11,257 research outputs found
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 and 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
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