538 research outputs found
On the Phase-Space Volume of Primordial Cosmological Perturbations
We show how to determine the typical phase space volume for
primordial gravitational waves produced during an inflationary stage, which is
invariant under squeezing. An expression for is found in the long
wavelength regime. The quasi-classical entropy of a pure vacuum initial state
defined as the logarithm of modulo a constant remains zero in spite of
the generation of fluctuations (creation of real gravitons).Comment: LaTeX (10 pages
Dark energy with non-adiabatic sound speed: initial conditions and detectability
Assuming that the universe contains a dark energy fluid with a constant
linear equation of state and a constant sound speed, we study the prospects of
detecting dark energy perturbations using CMB data from Planck,
cross-correlated with galaxy distribution maps from a survey like LSST. We
update previous estimates by carrying a full exploration of the mock data
likelihood for key fiducial models. We find that it will only be possible to
exclude values of the sound speed very close to zero, while Planck data alone
is not powerful enough for achieving any detection, even with lensing
extraction. We also discuss the issue of initial conditions for dark energy
perturbations in the radiation and matter epochs, generalizing the usual
adiabatic conditions to include the sound speed effect. However, for most
purposes, the existence of attractor solutions renders the perturbation
evolution nearly independent of these initial conditions.Comment: 16 pages, 2 figures, version accepted in JCA
On the Entropy and the Density Matrix of Cosmological Perturbations
We look at the transition to the semiclassical behaviour and the decoherence
process for the inhomogeneous perturbations in the inflationary universe. Two
different decoherence mechanisms appear: one dynamical, accompanied with a
negligible, if at all, entropy gain, and the other, effectively irreversible
dephasing, due to a rapid variation in time of the off-diagonal density matrix
elements in the post-inflationary epoch. We thus settle the discrepancies in
the entropy content of perturbations evaluated by different authors.Comment: LaTeX2e with the epsf packag
Cosmology of the Randall-Sundrum model after dilaton stabilization
We provide the first complete analysis of cosmological evolution in the Randall-Sundrum model with stabilized dilaton. We give the exact expansion law for matter densities on the two branes with arbitrary equations of state. The effective four-dimensional theory leads to standard cosmology at low energy. The limit of validity of the low energy theory and possible deviations from the ordinary expansion law in the high energy regime are finally discussed
Goldberger-Wise variations: stabilizing brane models with a bulk scalar
Braneworld scenarios with compact extra-dimensions need the volume of the
extra space to be stabilized. Goldberger and Wise have introduced a simple
mechanism, based on the presence of a bulk scalar field, able to stabilize the
radius of the Randall-Sundrum model. Here, we transpose the same mechanism to
generic single-brane and two-brane models, with one extra dimension and
arbitrary scalar potentials in the bulk and on the branes. The single-brane
construction turns out to be always unstable, independently of the bulk and
brane potentials. In the case of two branes, we derive some generic criteria
ensuring the stabilization or destabilization of the system.Comment: 8 pages, 2 figures. 1 figure and one subsection added. version
published on PR
Realistic sterile neutrino dark matter with keV mass does not contradict cosmological bounds
Previous fits of sterile neutrino dark matter models to cosmological data
assumed a peculiar production mechanism, which is not representative of the
best-motivated particle physics models given current data on neutrino
oscillations. These analyses ruled out sterile neutrino masses smaller than
8-10 keV. Here we focus on sterile neutrinos produced resonantly. We show that
their cosmological signature can be approximated by that of mixed Cold plus
Warm Dark Matter (CWDM). We use recent results on LambdaCWDM models to show
that for each mass greater than or equal to 2 keV, there exists at least one
model of sterile neutrino accounting for the totality of dark matter, and
consistent with Lyman-alpha and other cosmological data. Resonant production
occurs in the framework of the nuMSM (the extension of the Standard Model with
three right-handed neutrinos). The models we checked to be allowed correspond
to parameter values consistent with neutrino oscillation data, baryogenesis and
all other dark matter bounds.Comment: 4 pages, 4 figure
Current cosmological bounds on neutrino masses and relativistic relics
We combine the most recent observations of large-scale structure (2dF and
SDSS galaxy surveys) and cosmic microwave anisotropies (WMAP and ACBAR) to put
constraints on flat cosmological models where the number of massive neutrinos
and of massless relativistic relics are both left arbitrary. We discuss the
impact of each dataset and of various priors on our bounds. For the standard
case of three thermalized neutrinos, we find an upper bound on the total
neutrino mass sum m_nu < 1.0 (resp. 0.6) eV (at 2sigma), using only CMB and LSS
data (resp. including priors from supernovae data and the HST Key Project), a
bound that is quite insensitive to the splitting of the total mass between the
three species. When the total number of neutrinos or relativistic relics N_eff
is left free, the upper bound on sum m_nu (at 2sigma, including all priors)
ranges from 1.0 to 1.5 eV depending on the mass splitting. We provide an
explanation of the parameter degeneracy that allows larger values of the masses
when N_eff increases. Finally, we show that the limit on the total neutrino
mass is not significantly modified in the presence of primordial gravitational
waves, because current data provide a clear distinction between the
corresponding effects.Comment: 13 pages, 6 figure
Optimising Boltzmann codes for the Planck era
High precision measurements of the Cosmic Microwave Background (CMB)
anisotropies, as can be expected from the Planck satellite, will require
high-accuracy theoretical predictions as well. One possible source of
theoretical uncertainty is the numerical error in the output of the Boltzmann
codes used to calculate angular power spectra. In this work, we carry out an
extensive study of the numerical accuracy of the public Boltzmann code CAMB,
and identify a set of parameters which determine the error of its output. We
show that at the current default settings, the cosmological parameters
extracted from data of future experiments like Planck can be biased by several
tenths of a standard deviation for the six parameters of the standard
Lambda-CDM model, and potentially more seriously for extended models. We
perform an optimisation procedure that leads the code to achieve sufficient
precision while at the same time keeping the computation time within reasonable
limits. Our conclusion is that the contribution of numerical errors to the
theoretical uncertainty of model predictions is well under control -- the main
challenges for more accurate calculations of CMB spectra will be of an
astrophysical nature instead.Comment: 13 pages, 4 figure
Neutrinos and Future Concordance Cosmologies
We review the free parameters in the concordance cosmology, and those which
might be added to this set as the quality of astrophysical data improves. Most
concordance parameters encode information about otherwise unexplored aspects of
high energy physics, up to the GUT scale via the "inflationary sector," and
possibly even the Planck scale in the case of dark energy. We explain how
neutrino properties may be constrained by future astrophysical measurements.
Conversely, future neutrino physics experiments which directly measure these
parameters will remove uncertainty from fits to astrophysical data, and improve
our ability to determine the global properties of our universe.Comment: Proceedings of paper given at Neutrino 2008 meeting (by RE
Probing interactions within the dark matter sector via extra radiation contributions
The nature of dark matter is one of the most thrilling riddles for both cosmology and particle physics nowadays. While in the typical models the dark sector is composed only by weakly interacting massive particles, an arguably more natural scenario would include a whole set of gauge interactions which are invisible for the standard model but that are in contact with the dark matter. We present a method to constrain the number of massless gauge bosons and other relativistic particles that might be present in the dark sector using current and future cosmic microwave background data, and provide upper bounds on the size of the dark sector. We use the fact that the dark matter abundance depends on the strength of the interactions with both sectors, which allows one to relate the freeze-out temperature of the dark matter with the temperature of this cosmic background of dark gauge bosons. This relation can then be used to calculate how sizable is the impact of the relativistic dark sector in the number of degrees of freedom of the early Universe, providing an interesting and testable connection between cosmological data and direct/indirect detection experiments. The recent Planck data, in combination with other cosmic microwave background experiments and baryonic acoustic oscillations data, constrains the number of relativistic dark gauge bosons, when the freeze-out temperature of the dark matter is larger than the top mass, to be N<14 for the simplest scenarios, while those limits are slightly relaxed for the combination with the Hubble constant measurements to N<20. Future releases of Planck data are expected to reduce the uncertainty by approximately a factor of 3, which will reduce significantly the parameter space of allowed models
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