15 research outputs found
Interacting Dark Matter and Dark Energy
We discuss models for the cosmological dark sector in which the energy
density of a scalar field approximates Einstein's cosmological constant and the
scalar field value determines the dark matter particle mass by a Yukawa
coupling. A model with one dark matter family can be adjusted so the
observational constraints on the cosmological parameters are close to but
different from what is predicted by the Lambda CDM model. This may be a useful
aid to judging how tightly the cosmological parameters are constrained by the
new generation of cosmological tests that depend on the theory of structure
formation. In a model with two families of dark matter particles the scalar
field may be locked to near zero mass for one family. This can suppress the
long-range scalar force in the dark sector and eliminate evolution of the
effective cosmological constant and the mass of the nonrelativistic dark matter
particles, making the model close to Lambda CDM, until the particle number
density becomes low enough to allow the scalar field to evolve. This is a
useful example of the possibility for complexity in the dark sector.Comment: 15 pages, 6 figures; added a reference and a minor correctio
Scalar field-perfect fluid correspondence and nonlinear perturbation equations
The properties of dynamical Dark Energy (DE) and, in particular, the
possibility that it can form or contribute to stable inhomogeneities, have been
widely debated in recent literature, also in association to a possible coupling
between DE and Dark Matter (DM). In order to clarify this issue, in this paper
we present a general framework for the study of the nonlinear phases of
structure formation, showing the equivalence between two possible descriptions
of DE: a scalar field \phi self-interacting through a potential V(\phi) and a
perfect fluid with an assigned negative equation of state w(a). This enables us
to show that, in the presence of coupling, the mass of DE quanta may increase
where large DM condensations are present, so that also DE may partake to the
clustering process.Comment: 16 pages, accepted for publication in JCA
Clues from nearby galaxies to a better theory of cosmic evolution
The great advances in the network of cosmological tests show that the
relativistic Big Bang theory is a good description of our expanding universe.
But the properties of nearby galaxies that can be observed in greatest detail
suggest a still better theory would more rapidly gather matter into galaxies
and groups of galaxies. This happens in theoretical ideas now under discussion.Comment: published in Natur
Three-dimensional Topology-Independent Methods to Look for Global Topology
The space-like hypersurface of the Universe at the present cosmological time
is a three-dimensional manifold. A non-trivial global topology of this
space-like hypersurface would imply that the apparently observable universe
(the sphere of particle horizon radius) could contain several images of the
single, physical Universe. Recent three-dimensional techniques for constraining
and/or detecting this topology are reviewed. Initial applications of these
techniques using X-ray bright clusters of galaxies and quasars imply (weak)
candidates for a non-trivial topology.Comment: minor revision; 7 pages, 1 figure, accepted by Classical and Quantum
Gravit
Coupled dark matter-dark energy in light of near Universe observations
Cosmological analysis based on currently available observations are unable to
rule out a sizeable coupling among the dark energy and dark matter fluids. We
explore a variety of coupled dark matter-dark energy models, which satisfy
cosmic microwave background constraints, in light of low redshift and near
universe observations. We illustrate the phenomenology of different classes of
dark coupling models, paying particular attention in distinguishing between
effects that appear only on the expansion history and those that appear in the
growth of structure. We find that while a broad class of dark coupling models
are effectively models where general relativity (GR) is modified --and thus can
be probed by a combination of tests for the expansion history and the growth of
structure--, there is a class of dark coupling models where gravity is still
GR, but the growth of perturbations is, in principle modified. While this
effect is small in the specific models we have considered, one should bear in
mind that an inconsistency between reconstructed expansion history and growth
may not uniquely indicate deviations from GR. Our low redshift constraints
arise from cosmic velocities, redshift space distortions and dark matter
abundance in galaxy voids. We find that current data constrain the
dimensionless coupling to be |xi|<0.2, but prospects from forthcoming data are
for a significant improvement. Future, precise measurements of the Hubble
constant, combined with high-precision constraints on the growth of structure,
could provide the key to rule out dark coupling models which survive other
tests. We shall exploit as well weak equivalence principle violation arguments,
which have the potential to highly disfavour a broad family of coupled models.Comment: 34 pages, 6 figures; changes to match published versio
Dark Energy from Mass Varying Neutrinos
We show that mass varying neutrinos (MaVaNs) can behave as a negative
pressure fluid which could be the origin of the cosmic acceleration. We derive
a model independent relation between the neutrino mass and the equation of
state parameter of the neutrino dark energy, which is applicable for general
theories of mass varying particles. The neutrino mass depends on the local
neutrino density and the observed neutrino mass can exceed the cosmological
bound on a constant neutrino mass. We discuss microscopic realizations of the
MaVaN acceleration scenario, which involve a sterile neutrino. We consider
naturalness constraints for mass varying particles, and find that both ev
cutoffs and ev mass particles are needed to avoid fine-tuning. These
considerations give a (current) mass of order an eV for the sterile neutrino in
microscopic realizations, which could be detectable at MiniBooNE. Because the
sterile neutrino was much heavier at earlier times, constraints from big bang
nucleosynthesis on additional states are not problematic. We consider regions
of high neutrino density and find that the most likely place today to find
neutrino masses which are significantly different from the neutrino masses in
our solar system is in a supernova. The possibility of different neutrino mass
in different regions of the galaxy and the local group could be significant for
Z-burst models of ultra-high energy cosmic rays. We also consider the cosmology
of and the constraints on the ``acceleron'', the scalar field which is
responsible for the varying neutrino mass, and briefly discuss neutrino density
dependent variations in other constants, such as the fine structure constant.Comment: 26 pages, 3 figures, refs added, typos corrected, comment added about
possible matter effect
Hybrid dark sector: Locked quintessence and dark matter.
We present a unified model of dark matter and dark energy. The dark matter field is a modulus corresponding to a flat direction of supersymmetry, which couples, in a hybrid type potential, with the dark energy field. The latter is a light scalar, whose direction is stabilized by non-renormalizable terms. This quintessence field is kept 'locked' on top of a false vacuum due to the coupling with the oscillating dark matter field. It is shown that the model can satisfy the observations when we consider low-scale gauge-mediated supersymmetry breaking. The necessary initial conditions are naturally attained by the action of supergravity corrections on the potential, in the period following the end of primordial inflation
A search for anisotropy in the arrival directions of ultra high energy cosmic rays recorded at the Pierre Auger Observatory
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Contains fulltext :
93734-1.pdf (preprint version ) (Open Access