37 research outputs found
Variations on Kaluza-Klein Cosmology
We investigate the cosmological consequences of having quantum fields living
in a space with compactified dimensions. We will show that the equation of
state is not modified by topological effects and so the dynamics of the
universe remains as it is in the infinite volume limit. On the contrary the
thermal history of the universe depends on terms that are associated with
having non-trivial topology. In the conclusions we discuss some issues about
the relationship between the non-critical string-inspired cosmology and
the result obtained with matter given by a hot massless field in S^{1}\times
\mbox{\bf
R}.Comment: 22 pages, 7 figures in a uuencoded file (using uufiles), LaTeX,
FTUAM-93/13 (LaTeX errors corrected
Gravitational shocks as a key ingredient of Gamma-Ray Bursts
We identify a novel physical mechanism that may be responsible for energy
release in -ray bursts. Radial perturbations in the neutron core,
induced by its collision with collapsing outer layers during the early stages
of supernova explosions, can trigger a gravitational shock, which can readily
eject a small but significant fraction of the collapsing material at
ultra-relativistic speeds. The development of such shocks is a strong-field
effect arising in near-critical collapse in General Relativity and has been
observed in numerical simulations in various contexts, including in particular
radially perturbed neutron star collapse, albeit for a tiny range of initial
conditions. Therefore, this effect can be easily missed in numerical
simulations if the relevant parameter space is not exhaustively investigated.
In the proposed picture, the observed rarity of -ray bursts would be
explained if the relevant conditions for this mechanism appear in only about
one in every core collapse supernovae. We also mention the
possibility that near-critical collapse could play a role in powering the
central engines of Active Galactic Nuclei.Comment: 9 pages, 3 figure
Thermofield Dynamics of the Heterotic String - Physical Aspects of the Thermal Duality -
The thermofield dynamics of the D = 10 heterotic thermal string theory is
described in proper reference to the thermal duality symmetry as well as the
thermal stability of modular invariance in association with the global phase
structure of the D = 10 heterotic thermal string ensemble.Comment: 8 pages, LaTeX, Minor correction
Initial Conditions and the Structure of the Singularity in Pre-Big-Bang Cosmology
We propose a picture, within the pre-big-bang approach, in which the universe
emerges from a bath of plane gravitational and dilatonic waves. The waves
interact gravitationally breaking the exact plane symmetry and lead generically
to gravitational collapse resulting in a singularity with the Kasner-like
structure. The analytic relations between the Kasner exponents and the initial
data are explicitly evaluated and it is shown that pre-big-bang inflation may
occur within a dense set of initial data. Finally, we argue that plane waves
carry zero gravitational entropy and thus are, from a thermodynamical point of
view, good candidates for the universe to emerge from.Comment: 18 pages, LaTeX, epsfig. 3 figures included. Minor changes; paragraph
added in the introduction, references added and typos corrected. Final
version published in Classical and Quantum Gravit
A note on supersymmetric Yang-Mills thermodynamics
The thermodynamics of supersymmetric Yang-Mills theories is studied by
computing the two-loop correction to the canonical free energy and to the
equation of state for theories with 16, 8 and 4 supercharges in any dimension
, and in two dimensions at finite volume. In the
four-dimensional case we also evaluate the first non-analytic contribution in
the 't Hooft coupling to the free energy, arising from the resummation of ring
diagrams. To conclude, we discuss some applications to the study of the
Hagedorn transition in string theory in the context of Matrix strings and
speculate on the possible physical meaning of the transition.Comment: 19 pages, harvmac, epsf. 1 figure included. Minor changes: typos
corrected; references, a footonote and a note adde
Duality in Non-Trivially Compactified Heterotic Strings
We study the implications of duality symmetry on the analyticity properties
of the partition function as it depends upon the compactification length. In
order to obtain non-trivial compactifications, we give a physical prescription
to get the Helmholtz free energy for any heterotic string supersymmetric or
not. After proving that the free energy is always invariant under the duality
transformation and getting the zero temperature
theory whose partition function corresponds to the Helmholtz potential, we show
that the self-dual point is a generic singularity
as the Hagedorn one. The main difference between these two critical
compactification radii is that the term producing the singularity at the
self-dual point is finite for any . We see that this behavior at
actually implies a loss of degrees of freedom below that point.Comment: (Preprint No. FTUAM-92/12) 17 page
Stability of marginally outer trapped surfaces and symmetries
We study properties of stable, strictly stable and locally outermost
marginally outer trapped surfaces in spacelike hypersurfaces of spacetimes
possessing certain symmetries such as isometries, homotheties and conformal
Killings. We first obtain results for general diffeomorphisms in terms of the
so-called metric deformation tensor and then particularize to different types
of symmetries. In particular, we find restrictions at the surfaces on the
vector field generating the symmetry. Some consequences are discussed. As an
application we present a result on non-existence of stable marginally outer
trapped surfaces in slices of FLRW.Comment: 23 pages, 3 figure
Positioning systems in Minkowski space-time: from emission to inertial coordinates
The coordinate transformation between emission coordinates and inertial
coordinates in Minkowski space-time is obtained for arbitrary configurations of
the emitters. It appears that a positioning system always generates two
different coordinate domains, namely, the front and the back emission
coordinate domains. For both domains, the corresponding covariant expression of
the transformation is explicitly given in terms of the emitter world-lines.
This task requires the notion of orientation of an emitter configuration. The
orientation is shown to be computable from the emission coordinates for the
users of a `central' region of the front emission coordinate domain. Other
space-time regions associated with the emission coordinates are also outlined.Comment: 20 pages; 1 figur
Quintessential brane cosmology
We study a class of braneworlds where the cosmological evolution arises as
the result of the movement of a three-brane in a five-dimensional static
dilatonic bulk, with and without reflection symmetry. The resulting
four-dimensional Friedmann equation includes a term which, for a certain range
of the parameters, effectively works as a quintessence component, producing an
acceleration of the universe at late times. Using current observations and
bounds derived from big-bang nucleosynthesis we estimate the parameters that
characterize the model.Comment: 29 pages. LaTeX. 5 postscript figures included. v2 typos corrected
and references added. Final version to appear in Phys.Rev.