23 research outputs found
Dark energy and CMB bispectrum
We consider the CMB bispectrum signal induced by structure formation through
the correlation between the Integrated Sachs-Wolfe and the weak lensing effect.
We investigate how the bispectrum knowledge can improve our knowledge of the
most important cosmological parameters, focusing on the dark energy ones.
Preliminary results suggest a consistent improvement on the estimation of dark
energy abundance and on dynamical properties of the equation of state.Comment: 6 pages, 5 figures. To appear in "Impact of Gravitational Lensing on
Cosmology", IAU Symposium 225, Mellier & Meylan ed
The Dyer-Roeder distance in quintessence cosmology and the estimation of H_0 through time-delays
We calculate analytically and numerically the Dyer-Roeder distance in perfect
fluid quintessence models and give an accurate fit to the numerical solutions
for all the values of the density parameter and the quintessence equation of
state. Then we apply our solutions to the estimation of from multiple
image time delays and find that the inclusion of quintessence modifies sensibly
the likelihood distribution of , generally reducing the best estimate
with respect to a pure cosmological constant. Marginalizing over the other
parameters ( and the quintessence equation of state), we obtain
km/sec/Mpc for an empty beam and km/sec/Mpc for
a filled beam. We also discuss the future prospects for distinguishing
quintessence from a cosmological constant with time delays.Comment: 10 pages, 6 figures, submitted to MNRA
Rediscovering the scientific and didactic value of minor herbarium collections: the seeds and fruits collection by Gustavo Bonaventura
Seeds and fruits collections are very important from a systematic point of view and represent useful references in several disciplines and research fields. The Herbarium of Sapienza University of Rome (RO) hosts a Spermoteque/Carpoteque, which was organized by Gustavo Bonaventura (1902-1976). The purpose of this paper is to describe the heritage of Bonaventura's collection. It consists of 42 wooden boxes, globally hosting 3411 glass tubes containing seeds, fruits, and other materials. The collection was first of all catalogued; then, analysis were conducted regarding taxonomic composition, temporal and geographic coverage, institutions of provenience, collectors, content, and preservation status. The specimens refer to 2740 taxa, belonging to 890 genera and 135 families. Many genera of agricultural interest are present, each one with different cultivars. The collection spans across 130 years (1843-1975) and hosts specimens coming from all over the world. Materials were provided by several herbaria, botanical gardens and agrarian institutes, and by 50 collectors. The Bonaventura's collection is still a useful reference collection, testifying biodiversity over times and thus being useful for diachronic studies; moreover, it documents the interests of collectors and the past network activity between institutions
An investigation of gravitational lens determinations of H_o in quintessence cosmologies
There is growing evidence that the majority of the energy density of the
universe is not baryonic or dark matter, rather it resides in an exotic
component with negative pressure. The nature of this `quintessence' influences
our view of the universe, modifying angular diameter and luminosity distances.
Here, we examine the influence of a quintessence component upon gravitational
lens time delays. As well as a static quintessence component, an evolving
equation of state is also considered. It is found that the equation of state of
the quintessence component and its evolution influence the value of the
Hubble's constant derived from gravitational lenses. However, the differences
between evolving and non-evolving cosmologies are relatively small. We
undertake a suite of Monte Carlo simulations to examine the potential
constraints that can be placed on the universal equation of state from the
monitoring of gravitational lens system, and demonstrate that at least an order
of magnitude more lenses than currently known will have to be discovered and
analysed to accurately probe any quintessence component.Comment: 8pages, accepted for publication in MNRA
Probing Gravitation, Dark Energy, and Acceleration
The acceleration of the expansion of the universe arises from unknown
physical processes involving either new fields in high energy physics or
modifications of gravitation theory. It is crucial for our understanding to
characterize the properties of the dark energy or gravity through cosmological
observations and compare and distinguish between them. In fact, close
consistencies exist between a dark energy equation of state function w(z) and
changes to the framework of the Friedmann cosmological equations as well as
direct spacetime geometry quantities involving the acceleration, such as
``geometric dark energy'' from the Ricci scalar. We investigate these
interrelationships, including for the case of superacceleration or phantom
energy where the fate of the universe may be more gentle than the Big Rip.Comment: 12 pages, 8 figure
The angular size - redshift relation in power-law cosmologies
A linear evolution of the cosmological scale factor is a feature in several
models designed to solve the cosmological constant problem via a coupling
between scalar or tensor classical fields to the space-time curvature as well
as in some alternative gravity theories. In this paper, by assuming a general
time dependence of the scale factor, , we investigate
observational constraints on the dimensionless parameter from
measurements of the angular size for a large sample of milliarcsecond compact
radio sources. In particular, we find that a strictly linear evolution, i.e.,
is favoured by these data, which is also in agreement with
limits obtained from other independent cosmological tests. The dependence of
the critical redshift (at which a given angular size takes its minimal
value) with the index is briefly discussed.Comment: 5 pages, 4 figures, LaTe
Constraining the dark energy dynamics with the cosmic microwave background bispectrum
We consider the influence of the dark energy dynamics at the onset of cosmic
acceleration on the Cosmic Microwave Background (CMB) bispectrum, through the
weak lensing effect induced by structure formation. We study the line of sight
behavior of the contribution to the bispectrum signal at a given angular
multipole : we show that it is non-zero in a narrow interval centered at a
redshift satisfying the relation , where the
wavenumber corresponds to the scale entering the non-linear phase, and is
the cosmological comoving distance. The relevant redshift interval is in the
range 0.1\lsim z\lsim 2 for multipoles 1000\gsim\ell\gsim 100; the signal
amplitude, reflecting the perturbation dynamics, is a function of the
cosmological expansion rate at those epochs, probing the dark energy equation
of state redshift dependence independently on its present value. We provide a
worked example by considering tracking inverse power law and SUGRA Quintessence
scenarios, having sensibly different redshift dynamics and respecting all the
present observational constraints. For scenarios having the same present
equation of state, we find that the effect described above induces a projection
feature which makes the bispectra shifted by several tens of multipoles, about
10 times more than the corresponding effect on the ordinary CMB angular power
spectrum.Comment: 15 pages, 7 figures, matching version accepted by Physical Review D,
one figure improve
Weak lensing in generalized gravity theories
We extend the theory of weak gravitational lensing to cosmologies with generalized gravity, described in the Lagrangian by a generic function depending on the Ricci scalar and a nonminimal coupled scalar field. We work out the generalized Poisson equations relating the dynamics of the fluctuating components to the two gauge-invariant scalar gravitational potentials, fixing the contributions from the modified background expansion and fluctuations. We show how the lensing equation gets modified by the cosmic expansion as well as by the presence of anisotropic stress, which is non-null at the linear level both in scalar-tensor gravity and in theories where the gravitational Lagrangian term features a nonminimal dependence on the Ricci scalar. Starting from the geodesic deviation, we derive the generalized expressions for the shear tensor and projected lensing potential, encoding the spacetime variation of the effective gravitational constant and isolating the contribution of the anisotropic stress, which introduces a correction due to the spatial correlation between the gravitational potentials. Finally, we work out the expressions of the lensing convergence power spectrum as well as the correlation between the lensing potential and the integrated Sachs-Wolfe effect affecting cosmic microwave background total intensity and polarization anisotropies. To illustrate phenomenologically the effects, we work out approximate expressions for the quantities above in extended quintessence scenarios where the scalar field coupled to gravity plays the role of the dark energy
Malcolmia littorea (L.) R. Br.
Viene presentato lo stato di rischio di estinzione di Malcolmia littorea in Italia secondo il protocollo IUC