911 research outputs found
Do the cosmological observational data prefer phantom dark energy?
The dynamics of expansion and large scale structure formation of the Universe
are analyzed for models with dark energy in the form of a phantom scalar field
which initially mimics a -term and evolves slowly to the Big Rip
singularity. The discussed model of dark energy has three parameters -- the
density and the equation of state parameter at the current epoch,
and , and the asymptotic value of the equation of state parameter at
, . Their best-fit values are determined jointly
with all other cosmological parameters by the MCMC method using observational
data on CMB anisotropies and polarization, SNe Ia luminosity distances, BAO
measurements and more. Similar computations are carried out for CDM
and a quintessence scalar field model of dark energy. It is shown that the
current data slightly prefer the phantom model, but the differences in the
maximum likelihoods are not statistically significant. It is also shown that
the phantom dark energy with monotonically increasing density in future will
cause the decay of large scale linear matter density perturbations due to the
gravitational domination of dark energy perturbations long before the Big Rip
singularity.Comment: 13 pages, 8 figures, 5 tables; comments and references added; version
accepted for publication in Phys.Rev.
Reproducing the observed Cosmic microwave background anisotropies with causal scaling seeds
During the last years it has become clear that global O(N) defects and U(1)
cosmic strings do not lead to the pronounced first acoustic peak in the power
spectrum of anisotropies of the cosmic microwave background which has recently
been observed to high accuracy. Inflationary models cannot easily accommodate
the low second peak indicated by the data. Here we construct causal scaling
seed models which reproduce the first and second peak. Future, more precise CMB
anisotropy and polarization experiments will however be able to distinguish
them from the ordinary adiabatic models.Comment: 6 pages 2 figures, revtex; minor corrections and references adde
Reproducing Cosmic Microwave Background anisotropies with mixed isocurvature perturbations
Recently high quality data of the cosmic microwave background anisotropies
have been published. In this work we study to which extent the cosmological
parameters determined by using this data depend on assumptions about the
initial conditions. We show that for generic initial conditions, not only the
best fit values are very different but, and this is our main result, the
allowed parameter range enlarges dramatically.Comment: 4 pages, 5 figures, submitted to PRL; Major changes following
referees suggestions; the allowed cosmological parameter range enlarges
dramaticall
CMB anisotropies from pre-big bang cosmology
We present an alternative scenario for cosmic structure formation where
initial fluctuations are due to Kalb-Ramond axions produced during a pre-big
bang phase of inflation. We investigate whether this scenario, where the
fluctuations are induced by seeds and therefore are of isocurvature nature, can
be brought in agreement with present observations by a suitable choice of
cosmological parameters. We also discuss several observational signatures which
can distinguish axion seeds from standard inflationary models. We finally
discuss the gravitational wave background induced in this model and we show
that it may be well within the range of future observations.Comment: 33 pages, 18 figures, corrected some typo
The Cosmic Microwave Background and Helical Magnetic Fields: the tensor mode
We study the effect of a possible helicity component of a primordial magnetic
field on the tensor part of the cosmic microwave background temperature
anisotropies and polarization. We give analytical approximations for the tensor
contributions induced by helicity, discussing their amplitude and spectral
index in dependence of the power spectrum of the primordial magnetic field. We
find that an helical magnetic field creates a parity odd component of gravity
waves inducing parity odd polarization signals. However, only if the magnetic
field is close to scale invariant and if its helical part is close to maximal,
the effect is sufficiently large to be observable. We also discuss the
implications of causality on the magnetic field spectrum.Comment: We have corrected a normalisation error which was pointed out to us
by Antony Lewis. It enhances our limits on the magnetic fields by
(2\pi)^{3/4} ~
Are there static texture?
We consider harmonic maps from Minkowski space into the three sphere. We are
especially interested in solutions which are asymptotically constant, i.e.
converge to the same value in all directions of spatial infinity. Physical
3-space can then be compactified and can be identified topologically (but not
metrically!) with a three sphere. Therefore, at fixed time, the winding of the
map is defined. We investigate whether static solutions with non-trivial
winding number exist. The answer which we can proof here is only partial: We
show that within a certain family of maps no static solutions with non-zero
winding number exist. We discuss the existing static solutions in our family of
maps. An extension to other maps or a proof that our family of maps is
sufficiently general remains an open problem.Comment: 12 page Latex file, 1 postscript figure, submitted to PR
Detection of gravitational waves from the QCD phase transition with pulsar timing arrays
If the cosmological QCD phase transition is strongly first order and lasts
sufficiently long, it generates a background of gravitational waves which may
be detected via pulsar timing experiments. We estimate the amplitude and the
spectral shape of such a background and we discuss its detectability prospects.Comment: 7 pages, 5 figs. Version accepted by PR
Dynamical Casimir effect for gravitons in bouncing braneworlds
We consider a two-brane system in a five-dimensional anti-de Sitter
spacetime. We study particle creation due to the motion of the physical brane
which first approaches the second static brane (contraction) and then recedes
from it(expansion). The spectrum and the energy density of the generated
gravitons are calculated. We show that the massless gravitons have a blue
spectrum and that their energy density satisfies the nucleosynthesis bound with
very mild constraints on the parameters. We also show that the Kaluza-Klein
modes cannot provide the dark matter in an anti-de-Sitter braneworld. However,
for natural choices of parameters, backreaction from the Kaluza-Klein gravitons
may well become important. The main findings of this work have been published
in the form of a Letter [R. Durrer and M. Ruser, Phys. Rev. Lett. 99, 071601
(2007), arXiv:0704.0756].Comment: 40 pages, 34 figures, improved and extended version, matches
published versio
Skewness as a probe of non-Gaussian initial conditions
We compute the skewness of the matter distribution arising from non-linear
evolution and from non-Gaussian initial perturbations. We apply our result to a
very generic class of models with non-Gaussian initial conditions and we
estimate analytically the ratio between the skewness due to non-linear
clustering and the part due to the intrinsic non-Gaussianity of the models. We
finally extend our estimates to higher moments.Comment: 5 pages, 2 ps-figs., accepted for publication in PRD, rapid com
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