4 research outputs found
The Adiabatic Instability on Cosmology's Dark Side
We consider theories with a nontrivial coupling between the matter and dark
energy sectors. We describe a small scale instability that can occur in such
models when the coupling is strong compared to gravity, generalizing and
correcting earlier treatments. The instability is characterized by a negative
sound speed squared of an effective coupled dark matter/dark energy fluid. Our
results are general, and applicable to a wide class of coupled models and
provide a powerful, redshift-dependent tool, complementary to other
constraints, with which to rule many of them out. A detailed analysis and
applications to a range of models are presented in a longer companion paper.Comment: 4 pages, 1 figur
Bound-State Effects on Light-Element Abundances in Gravitino Dark Matter Scenarios
If the gravitino is the lightest supersymmetric particle and the long-lived
next-to-lightest sparticle (NSP) is the stau, the charged partner of the tau
lepton, it may be metastable and form bound states with several nuclei. These
bound states may affect the cosmological abundances of Li6 and Li7 by enhancing
nuclear rates that would otherwise be strongly suppressed. We consider the
effects of these enhanced rates on the final abundances produced in Big-Bang
nucleosynthesis (BBN), including injections of both electromagnetic and
hadronic energy during and after BBN. We calculate the dominant two- and
three-body decays of both neutralino and stau NSPs, and model the
electromagnetic and hadronic decay products using the PYTHIA event generator
and a cascade equation. Generically, the introduction of bound states drives
light element abundances further from their observed values; however, for small
regions of parameter space bound state effects can bring lithium abundances in
particular in better accord with observations. We show that in regions where
the stau is the NSP with a lifetime longer than 10^3-10^4 s, the abundances of
Li6 and Li7 are far in excess of those allowed by observations. For shorter
lifetimes of order 1000 s, we comment on the possibility in minimal
supersymmetric and supergravity models that stau decays could reduce the Li7
abundance from standard BBN values while at the same time enhancing the Li6
abundance.Comment: 22 pages 6 figure
Dark Matter from Gaugino Mediation.
We study dark matter for gaugino-mediated supersymmetry breaking and compact
dimensions of order the grand unification scale. Higgs fields are bulk fields,
and in general their masses differ from those of squarks and sleptons at the
unification scale. As a consequence, at different points in parameter space,
the gravitino, a neutralino or a scalar lepton can be the lightest (LSP) or
next-to-lightest (NLSP) superparticle. We investigate the constraints from
primordial nucleosynthesis on the different scenarios. While neutralino and
gravitino dark matter with a sneutrino NLSP are consistent for a wide range of
parameters, gravitino dark matter with a stau NLSP is strongly constrained.Comment: 18 pages, updated to published version (minor modifications,
reference added
Neutrino Dark Energy -- Revisiting the Stability Issue.
A coupling between a light scalar field and neutrinos has been widely
discussed as a mechanism for linking (time varying) neutrino masses and the
present energy density and equation of state of dark energy. However, it has
been pointed out that the viability of this scenario in the non-relativistic
neutrino regime is threatened by the strong growth of hydrodynamic
perturbations associated with a negative adiabatic sound speed squared. In this
paper we revisit the stability issue in the framework of linear perturbation
theory in a model independent way. The criterion for the stability of a model
is translated into a constraint on the scalar-neutrino coupling, which depends
on the ratio of the energy densities in neutrinos and cold dark matter. We
illustrate our results by providing meaningful examples both for stable and
unstable models.Comment: 24 pages, 8 figures, stable scenario with negative sound speed
squared included, figures added, references added, conclusions unchanged.
Matches version to be published in JCA