5,754 research outputs found
Curvature perturbations from dimensional decoupling
The scalar modes of the geometry induced by dimensional decoupling are
investigated. In the context of the low energy string effective action,
solutions can be found where the spatial part of the background geometry is the
direct product of two maximally symmetric Euclidean manifolds whose related
scale factors evolve at a dual rate so that the expanding dimensions first
accelerate and then decelerate while the internal dimensions always contract.
After introducing the perturbative treatment of the inhomogeneities, a class of
five-dimensional geometries is discussed in detail. Quasi-normal modes of the
system are derived and the numerical solution for the evolution of the metric
inhomogeneities shows that the fluctuations of the internal dimensions provide
a term that can be interpreted, in analogy with the well-known four-dimensional
situation, as a non-adiabatic pressure density variation. Implications of this
result are discussed with particular attention to string cosmological
scenarios.Comment: 25 pages, 3 figure
Generalized CMB initial conditions with pre-equality magnetic fields
The most general initial conditions of CMB anisotropies, compatible with the
presence of pre-equality magnetic fields, are derived. When the plasma is
composed by photons, baryons, electrons, CDM particles and neutrinos, the
initial data of the truncated Einstein-Boltzmann hierarchy contemplate one
magnetized adiabatic mode and four (magnetized) non-adiabatic modes. After
obtaining the analytical form of the various solutions, the Einstein-Boltzmann
hierarchy is numerically integrated for the corresponding sets of initial data.
The TT, TE and EE angular power spectra are illustrated and discussed for the
magnetized generalization of the CDM-radiation mode, of the baryon-radiation
mode and of the non-adiabatic mode of the neutrino sector. Mixtures of initial
conditions are examined by requiring that the magnetized adiabatic mode
dominates over the remaining non-adiabatic contributions. In the latter case,
possible degeneracies between complementary sets of initial data might be
avoided through the combined analysis of the TT, TE and EE angular power
spectra at high multipoles (i.e. ).Comment: 28 pages, 24 included figures in eps styl
Entropy Production in the Cosmological Amplification of the Vacuum Fluctuations
We estimate the entropy associated to a background of squeezed cosmic
gravitons, and we argue that the process of cosmological pair production from
the vacuum may explain the large amount of entropy of our present universe.Comment: 10 pages, plain tex, to appear in Phys.Lett.B, DFTT-63/9
Bianchi type I universes with dilaton and magnetic fields
We consider the dynamics of a Bianchi type I spacetime in the presence of
dilaton and magnetic fields. The general solution of the Einstein-Maxwell
dilaton field equations can be obtained in an exact parametric form. Depending
on the numerical values of the parameters of the model there are three distinct
classes of solutions. The time evolution of the mean anisotropy, shear and
deceleration parameter is considered in detail and it is shown that a
magnetic-dilaton anisotropic Bianchi type I geometry does not isotropize, the
initial anisotropy being present in the universe for all times.Comment: 10 pages, 4 figure
On Statistical Mechanics Developments of Clan Concept in Multiparticle Production
Clan concept has been introduced in multiparticle dynamics in order to
interpret the wide occurrence of negative binomial (NB) regularity in n-charged
particle multiplicity distributions (MDs) in various high energy collisions.
The centrality of clan concept led to the attempt to justify its occurrence
within a statistical model of clan formation and evolution. In this framework
all thermodynamical potentials have been explicitly calculated in terms of NB
parameters. Interestingly it was found that NB parameter k corresponds to the
one particle canonical partition function. The goal of this paper is to explore
a possible temperature and volume dependence of parameter k in various classes
of events in high energy hadron-hadron collisions. It is shown that the
existence of a phase transition at parton level from the ideal clan gas
associated to the semihard component with k>1 to the ideal clan gas of the hard
component with k<1 implies a discontinuity in the average number of particles
at hadron level.Comment: 20 pages, latex, no figures; v2: the description of the framework has
been considerably expanded, and the main body has been reorganized for
clarit
Squeezed Thermal Vacuum and the Maximum Scale for Inflation
We consider the stimulated emission of gravitons from an initial state of
thermal equilibrium, under the action of the cosmic gravitational background
field. We find that the low-energy graviton spectrum is enhanced if compared
with spontaneous creation from the vacuum; as a consequence, the scale of
inflation must be lowered, in order not to exceed the observed CMB quadrupole
anisotropy. This effect is particularly important for models based on a
symmetry-breaking transition which require, as initial condition, a state of
thermal equilibrium at temperatures of the order of the inflation scale.Comment: 13 pages, plain tex, three figures available upon request, to appear
in Phys.Rev.D, CERN-TH.6836/9
Primordial Magnetic Fields From String Cosmology
Sufficiently large seeds for generating the observed (inter)galactic magnetic
fields emerge naturally in string cosmology from the amplification of
electromagnetic vacuum fluctuations due to a dynamical dilaton background. The
success of the mechanism depends crucially on two features of the so-called
pre-big-bang scenario, an early epoch of dilaton-driven inflation at very small
coupling, and a sufficiently long intermediate stringy era preceding the
standard radiation-dominated evolution.Comment: 12 pages, latex, two figures available by fax upon reques
Homogeneous magnetic fields in fully anisotropic string cosmological backgrounds
We present new solutions of the string cosmological effective action in the
presence of a homogeneous Maxwell field with pure magnetic component. Exact
solutions are derived in the case of space-independent dilaton and vanishing
torsion background. In our examples the four dimensional metric is either of
Bianchi-type III and VI or Kantowski-Sachs.Comment: 4 page
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