13 research outputs found
On the Thermodynamics of Simple Non-Isentropic Perfect Fluids in General Relativity
We examine the consistency of the thermodynamics of irrotational and
non-isentropic perfect fluids complying with matter conservation by looking at
the integrability conditions of the Gibbs-Duhem relation. We show that the
latter is always integrable for fluids of the following types: (a) static, (b)
isentropic (admits a barotropic equation of state), (c) the source of a
spacetime for which , where is the dimension of the orbit of the
isometry group. This consistency scheme is tested also in two large classes of
known exact solutions for which , in general: perfect fluid Szekeres
solutions (classes I and II). In none of these cases, the Gibbs-Duhem relation
is integrable, in general, though specific particular cases of Szekeres class
II (all complying with ) are identified for which the integrability of
this relation can be achieved. We show that Szekeres class I solutions satisfy
the integrability conditions only in two trivial cases, namely the spherically
symmetric limiting case and the Friedman-Roberson-Walker (FRW) cosmology.
Explicit forms of the state variables and equations of state linking them are
given explicitly and discussed in relation to the FRW limits of the solutions.
We show that fixing free parameters in these solutions by a formal
identification with FRW parameters leads, in all cases examined, to unphysical
temperature evolution laws, quite unrelated to those of their FRW limiting
cosmologies.Comment: 29 pages, Plain.Te
Back-reaction and effective acceleration in generic LTB dust models
We provide a thorough examination of the conditions for the existence of
back-reaction and an "effective" acceleration (in the context of Buchert's
averaging formalism) in regular generic spherically symmetric
Lemaitre-Tolman-Bondi (LTB) dust models. By considering arbitrary spherical
comoving domains, we verify rigorously the fulfillment of these conditions
expressed in terms of suitable scalar variables that are evaluated at the
boundary of every domain. Effective deceleration necessarily occurs in all
domains in: (a) the asymptotic radial range of models converging to a FLRW
background, (b) the asymptotic time range of non-vacuum hyperbolic models, (c)
LTB self-similar solutions and (d) near a simultaneous big bang. Accelerating
domains are proven to exist in the following scenarios: (i) central vacuum
regions, (ii) central (non-vacuum) density voids, (iii) the intermediate radial
range of models converging to a FLRW background, (iv) the asymptotic radial
range of models converging to a Minkowski vacuum and (v) domains near and/or
intersecting a non-simultaneous big bang. All these scenarios occur in
hyperbolic models with negative averaged and local spatial curvature, though
scenarios (iv) and (v) are also possible in low density regions of a class of
elliptic models in which local spatial curvature is negative but its average is
positive. Rough numerical estimates between -0.003 and -0.5 were found for the
effective deceleration parameter. While the existence of accelerating domains
cannot be ruled out in models converging to an Einstein de Sitter background
and in domains undergoing gravitational collapse, the conditions for this are
very restrictive. The results obtained may provide important theoretical clues
on the effects of back-reaction and averaging in more general non-spherical
models.Comment: Final version accepted for publication in Classical and Quantum
Gravity. 47 pages in IOP LaTeX macros, 12 pdf figure
Implications for the Constrained MSSM from a new prediction for b to s gamma
We re-examine the properties of the Constrained MSSM in light of updated
constraints, paying particular attention to the impact of the recent
substantial shift in the Standard Model prediction for BR(B to X_s gamma). With
the help of a Markov Chain Monte Carlo scanning technique, we vary all relevant
parameters simultaneously and derive Bayesian posterior probability maps. We
find that the case of \mu>0 remains favored, and that for \mu<0 it is
considerably more difficult to find a good global fit to current constraints.
In both cases we find a strong preference for a focus point region. This leads
to improved prospects for detecting neutralino dark matter in direct searches,
while superpartner searches at the LHC become more problematic, especially when
\mu<0. In contrast, prospects for exploring the whole mass range of the
lightest Higgs boson at the Tevatron and the LHC remain very good, which
should, along with dark matter searches, allow one to gain access to the
otherwise experimentally challenging focus point region. An alternative measure
of the mean quality-of-fit which we also employ implies that present data are
not yet constraining enough to draw more definite conclusions. We also comment
on the dependence of our results on the choice of priors and on some other
assumptions.Comment: JHEP versio