197 research outputs found
Weak lensing in scalar-tensor theories of gravity
This article investigates the signatures of various models of dark energy on
weak gravitational lensing, including the complementarity of the linear and
non-linear regimes. It investigates quintessence models and their extension to
scalar-tensor gravity. The various effects induced by this simplest extension
of general relativity are discussed. It is shown that, given the constraints in
the Solar System, models such as a quadratic nonminimal coupling do not leave
any signatures that can be detected while other models, such as a runaway
dilaton, which include attraction toward general relativity can let an imprint
of about 10%.Comment: 25 pages, 29 figure
The No-defect Conjecture: Cosmological Implications
When the topology of the universe is non trivial, it has been shown that
there are constraints on the network of domain walls, cosmic strings and
monopoles. I generalize these results to textures and study the cosmological
implications of such constraints. I conclude that a large class of
multi-connected universes with topological defects accounting for structure
formation are ruled out by observation of the cosmic microwave background.Comment: 4 pages, 1 figure, accepted for publication as a brief report in
Phys. Rev.
Quintessence and Gravitational Waves
We investigate some aspects of quintessence models with a non-minimally
coupled scalar field and in particular we show that it can behave as a
component of matter with . We study the
properties of gravitational waves in this class of models and discuss their
energy spectrum and the cosmic microwave background anisotropies they induce.
We also show that gravitational waves are damped by the anisotropic stress of
the radiation and that their energy spectrum may help to distinguish between
inverse power law potential and supergravity motivated potential. We finish by
a discussion on the constraints arising from their density parameter
\Omega_\GW.Comment: 21 pages, 18 figures, fianl version, accepted for publication in PR
Bacterial Symbiosis Maintenance in the Asexually Reproducing and Regenerating Flatworm Paracatenula galateia
Bacteriocytes set the stage for some of the most intimate interactions between animal and bacterial cells. In all bacteriocyte possessing systems studied so far, de novo formation of bacteriocytes occurs only once in the host development, at the time of symbiosis establishment. Here, we present the free-living symbiotic flatworm Paracatenula galateia and its intracellular, sulfur-oxidizing bacteria as a system with previously undescribed strategies of bacteriocyte formation and bacterial symbiont transmission. Using thymidine analogue S-phase labeling and immunohistochemistry, we show that all somatic cells in adult worms – including bacteriocytes – originate exclusively from aposymbiotic stem cells (neoblasts). The continued bacteriocyte formation from aposymbiotic stem cells in adult animals represents a previously undescribed strategy of symbiosis maintenance and makes P. galateia a unique system to study bacteriocyte differentiation and development. We also provide morphological and immunohistochemical evidence that P. galateia reproduces by asexual fragmentation and regeneration (paratomy) and, thereby, vertically transmits numerous symbiont-containing bacteriocytes to its asexual progeny. Our data support the earlier reported hypothesis that the symbiont population is subjected to reduced bottleneck effects. This would justify both the codiversification between Paracatenula hosts and their Candidatus Riegeria symbionts, and the slow evolutionary rates observed for several symbiont genes
(Mis-)Interpreting supernovae observations in a lumpy universe
Light from `point sources' such as supernovae is observed with a beam width
of order of the sources' size - typically less than 1 AU. Such a beam probes
matter and curvature distributions that are very different from coarse-grained
representations in N-body simulations or perturbation theory, which are
smoothed on scales much larger than 1 AU. The beam typically travels through
unclustered dark matter and hydrogen with a mean density much less than the
cosmic mean, and through dark matter halos and hydrogen clouds. Using N-body
simulations, as well as a Press-Schechter approach, we quantify the density
probability distribution as a function of beam width and show that, even for
Gpc-length beams of 500 kpc diameter, most lines of sight are significantly
under-dense. From this we argue that modelling the probability distribution for
AU-diameter beams is absolutely critical. Standard analyses predict a huge
variance for such tiny beam sizes, and nonlinear corrections appear to be
non-trivial. It is not even clear whether under-dense regions lead to dimming
or brightening of sources, owing to the uncertainty in modelling the expansion
rate which we show is the dominant contribution. By considering different
reasonable approximations which yield very different cosmologies we argue that
modelling ultra-narrow beams accurately remains a critical problem for
precision cosmology. This could appear as a discordance between angular
diameter and luminosity distances when comparing SN observations to BAO or CMB
distances.Comment: 20 pages and 6 figures. v3 is a substantially revised version, now
including detailed analysis of N-body and Press-Schechter predictions which
indicate that even for 1Gpc/h length beams, the mean density sampled is
significantly below the cosmic mea
Exhaustive Study of Cosmic Microwave Background Anisotropies in Quintessential Scenarios
Recent high precision measurements of the CMB anisotropies performed by the
BOOMERanG and MAXIMA-1 experiments provide an unmatched set of data allowing to
probe different cosmological models. Among these scenarios, motivated by the
recent measurements of the luminosity distance versus redshift relation for
type Ia supernovae, is the quintessence hypothesis. It consists in assuming
that the acceleration of the Universe is due to a scalar field whose final
evolution is insensitive to the initial conditions. Within this framework we
investigate the cosmological perturbations for two well-motivated potentials:
the Ratra-Peebles and the SUGRA tracking potentials. We show that the solutions
of the perturbed equations possess an attractor and that, as a consequence, the
insensitivity to the initial conditions is preserved at the perturbed level.
Then, we study the predictions of these two models for structure formation and
CMB anisotropies and investigate the general features of the multipole moments
in the presence of quintessence. We also compare the CMB multipoles calculated
with the help of a full Boltzmann code with the BOOMERanG and MAXIMA-1 data. We
pay special attention to the location of the second peak and demonstrate that
it significantly differs from the location obtained in the cosmological
constant case. Finally, we argue that the SUGRA potential is compatible with
all the recent data with a standard values of the cosmological parameters. In
particular, it fits the MAXIMA-1 data better than a cosmological constant or
the Ratra-Peebles potential.Comment: 18 pages, 20 figures, submitted to PR
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