48,736 research outputs found
WMAP Constraints on a Quintessence Model
We use the results from the Wilkinson Microwave Anisotropy Probe (WMAP) for
the locations of peaks and troughs of the Cosmic Microwave Background (CMB)
power spectrum, together with constraints from large-scale structure, to study
a quintessence model in which the pure exponential potential is modified by a
polynomial factor. Our analysis, in the cosmological
parameters space shows that this quintessence model is favoured compared to
CDM for and relatively high values of early
quintessence; for , quintessence and CDM give similar results,
except for high values of early quintessence, in which case CDM is
favoured.Comment: 3 pages. Talk presented by N. M. C. Santos at the Tenth Marcel
Grossmann Meeting on General Relativity, Rio de Janeiro, July 200
An exterior for the G\"{o}del spacetime
We match the vacuum, stationary, cylindrically symmetric solution of
Einstein's field equations with , in a form recently given by Santos,
as an exterior to an infinite cylinder of dust cut out of a G\"{o}del universe.
There are three cases, depending on the radius of the cylinder. Closed timelike
curves are present in the exteriors of some of the solutions. There is a
considerable similarity between the spacetimes investigated here and those of
van Stockum referring to an infinite cylinder of rotating dust matched to
vacuum, with .Comment: 11 pages, LaTeX 2.09, no figures. Submitted to Classical and Quantum
Gravit
Bulk-Edge correspondence of entanglement spectrum in 2D spin ground states
General local spin ground states, described by a Valence Bond Solid (VBS)
on a two dimensional lattice are studied. The norm of these ground states is
mapped to a classical O(3) model on the same lattice. Using this
quantum-to-classical mapping we obtain the partial density matrix
associated with a subsystem of the original ground state. We show that
the entanglement spectrum of in a translation invariant lattice
is given by the spectrum of a quantum spin chain at the boundary of region ,
with local Heisenberg type interactions between spin 1/2 particles.Comment: 8 pages, 4 figures, one section and references adde
Supernovae constraints on dark energy and modified gravity models
We use the Type Ia Supernova gold sample to constrain the parameters of dark
energy models namely the Cardassian, Dvali-Turner (DT) and generalized
Chaplygin gas (GCG) models. In our best fit analysis for these dark energy
proposals we consider flat and the non-flat priors. For all models, we find
that relaxing the flatness condition implies that data favors a positive
curvature; moreover, the GCG model is nearly flat, as required by Cosmic
Microwave Background (CMB) observations.Comment: 6 pages, Latex file + 9 eps figures + (jpconf.cls,jpconf11.clo), to
appear in the Proceedings of the Fourth Meeting on Constrained Dynamics and
Quantum Gravity (QG05), Cala Gonone (Italy) September 12-16 200
Abundance ratios of volatile vs. refractory elements in planet-harbouring stars: hints of pollution?
We present the [X/H] trends as function of the elemental condensation
temperature Tc in 88 planet host stars and in a volume-limited comparison
sample of 33 dwarfs without detected planetary companions. We gathered
homogeneous abundance results for many volatile and refractory elements
spanning a wide range of Tc, from a few dozens to several hundreds kelvin. We
investigate possible anomalous trends of planet hosts with respect to
comparison sample stars in order to detect evidence of possible pollution
events. No significant differences are found in the behaviour of stars with and
without planets. This result is in agreement with a ``primordial'' origin of
the metal excess in planet host stars. However, a subgroup of 5 planet host and
1 comparison sample stars stands out for having particularly high [X/H] vs. Tc
slopes.Comment: 10 pages, 7 figures, accepted for publication in A&A. Figures with
higher resolution are available at www.iac.es/proyect/abuntes
Third and fourth degree collisional moments for inelastic Maxwell models
The third and fourth degree collisional moments for -dimensional inelastic
Maxwell models are exactly evaluated in terms of the velocity moments, with
explicit expressions for the associated eigenvalues and cross coefficients as
functions of the coefficient of normal restitution. The results are applied to
the analysis of the time evolution of the moments (scaled with the thermal
speed) in the free cooling problem. It is observed that the characteristic
relaxation time toward the homogeneous cooling state decreases as the
anisotropy of the corresponding moment increases. In particular, in contrast to
what happens in the one-dimensional case, all the anisotropic moments of degree
equal to or less than four vanish in the homogeneous cooling state for .Comment: 15 pages, 3 figures; v2: addition of two new reference
Kerr Geodesics, the Penrose Process and Jet Collimation by a Black Hole
We re-examine the possibility that astrophysical jet collimation may arise
from the geometry of rotating black holes and the presence of high-energy
particles resulting from a Penrose process, without the help of magnetic
fields. Our analysis uses the Weyl coordinates, which are revealed better
adapted to the desired shape of the jets. We numerically integrate the
2D-geodesics equations. We give a detailed study of these geodesics and give
several numerical examples. Among them are a set of perfectly collimated
geodesics with asymptotes parallel to the axis, with
only depending on the ratios and
, where and are the parameters of the Kerr black hole,
the particle energy and the Carter's constant.Comment: Accepted by Astronomy and Astrophysics. AA style with 3 EPS figures.
Content amended after AA's refereeing. Discussion of geodesics also corrected
and expanded earlier. Conclusions amended accordingl
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