70 research outputs found
Dark Matter Decaying into a Fermi Sea of Neutrinos
We study the possible decay of a coherently oscillating scalar field,
interpreted as dark matter, into light fermions. Specifically, we consider a
scalar field with sub-eV mass decaying into a Fermi sea of neutrinos. We
recognize the similarity between our scenario and inflationary preheating where
a coherently oscillating scalar field decays into standard model particles.
Like the case of fermionic preheating, we find that Pauli blocking controls the
dark matter decay into the neutrino sea. The radius of the Fermi sphere depends
on the expansion of the universe leading to a time varying equation of state of
dark matter. This makes the scenario very rich and we show that the decay rate
might be different at different cosmological epochs. We categorize this in two
interesting regimes and then study the cosmological perturbations to find the
impact on structure formation. We find that the decay may help alleviating some
of the standard problems related to cold dark matter.Comment: 8 pages, 3 figures, accepted for publication in Phys. Rev.
Dark energy properties from large future galaxy surveys
We perform a detailed forecast on how well a {\sc Euclid}-like survey will be
able to constrain dark energy and neutrino parameters from a combination of its
cosmic shear power spectrum, galaxy power spectrum, and cluster mass function
measurements. We find that the combination of these three probes vastly
improves the survey's potential to measure the time evolution of dark energy.
In terms of a dark energy figure-of-merit defined as , we find a value of 690 for {\sc Euclid}-like data combined
with {\sc Planck}-like measurements of the cosmic microwave background (CMB)
anisotropies in a 10-dimensional cosmological parameter space, assuming a
CDM fiducial cosmology. For the more commonly used 7-parameter model,
we find a figure-of-merit of 1900 for the same data combination. We consider
also the survey's potential to measure dark energy perturbations in models
wherein the dark energy is parameterised as a fluid with a nonstandard
non-adiabatic sound speed, and find that in an \emph{optimistic} scenario in
which deviates by as much as is currently observationally allowed from
, models with and can be distinguished at more than significance. We emphasise that
constraints on the dark energy sound speed from cluster measurements are
strongly dependent on the modelling of the cluster mass function; significantly
weaker sensitivities ensue if we modify our model to include fewer features of
nonlinear dark energy clustering. Finally, we find that the sum of neutrino
masses can be measured with a precision of 0.015~eV, (abridged)Comment: 26 pages, 5 figures, matches JCAP versio
Phenomenology of Hybrid Scenarios of Neutrino Dark Energy
We study the phenomenology of hybrid scenarios of neutrino dark energy, where
in addition to a so-called Mass Varying Neutrino (MaVaN) sector a cosmological
constant (from a false vacuum) is driving the accelerated expansion of the
universe today. For general power law potentials we calculate the effective
equation of state parameter w_{eff}(z) in terms of the neutrino mass scale. Due
to the interaction of the dark energy field (acceleron) with the neutrino
sector, w_{eff}(z) is predicted to become smaller than -1 for z>0, which could
be tested in future cosmological observations. For the considered scenarios,
the neutrino mass scale additionally determines which fraction of the dark
energy is dynamical, and which originates from the cosmological constant like
vacuum energy of the false vacuum. On the other hand, the field value of the
acceleron field today as well as the masses of the right-handed neutrinos,
which appear in the seesaw-type mechanism for small neutrino masses, are not
fixed. This, in principle, allows to realise hybrid scenarios of neutrino dark
energy with a high-scale seesaw where the right-handed neutrino masses are
close to the GUT scale. We also comment on how MaVaN Hybrid Scenarios with
high-scale seesaw might help to resolve stability problems of dark energy
models with non-relativistic neutrinos.Comment: 22 pages, 5 figures; references and comments added; version to be
published in JCA
Are cosmological neutrinos free-streaming?
Precision data from cosmology suggest neutrinos stream freely and hence
interact very weakly around the epoch of recombination. We study this issue in
a simple framework where neutrinos recouple instantaneously and stop streaming
freely at a redshift z_i. The latest cosmological data imply z_i < 1500, the
exact constraint depending somewhat on the assumed prior on z_i. This bound
translates into a limit on the coupling strength between neutrinos and
majoron-like particles phi, implying tau > 1 x 10^10 s (m_2/50 meV)^3 for the
decay nu_2 -> nu_1+phi.Comment: 4 pages, 2 figure
Coupling active and sterile neutrinos in the cosmon plus seesaw framework
The cosmological evolution of neutrino energy densities driven by cosmon-type
field equations is introduced assuming that active and sterile neutrinos are
intrinsically connected by cosmon fields through the {\em seesaw} mechanism.
Interpreting sterile neutrinos as dark matter adiabatically coupled with dark
energy results in a natural decoupling of (active) mass varying neutrino
(MaVaN) equations. Identifying the dimensionless scale of the {\em seesaw}
mechanism, , with a power of the cosmological scale factor, , allows
for embedding the resulting masses into the generalized Chaplygin gas (GCG)
scenario for the dark sector. Without additional assumptions, our findings
establish a precise connection among three distinct frameworks: the cosmon
field dynamics for MaVaN's, the {\em seesaw} mechanism for dynamical mass
generation and the GCG scenario. Our results also corroborate with previous
assertions that mass varying particles can be the right responsible for the
stability issue and for the cosmic acceleration of the universe.Comment: 12 pages, 2 figure
Mass-varying neutrino in light of cosmic microwave background and weak lensing
We aim to constrain mass-varying neutrino models using large scale structure
observations and produce forecast for the Euclid survey. We investigate two
models with different scalar field potential and both positive and negative
coupling parameters \beta. These parameters correspond to growing or decreasing
neutrino mass, respectively. We explore couplings up to |\beta|<5. In the case
of the exponential potential, we find an upper limit on <0.004
at 2- level. In the case of the inverse power law potential the null
coupling can be excluded with more than 2-\sigma significance; the limits on
the coupling are \beta>3 for the growing neutrino mass and \beta<-1.5 for the
decreasing mass case. This is a clear sign for a preference of higher
couplings. When including a prior on the present neutrino mass the upper limit
on the coupling becomes |\beta|<3 at 2- level for the exponential
potential. Finally, we present a Fisher forecast using the tomographic weak
lensing from an Euclid-like experiment and we also consider the combination
with the cosmic microwave background (CMB) temperature and polarisation spectra
from a Planck-like mission. If considered alone, lensing data is more efficient
in constraining with respect to CMB data alone. There is, however,
a strong degeneracy in the \beta- plane. When the two data sets
are combined, the latter degeneracy remains, but the errors are reduced by a
factor ~2 for both parameters.Comment: 5 pages, 6 figures. Now published in A&A 500, 657-665 (2009
Dynamical neutrino masses in the generalized Chaplygin gas scenario with mass varying CDM
Neutrinos coupled to an underlying scalar field in the scenario for
unification of mass varying dark matter and cosmon-{\em like} dark energy is
examined. In the presence of a tiny component of mass varying neutrinos, the
conditions for the present cosmic acceleration and for the stability issue are
reproduced. It is assumed that {\em sterile} neutrinos behave like mass varying
dark matter coupled to mass varying {\em active} neutrinos through the {\em
seesaw} mechanism, in a kind of {\em mixed} dark matter sector. The crucial
point is that the dark matter mass may also exhibit a dynamical behavior driven
by the scalar field. The scalar field mediates the nontrivial coupling between
the mixed dark matter and the dark energy responsible for the accelerated
expansion of the universe. The equation of state of perturbations reproduce the
generalized Chaplygin gas (GCG) cosmology so that all the effective results
from the GCG paradigm are maintained, being perturbatively modified by
neutrinos.Comment: 19 pages, 5 figure
Cosmology when living near the Great Attractor
If we live in the vicinity of the hypothesized Great Attractor, the age of
the universe as inferred from the local expansion rate can be off by three per
cent. We study the effect that living inside or near a massive overdensity has
on cosmological parameters induced from observations of supernovae, the Hubble
parameter and the Cosmic Microwave Background. We compare the results to those
for an observer in a perfectly homogeneous LCDM universe. We find that for
instance the inferred value for the global Hubble parameter changes by around
three per cent if we happen to live inside a massive overdensity such as the
hypothesized Great Attractor. Taking into account the effect of such structures
on our perception of the universe makes cosmology perhaps less precise, but
more accurate.Comment: 8 pages, 6 figures, Submitted to MNRA
Higgs-Dilaton cosmology: Are there extra relativistic species?
Recent analyses of cosmological data suggest the presence of an extra
relativistic component beyond the Standard Model content. The Higgs-Dilaton
cosmological model predicts the existence of a massless particle -the dilaton-
associated with the spontaneous symmetry breaking of scale invariance and
undetectable by any accelerator experiment. Its ultrarelativistic character
makes it a suitable candidate for contributing to the effective number of light
degrees of freedom in the Universe. In this Letter we analyze the dilaton
production at the (p)reheating stage right after inflation and conclude that no
extra relativistic degrees of freedom beyond those already present in the
Standard Model are expected within the simplest Higgs-Dilaton scenario. The
elusive dilaton remains thus essentially undetectable by any particle physics
experiment or cosmological observation.Comment: 7 pages, 1 figure, corrected typos; added reference
- …