216 research outputs found
Cosmographic analysis of dark energy
The Hubble relation between distance and redshift is a purely cosmographic
relation that depends only on the symmetries of a FLRW spacetime, but does not
intrinsically make any dynamical assumptions. This suggests that it should be
possible to estimate the parameters defining the Hubble relation without making
any dynamical assumptions. To test this idea, we perform a number of
inter-related cosmographic fits to the legacy05 and gold06 supernova datasets,
paying careful attention to the systematic uncertainties. Based on this
supernova data, the "preponderance of evidence" certainly suggests an
accelerating universe. However we would argue that (unless one uses additional
dynamical and observational information, and makes additional theoretical
assumptions) this conclusion is not currently supported "beyond reasonable
doubt". As part of the analysis we develop two particularly transparent
graphical representations of the redshift-distance relation -- representations
in which acceleration versus deceleration reduces to the question of whether
the relevant graph slopes up or down.Comment: 14 pages. To appear in the Proceedings of DARK 2009 -- The Seventh
International Heidelberg Conference on Dark Matter in Astro and Particle
Physics, Christchurch, New Zealand, January 200
Cosmological milestones and gravastars - topics in general relativity
In this thesis, we consider two different problems relevant to general
relativity. Over the last few years, opinions on physically relevant
singularities occurring in FRW cosmologies have considerably changed. We
present an extensive catalogue of such cosmological milestones using
generalized power series both at the kinematical and dynamical level. We define
the notion of "scale factor singularity" and explore its relation to polynomial
and differential curvature singularities. We also extract dynamical information
using the Friedmann equations and derive necessary and sufficient conditions
for the existence of cosmological milestones such as big bangs, big crunches,
big rips, sudden singularities and extremality events. Specifically, we provide
a complete characterization of cosmological milestones for which the dominant
energy condition is satisfied. The second problem looks at one of the very
small number of serious alternatives to the usual concept of an astrophysical
black hole, that is, the gravastar model developed by Mazur and Mottola. By
considering a generalized class of similar models with continuous pressure (no
infinitesimally thin shells) and negative central pressure, we demonstrate that
gravastars cannot be perfect fluid spheres: anisotropic pressures are
unavoidable. We provide bounds on the necessary anisotropic pressure and show
that these transverse stresses that support a gravastar permit a higher
compactness than is given by the Buchdahl-Bondi bound for perfect fluid stars.
We also comment on the qualitative features of the equation of state that such
gravastar-like objects without any horizon must have.Comment: 171 pages; MSc thesi
Cosmodynamics: Energy conditions, Hubble bounds, density bounds, time and distance bounds
We refine and extend a programme initiated by one of the current authors
[Science 276 (1997) 88; Phys. Rev. D56 (1997) 7578] advocating the use of the
classical energy conditions of general relativity in a cosmological setting to
place very general bounds on various cosmological parameters. We show how the
energy conditions can be used to bound the Hubble parameter H(z), Omega
parameter Omega(z), density rho(z), distance d(z), and lookback time T(z) as
(relatively) simple functions of the redshift z, present-epoch Hubble parameter
H_0, and present-epoch Omega parameter Omega_0. We compare these results with
related observations in the literature, and confront the bounds with the recent
supernova data.Comment: 21 pages, 2 figure
Stable Gravastars of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of phantom energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy (or phantom)
fluid, and the external Schwarzschild region. It is found that in some cases
the models represent the "bounded excursion" stable gravastars, where the thin
shell is oscillating between two finite radii, while in other cases they
collapse until the formation of black holes or normal stars. In the phase
space, the region for the "bounded excursion" gravastars is very small in
comparison to that of black holes, but not empty, as found in our previous
papers. Therefore, although the existence of gravastars can not be completely
excluded from current analysis, the opposite is not possible either, that is,
even if gravastars exist, they do not exclude the existence of black holes.Comment: 35 pages, 43 figures, added some clarifying texts and corrected some
typos, accepted for publication in JCA
How the Charge Can Affect the Formation of Gravastars
In recent work we physically interpreted a special gravastar solution
characterized by a zero Schwarzschild mass. In fact, in that case, none
gravastar was formed and the shell expanded, leaving behind a de Sitter or a
Minkowski spacetime, or collapsed without forming an event horizon, originating
what we called a massive non-gravitational object. This object has two
components of non zero mass but the exterior spacetime is Minkowski or de
Sitter. One of the component is a massive thin shell and the other one is de
Sitter spacetime inside. The total mass of this object is zero Schwarzschild
mass, which characterizes an exterior vacuum spacetime. Here, we extend this
study to the case where we have a charged shell. Now, the exterior is a
Reissner-Nordstr\"om spacetime and, depending on the parameter
of the equation of state of the shell, and the charge, a
gravastar structure can be formed. We have found that the presence of the
charge contributes to the stability of the gravastar, if the charge is greater
than a critical value. Otherwise, a massive non-gravitational object is formed
for small charges.Comment: 17 pages and 7 figures, several typos corrected, accepted for
publication in JCA
High-Redshift Cosmography
We constrain the parameters describing the kinematical state of the universe
using a cosmographic approach, which is fundamental in that it requires a very
minimal set of assumptions (namely to specify a metric) and does not rely on
the dynamical equations for gravity. On the data side, we consider the most
recent compilations of Supernovae and Gamma Ray Bursts catalogues. This allows
to further extend the cosmographic fit up to , i.e. up to redshift for
which one could start to resolve the low z degeneracy among competing
cosmological models. In order to reliably control the cosmographic approach at
high redshifts, we adopt the expansion in the improved parameter .
This series has the great advantage to hold also for and hence it is
the appropriate tool for handling data including non-nearby distance
indicators. We find that Gamma Ray Bursts, probing higher redshifts than
Supernovae, have constraining power and do require (and statistically allow) a
cosmographic expansion at higher order than Supernovae alone. Exploiting the
set of data from Union and GRBs catalogues, we show (for the first time in a
purely cosmographic approach parametrized by deceleration , jerk ,
snap ) a definitively negative deceleration parameter up to the
3 confidence level. We present also forecasts for realistic data sets
that are likely to be obtained in the next few years.Comment: 16 pages, 6 figures, 3 tables. Improved version matching the
published one, additional comments and reference
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