV. ∼ , a vector-like DM 0 ]. It includes a dark Θ N 6 m and uncharged under the physics, which is the focus X ]. ) 7 1 ( eV and ], and the distribution of galax- U to the chiral dark fermions. 1 ∼ N particle N X m − CDM [ = 0 Λ N X (dark photon), a scalar CDM standard model of cosmology, a flat uni- V and 1 Λ N X with eV-scale and MeV-scale masses, respectively. The 0 N CDM simulations, which should be too big to fail at form- Λ and N We choose parameters such that 1 gauge interaction, which is spontaneously broken by the vacuum expec- X ] includes a second dark scalar, but this is not necessary [ ) ]. The number of small satellite galaxies predicted for a Milky-Way-size 6 1 3 ( MeV. The dark fermions are charged under U is broken. i ∼ ] but we will focus on a particular model first proposed in [ X 5 Θ ) h 1 CDM far exceeds the number of known Milky Way satellites (missing satellite prob- ( , and chiral fermions Λ U χ ]. However, looking more closely one realizes that these observations probe cosmology 2 The original model [ The approach presented here employs a combination of DM self-interactions and late kinetic Ever more precise observations probe the Effective operators that can be generated by the exchange of heavy singlet fermions lead to 1 CDM will eventually be explained by effects from the domain of astrophysics. Nevertheless, it is order to cancel anomalies, we assign charges That said, it certainly remains possible,Λ and probably even likely, thatinteresting the to small-scale wonder problems if of and how they can be addressed by new sector consisting of anfermion MeV-scale gauge boson boson mediates a decoupling due to interactionsclassified between in DM [ and dark radiation. Viable simplified models were tation value ies predicted by numericaltions simulations [ of structure formation closely matches actual observa- lem). Small, DM-dominated galaxiesthat seem the to density possess becomes cored nearly inner constant density towards profiles, the which center. means This is inconsistent with the of this contribution. 2. Dark Matter Interacting with Itself and with Dark Radiation Standard Model gauge group, while the Standard Model particles are neutral under at large scales.nificant On small challenges scales, [ i.e.,galaxy in in observations of dwarf galaxies, the scenario faces sig- Small-Scale Crisis in Cosmology – Sterile Neutrinos to the Rescue? 1. Introduction prediction of cuspy profiles, wherecenter the (cusp-core density problem). is Comparing inverselythe proportional the most to brightest massive the satellites observed found distance satellitesing in from of stars, the the one finds Milky that Waythan the with central their regions observed of counterparts theulations (too-big-to-fail simulated used dwarf problem). galaxies to are An identify more importantthe these concentrated simulations caveat problems can is considered be that brought only theStandard into cold Model agreement sim- DM. baryons. with However, observations this It by possibilityfined including is is simulations the called therefore which matter into do possible question made include by that of baryons thedwarf lead discovery rotation to that curves an re- in agreement some between cases simulated and while observed discrepancies remain in others (diversity problem) [ verse dominated by a cosmologicalingly constant and confirm non-baryonic this cold relatively dark minimalmicrowave matter background scenario. (DM), is and in For seem- excellent example, agreement the with power spectrum of the cosmic Majorana masses for the chiral darknos fermions after and to their mixing with the Standard Model neutri- PoS(NOW2018)088 . ]. & N X 17 eff N 25. As g. This ]. ∆ / / 2 12 charge . DM , Jörn Kersten X m ) DM 11 1cm 1 . This coupling , m ∼ ( . Assuming that ∼ H 10 U T eff , ]. This solves the CDM. As the dark , N DM 9 Λ 2 , 15 | m 8 , / Θ | σ 2 14 | , H | 13 κ + 1, this happens if the dark photon
0 forest, analogous to the warm DM = N interaction, and the N α µ X γ X 0 ) , the relevant free parameters are the DM 1 N ( N kd T U X weaken the interaction between DM and dark as a function of the DM mass −  and the Standard Model Higgs N X N g X N 1eV. The new gauge interaction leads to efficient 2 Θ µ m γ & ]. One should caution that the suppression of small- N T N 16 X , + 13 χ µ increase the energy density of relativistic particles in the early χγ stays relativistic in the early universe until the temperature falls N µ N . Smaller values of V ]. If this result stands, the model presented here can still contribute to gauge coupling. Apart from the gauge couplings of the dark fermions, 1 X X g 17 ) 1 ( ⊃ − U of the gauge boson mediating the L med slightly below 1keV [ m kd T 1, the parameter space region in which the missing satellites problem is solved is shown denotes the X = g N With this qualitative understanding of the features of the model, we can now discuss which The light chiral fermion The light chiral fermions For our purposes, the terms of interest in the Lagrangian are If the dark gauge boson is much lighter than the DM, it mediates a long-range DM self- X For by the green bands in figure gauge coupling is fixed by theis observed only DM weakly density constrained and by as the the requirement mass of the dark radiation particle solving the missing satellites problem, but additional contributions from astrophysics are required. regions in parameter space provide solutions to the small-scale problems of scattering between the DM andequilibrium the dark long radiation, after which the causes chemical the decoupling two (freeze-out) species of to the stay DM in around kinetic mass, the mass universe, which is parameterized by the effective number of neutrino species there is a portal coupling between the dark scalar where below its mass.photons Thus, and it Standard Model acts neutrinos) as for dark radiation (a relativistic degree of freedoma different consequence, from structure formationmissing is satellites suppressed problem at iftemperature the small kinetic scales decoupling [ betweenscale DM structure and is dark constrained radiation byscenario. occurs observations of at the a A Lyman- recentsatellites study by at concluded most 30% that [ this allows a reduction of the number of Milky Way radiation, which has to be compensated byformation lowering is the dark to photon stay mass if thethe the same; impact figure on consequently, indicate structure the the green parameter bandsto space move solve region to the where the cusp-core, DM left. too-big-to-fail, self-interactionssmall-scale and problems The have diversity can the blue problems. be right bands addressed As simultaneously. strength in For themass different is bands of overlap, order all hundred MeV MeV. and The region if with lighter the DM order may be of favored by magnitude observations of of galaxy the clusters [ DM massno is other either dark-sector a particles remain TeV relativistic or during a big bang few nucleosynthesis, we find interaction. Its strength depends on the velocityobtain of the sufficiently DM weak particles. self-interactions Hence, on forclusters, the the one where hand large stringent we velocities can bounds occurring exist. onvelocities On are the the typical, scale the other of self-interaction hand, cross galaxy in section dwarf can galaxies, become where large, much smaller Small-Scale Crisis in Cosmology – Sterile Neutrinos to the Rescue? ensures that the darkvery sector early universe. is The in DM thermalSetting relic it density equilibrium equal is with to thus the the obtained observed by Standard value the determines Model standard freeze-out particles mechanism. in the enables a solution of the cusp-core, too-big-to-fail, and diversity problems [ PoS(NOW2018)088 ]. at 8 18 ]. A σ leads means 22 N

, M 21 Jörn Kersten interactions become X ) 1 ( 0 U 10 ξ=0.76 -1 wave ]. An interesting detail of this re- - 10 ], which is sufficient to equalize the s ia DM Dirac 26 6 , etrmediator) (vector ]. Hence, there is a chance to avoid the 25 -2 interaction of the sterile neutrinos mixing with the Standard Model neutrinos, 1 24 10

- ) X ) , 2 g 1 . [GeV] 1 1

- 0

cm ( 23

⊙ (

1 2 g M

med - 10 - U 9 3 -3

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0.1 cm m ⊙

M 10

11

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10 10 10 10 10

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DM ] [ m GeV ]. On the other hand, it is in tension with big bang nucleosynthesis constraints ]. One way to see this is realizing that the process is irreversible, which implies 19 ]. 6 ]. However, in our scenario the 1 20 1eV [ level [ Parameter space regions yielding the correct DM relic density as well as a solution of the missing . 0 σ , for example). The numbers in the blue bands correspond to the DM self-interaction cross section at .

At temperatures below the mass of the dark photon, however, the A tempting possibility is identifying the new eV-scale fermion with the sterile neutrino as- ν M 9 m 33. On the one hand, this is of the correct order of magnitude to address tensions between . weaker, and consequently thevalue. active-sterile neutrino Then mixing additional returns sterile to neutrino its production unsuppressed occurs vacuum [ to an effective matter potential thattrino suppresses production active-sterile at neutrino temperatures mixing andcosmological above neutrino thus an mass sterile MeV bound. neu- [ temperatures of Standard Modelthermalization and or recoupling sterile process neutrinos is that [ itworks, does including not [ conserve entropy, contrary to claims in some 3. Sterile Neutrinos as Dark Radiation sociated with the anomalies observed in experiments such as LSND and MiniBooNE [ The numbers in the green band10 indicate the mass cutoff below which no galaxies can form (and 9 at the 2 standard sterile neutrino with Figure 1: satellites (green bands) and other small-scale problems (blue bands), as determined by DarkSUSY 6.1 [ dwarf-galaxy scales divided by the DM mass. 0 measurements of the Hubble parameter as indicated by the anomalies,by is strongly oscillations disfavored in by cosmology the since∑ early it universe, is copiously which produced violates the bound on the sum of neutrino masses, Small-Scale Crisis in Cosmology – Sterile Neutrinos to the Rescue? different redshifts [ PoS(NOW2018)088 CDM 440 Λ , Jörn Kersten Ann. Rev. Nature , , CDM Paradigm ] found that a sterile neutrino Λ 30 small-scale problems of all ]. 27 ]. 4 . I am grateful to Alessandro Mirizzi, Basudeb 1 The large-scale structure of the Universe Planck 2018 results. VI. Cosmological parameters ]. The most recent work [ Small-Scale Challenges to the ]. 30 , 1707.04256 29 [ , 28 , 27 , (2017) 343 26 , 55 25 . astro-ph/0604561 [ collaboration, N. Aghanim et al., LANCK Astron. Astrophys. (2006) 1137 1807.06209 A number of studies considered re-thermalization and its impact on cosmology, arriving at Discrepancies between observations of dwarf galaxies and simulations of structure formation It would be tempting to identify the new eV-scale fermion with a sterile neutrino that mixes [2] V. Springel, C. S. Frenk and S. D. M. White, [3] J. S. Bullock and M. Boylan-Kolchin, [1]P Dasgupta, Joachim Kopp, and RasmusBesides, Hansen I for thank very the helpful Abdus Salam discussionsacknowledge ICTP during travel for the support hospitality workshop. from during the the writing ResearchSpecial of thanks Council these are of proceedings due Norway and to under theorganization project NOW and number organizers, hospitality. 255182. in particular Eligio Lisi, for their truly outstanding References varying conclusions [ may point to deviations from the standarda picture model of that collisionless introduces cold a dark new matter. gaugeto I interaction have mediated dark presented by matter an MeV-scale self-interactions gauge boson. solvingaddition, This the leads a cusp-core, new too-big-to-fail, eV-scale particle andresults species diversity in the problems. interacts suppression with of In structure thelem. formation dark at Thus, small matter scales, a by solving simple the the particle missing new physics satellites force, prob- model is which able to address Small-Scale Crisis in Cosmology – Sterile Neutrinos to the Rescue? an increase in entropy.As a Instead, consequence, the the totaltemperature momentum and neutrino distribution a number functions non-vanishing chemical are and potential characterized energy [ by densities a are common conserved. kinetic with significant mixing with theradiation Standard particle. Model Depending neutrinos on is thebound not on values the a of sum viable of the neutrino candidate model massesto is for parameters, violated be the the or compatible that dark reason with neutrino cosmic is free-streaming is microwave either too background that much observations. reduced the 4. Conclusions with the Standard Model neutrinos and explainsHowever, the this anomalies option observed in does oscillation experiments. notcase, seem the new to light be particlesStandard compatible have Model with to neutrinos. constraints be generic from dark cosmology. fermions that In mix this Acknowledgements at most very weakly with I would like to thank Torsten Bringmannbuilding for the and collaboration on for self-interacting dark providing matter model the plot in figure cosmology. PoS(NOW2018)088 , , , ]. , ]. D71 Phys. , 104 Phys. Rev. , Jörn Kersten 1611.02716 [ Phys. Rev. ]. , (2012) 231301 (2005) 003 Mon. Not. Roy. Astron. hep-ex/0104049 08 109 [ , Phys. Rev. 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