Dilaton dominance in the early Universe dilutes Dark Matter relic abundances

The role of the dilaton field and its coupling to matter may result to a dilution of Dark Matter (DM) relic densities. This is to be contrasted with quintessence scenarios in which relic densities are augmented, due to modification of the expansion rate, since Universe is not radiation dominated at DM decoupling. Dilaton field, besides this, affects relic densities through its coupling to dust which tends to decrease relic abundances. Thus two separate mechanisms compete each other resulting, in general, to a decrease of the relic density. This feature may be welcome and can rescue the situation if Direct Dark Matter experiments point towards small neutralino-nucleon cross sections, implying small neutralino annihilation rates and hence large relic densities, at least in the popular supersymmetric scenarios. In the presence of a diluting mechanism both experimental constraints can be met. The role of the dilaton for this mechanism has been studied in the context of the non-critical string theory but in this work we follow a rather general approach assuming that the dilaton dominates only at early eras long before Big Bang Nucleosynthesis.Comment: 11 pages, Latex, 4 figures: Comments and references added, version to appear in Phys. Rev.

Asymmetric Dark Matter and Effective Operators

In order to annihilate in the early Universe to levels well below the measured dark matter density, asymmetric dark matter must possess large couplings to the Standard Model. In this paper, we consider effective operators which allow asymmetric dark matter to annihilate into quarks. In addition to a bound from requiring sufficient annihilation, the energy scale of such operators can be constrained by limits from direct detection and monojet searches at colliders. We show that the allowed parameter space for these operators is highly constrained, leading to non-trivial requirements that any model of asymmetric dark matter must satisfy.Comment: 6 pages, 1 figure. V2 replacement: Citations added. Shading error in Fig. 1 (L_FV panel) corrected. Addition of direct detection bounds on m_chi <5 GeV added, minor alterations in text to reflect these change

Bulk viscosity of a gas of neutrinos and coupled scalar particles, in the era of recombination

Bulk viscosity may serve to damp sound waves in a system of neutrinos coupled to very light scalar particles, in the era after normal neutrino decoupling but before recombination. We calculate the bulk viscosity parameter in a minimal scheme involving the coupling of the two systems. We add some remarks on the bulk viscosity of a system of fully ionized hydrogen plus photons.Comment: 6 pages, 4 figure

Nonthermal Supermassive Dark Matter

We discuss several cosmological production mechanisms for nonthermal supermassive dark matter and argue that dark matter may be elementary particles of mass much greater than the weak scale. Searches for dark matter should not be limited to weakly interacting particles with mass of the order of the weak scale, but should extend into the supermassive range as well.Comment: 11 page LaTeX file. No major changes. Version accepted by PR

Gravitational waves and cosmological braneworlds: a characteristic evolution scheme

Motivated by the problem of the evolution of bulk gravitational waves in Randall-Sundrum cosmology, we develop a characteristic numerical scheme to solve 1+1 dimensional wave equations in the presence of a moving timelike boundary. The scheme exhibits quadratic convergence, is capable of handling arbitrary brane trajectories, and is easily extendible to non-AdS bulk geometries. We use our method to contrast two different prescriptions for the bulk fluctuation initial conditions found in the literature; namely, those of Hiramatsu et al. (hep-th/0410247) and Ichiki and Nakamura (astro-ph/0406606). We find that if the initial data surface is set far enough in the past, the late time waveform on the brane is insensitive to the choice between the two possibilities; and we present numeric and analytic evidence that this phenomenon generalizes to more generic initial data. Observationally, the main consequence of this work is to re-affirm previous claims that the stochastic gravitational wave spectrum is predominantly flat, in contradiction with naive predictions from the effective 4-dimensional theory. Furthermore, this flat spectrum result is predicted to be robust against uncertainties in (or modifications of) the bulk initial data, provided that the energy scale of brane inflation is high enough.Comment: 21 pages, 18 figures. Points of clarification added to Secs. V and VIIA concerning initial conditions and basis functions, respectively. Other minor typos corrected, references updated. To appear in PR

Femtolensing and Picolensing by Axion Miniclusters

Non-linear effects in the evolution of the axion field in the early Universe may lead to the formation of gravitationally bound clumps of axions, known as ``miniclusters.'' Minicluster masses and radii should be in the range $M_{\rm mc}\sim10^{-12} M_\odot$ and $R_{\rm mc} \sim 10^{10}$cm, and in plausible early-Universe scenarios a significant fraction of the mass density of the Universe may be in the form of axion miniclusters. If such axion miniclusters exist, they would have the physical properties required to be detected by ``femtolensing.''Comment: 7 pages plus 2 figures (Fig.1 avalible upon request), LaTe

The Higgs portal and an unified model for dark energy and dark matter

We examine a scenario where the Higgs boson is coupled to an additional singlet scalar field which we identify with a quintessence field. We show that this results in an unified picture of dark matter and dark energy, where dark energy is the zero-mode classical field rolling the usual quintessence potential and the dark matter candidate is the quantum excitation (particle) of the field, which is produced in the universe due to its coupling to the Higgs boson.Comment: 5 pages, no figures. New references and acknowledgment adde

Nonabelian dark matter: models and constraints

Numerous experimental anomalies hint at the existence of a dark matter (DM) multiplet chi_i with small mass splittings. We survey the simplest such models which arise from DM in the low representations of a new SU(2) gauge symmetry, whose gauge bosons have a small mass mu < 1 GeV. We identify preferred parameters M_chi ~ 1 TeV, mu ~ 100 MeV, alpha_g ~ 0.04 and the chi chi -> 4e annihilation channel, for explaining PAMELA, Fermi, and INTEGRAL/SPI lepton excesses, while remaining consistent with constraints from relic density, diffuse gamma rays and the CMB. This consistency is strengthened if DM annihilations occur mainly in subhalos, while excitations (relevant to the excited DM proposal to explain the 511 keV excess) occur in the galactic center (GC), due to higher velocity dispersions in the GC, induced by baryons. We derive new constraints and predictions which are generic to these models. Notably, decays of excited DM states chi' -> chi gamma arise at one loop and could provide a new signal for INTEGRAL/SPI; big bang nucleosynthesis (BBN) constraints on the density of dark SU(2) gauge bosons imply a lower bound on the mixing parameter epsilon between the SU(2) gauge bosons and photon. These considerations rule out the possibility of the gauge bosons that decay into e^+e^- being long-lived. We study in detail models of doublet, triplet and quintuplet DM, showing that both normal and inverted mass hierarchies can occur, with mass splittings that can be parametrically smaller, e.g., O(100) keV, than the generic MeV scale of splittings. A systematic treatment of Z_2 symmetry which insures the stability of the intermediate DM state is given for cases with inverted mass hierarchy, of interest for boosting the 511 keV signal from the excited dark matter mechanism.Comment: 28 pages, 17 figures; v2. added brief comment, reference