115 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
On Nieh-Yan transport
[EN] We study nondissipative transport induced by the Nieh-Yan anomaly. After computing the torsional terms in the equilibrium partition function using transgression, we find the constitutive relations for the covariant axial-vector, heat, stress, and spin currents. A number of new transport effects are found, driven by background torsion and the spin chemical potential. Torsional constitutive relations in two-dimensional systems are also analyzed.This work has been supported by Spanish Science Ministry grants PGC2018-094626-B-C21 (MCIU/AEI/FEDER, EU) and PGC2018-094626-B-C22 (MCIU/AEI/FEDER, EU), as well as by Basque Government grant IT979-16
Comments on noncommutative gravity
We study the possibility of obtaining noncommutative gravity dynamics from
string theory in the Seiberg-Witten limit. We find that the resulting
low-energy theory contains more interaction terms than those proposed in
noncommutative deformations of gravity. The role of twisted diffeomorphisms in
string theory is studied and it is found that they are not standard physical
symmetries. It is argued that this might be the reason why twisted
diffeomorphisms are not preserved by string theory in the low energy limit.
Twisted gauge transformations are also discussed.Comment: 37 pages. Typos corrected. Final version to appear in Nuclear Physics
Can the Type-IIB axion prevent Pre-big Bang inflation?
We look at the possibility of superinflationary behavior in a class of
anisotropic Type-IIB superstring cosmologies in the context of Pre-big Bang
scenario and find that there exists a rather narrow range of parameters for
which these models inflate. We then show that, although in general this
behavior is left untouched by the introduction of a Ramond-Ramond axion field
through a SL(2,R) rotation, there exists a particular class of axions for which
inflation disappears completely. Asymptotic past initial conditions are briefly
discussed, and some speculations on the possible extension of Pre-big Bang
ideas to gravitational collapse are presented.Comment: harvmac, epsf. 3 figures include
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
Entropy generation and inflation in wave collision induced pre-big-bang cosmology
We study inflation and entropy generation in a recently proposed pre-big-bang model universe produced in a collision of gravitational and dilaton waves. It is shown that enough inflation occurs provided the incoming waves are sufficiently weak. We also find that entropy in this model is dynamically generated as the result of the nonlinear interaction of the incoming waves, before the universe enters the phase of dilaton driven inflation. In particular, we introduce a measure for the entropy produced in the collision in terms of the focusing lengths of the incoming waves
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
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