Dark Radiation Alleviates Problems with Dark Matter Halos

Abstract

We show that a scalar and a fermion charged under a global U (1) symmetry can not only explain the existence and abundance of dark matter (DM) and dark radiation (DR), but also imbue DM with improved scattering properties at galactic scales, while remaining consistent with all other observations. Delayed DM-DR kinetic decoupling eases the missing satellites problem, while scalarmediated self-interactions of DM ease the cusp vs. core and too big to fail problems. In this scenario, DM is expected to be pseudo-Dirac and have a mass 100 keV m χ 10 GeV. The predicted DR may be measurable using the primordial elemental abundances from big bang nucleosynthesis (BBN), and using the cosmic microwave background (CMB). Introduction.-Cosmological and astrophysical data now firmly point towards the existence of new nonrelativistic particles, dubbed dark matter (DM), and there is a vigorous experimental program underway to discover these particles and measure their properties. Dark radiation (DR), on the other hand, refers to new relativistic particles that contribute to the cosmological energy density but are otherwise decoupled from ordinary matter and radiation. There is neither clear evidence nor a definitive exclusion, but several independent analyses of cosmological data show tantalizing hints for DR [1][2][3][4][5], most recently to reconcile the results from the Planck Collaboration [6] with those from BICEP2 Collaboration [7].

Galactic positron excess from selectively enhanced dark matter annihilation

“…Therefore, p-wave annihilation process contributes more by a factor of ∼ 2 than coannihilation in the early Universe. This more carefully computed result disagrees somewhat with the conclusions made in previous papers [25,27,28]. However, we must note that at this large velocity v ∼ 0.2, the partial wave expansion of the cross section may not be very accurate as higher partial wave contributions become important.…”

“…However, as we will show later, this not the case in the region of the parameter space we are interested in. If the dark sector decouples from the visible sector early enough then ρ, η do not affect the predictions for N eff and the Hubble parameter much during BBN, as was shown in [27]. Dark sector temperature T d rises after the DM decouples from the thermal bath.…”

“…It was already pointed out in Ref. [27] that for m χ 1 GeV, ∆/m χ 10 −4.5 and massless η (acting as dark radiation), the kinetic decoupling temperature is ∼ 0.5 keV which is essential to solve the missing satellite problem. This temperature decreases further by a factor of a few when a more exact analytic expression for the cross section is used and the thermal distributions of χ 1 and η are taken into account, as shown in Fig.…”

Unveiling secret interactions among sterile neutrinos with big-bang nucleosynthesis

“…As in the case of neutrino asymmetries, the self-interactions would generate a matter potential in the flavor evolution equations which suppresses the sterile neutrino abundance. If the new interaction mediator X also couples to dark matter, this might also possibly relieve some of the small-scale structure problems associated with cold dark matter [27,28] (nonstandard interactions were also introduced to alleviate problems related to cold dark matter in [29][30][31][32][33][34] and in the references therein).…”

Dark matter–radiation interactions: the structure of Milky Way satellite galaxies

“…A range of alternatives to vanilla CDM have also been proposed e.g. warm DM (Schaeffer & Silk 1988), interacting DM (Boehm et al 2001;Boehm & Schaeffer 2005;Cyr-Racine & Sigurdson 2013;Chu & Dasgupta 2014), selfinteracting DM (Spergel & Steinhardt 2000;Rocha et al 2013;Vogelsberger et al 2014;Buckley et al 2014), decaying DM (Wang et al 2014) and late-forming DM (Agarwal et al 2015). These "beyond CDM" models generally exhibit a cut-off in the linear matter power spectrum at small scales (high wavenumbers) that translates into a reduced number of low-mass DM haloes compared to collisionless CDM at late times.…”