387 research outputs found
Axion perturbation spectra in string cosmologies
We discuss the semi-classical perturbation spectra produced in the massless
fields of the low energy string action in a pre big bang type scenario. Axion
fields may possess an almost scale-invariant spectrum on large scales dependent
upon the evolution of the dilaton and moduli fields to which they are coupled.
As an example we calculate the spectra for three axion fields present in a
truncated type IIB model and show that they are related with at least one of
the fields having a scale-invariant or red perturbation spectrum. In the
simplest pre big bang scenario this may be inconsistent with the observed
isotropy of the microwave background. More generally, relations between the
perturbation spectra in low energy string cosmologies reflect the symmetries of
the theory.Comment: 9 pages, latex with epsf, 1 figure. Revised estimate of amplitude of
density perturbations and extended discussion of possible conflict with
isotropy of cosmic microwave background. To appear in Physics Letters
Inflationary cosmology in the central region of String/M-theory moduli space
The "central" region of moduli space of M- and string theories is where the
string coupling is about unity and the volume of compact dimensions is about
the string volume. Here we argue that in this region the non-perturbative
potential which is suggested by membrane instanton effects has the correct
scaling and shape to allow for enough slow-roll inflation, and to produce the
correct amplitude of CMB anisotropies. Thus, the well known theoretical
obstacles for achieving viable slow-roll inflation in the framework of
perturbative string theory are overcome. Limited knowledge of some generic
properties of the induced potential is sufficient to determine the simplest
type of consistent inflationary model and its predictions about the spectrum of
cosmic microwave background anisotropies: a red spectrum of scalar
perturbations, and negligible amount of tensor perturbations.Comment: 9 pages, 1 figur
Causal Entropy Bound for Non-Singular Cosmologies
The conditions for validity of the Causal Entropy Bound (CEB) are verified in
the context of non-singular cosmologies with classical sources. It is shown
that they are the same conditions that were previously found to guarantee
validity of the CEB: the energy density of each dynamical component of the
cosmic fluid needs to be sub-Planckian and not too negative, and its equation
of state needs to be causal. In the examples we consider, the CEB is able to
discriminate cosmologies which suffer from potential physical problems more
reliably than the energy conditions appearing in singularity theorems.Comment: 16 pages, no figures, acknowledgments adde
Duality in String Cosmology
Scale factor duality, a truncated form of time dependent T-duality, is a
symmetry of string effective action in cosmological backgrounds interchanging
small and large scale factors. The symmetry suggests a cosmological scenario
("pre-big-bang") in which two duality related branches, an inflationary branch
and a decelerated branch are smoothly joined into one non-singular cosmology.
The use of scale factor duality in the analysis of the higher derivative
corrections to the effective action, and consequences for the nature of exit
transition, between the inflationary and decelerated branches, are outlined. A
new duality symmetry is obeyed by the lowest order equations for inhomogeneity
perturbations which always exist on top of the homogeneous and isotropic
background. In some cases it corresponds to a time dependent version of
S-duality, interchanging weak and strong coupling and electric and magnetic
degrees of freedom, and in most cases it corresponds to a time dependent
mixture of both S-, and T-duality.
The energy spectra obtained by using the new symmetry reproduce known results
of produced particle spectra, and can provide a useful lower bound on particle
production when our knowledge of the detailed dynamical history of the
background is approximate or incomplete.Comment: 6 pages, no figures, latex2e using ltwol2e.sty. Based on talks at the
44'th annual meeting of the Israel Physical Society, Apr 8, 1998, Rehovot,
Israel, and ICHEP98, 23-29 July, Vancouver, BC, Canada, and second conf. on
Quantum Aspects of Gauge Theories, Supersymmetry and Unification, Sept 21-26,
1998, Corfu, Greece. To be published in the proceeding
Cosmological perturbation spectra from SL(4,R)-invariant effective actions
We investigate four-dimensional cosmological vacuum solutions derived from an
effective action invariant under global SL(n,R) transformations. We find the
general solutions for linear axion field perturbations about homogeneous
dilaton-moduli-vacuum solutions for an SL(4,R)-invariant action and find the
spectrum of super-horizon perturbations resulting from vacuum fluctuations in a
pre big bang scenario. We show that for SL(n,R)-invariant actions with n>3
there exists a regime of parameter space of non-zero measure where all the
axion field spectra have positive spectral tilt, as required if light axion
fields are to provide a seed for anisotropies in the microwave background and
large-scale structure in the universe.Comment: 8 pages, 3 figures, revtex plus epsf, minor typos corrected, version
to appear in Physical Review
Particle production in string cosmology models
We compute spectra of particles produced during a dilaton-driven kinetic
inflation phase within string cosmology models. The resulting spectra depend on
the parameters of the model and on the type of particle and are quite varied,
some increasing and some decreasing with frequency. We use an approximation
scheme in which all spectra can be expressed in a nice symmetric form, perhaps
hinting at a deeper symmetry of the underlying physics. Our results may serve
as a starting point for detailed studies of relic abundances, dark matter
candidates, and possible sources of large scale anisotropy.Comment: 20 pages, no figures, latex, RevTe
Unitarity constraints on the ratio of shear viscosity to entropy density in higher derivative gravity
We discuss corrections to the ratio of shear viscosity to entropy density
in higher-derivative gravity theories. Generically, these theories
contain ghost modes with Planck-scale masses. Motivated by general
considerations about unitarity, we propose new boundary conditions for the
equations of motion of the graviton perturbations that force the amplitude of
the ghosts modes to vanish. We analyze explicitly four-derivative perturbative
corrections to Einstein gravity which generically lead to four-derivative
equations of motion, compare our choice of boundary conditions to previous
proposals and show that, with our new prescription, the ratio remains
at the Einstein-gravity value of to leading order in the corrections.
It is argued that, when the new boundary conditions are imposed on six and
higher-derivative equations of motion, can only increase from the
Einstein-gravity value. We also recall some general arguments that support the
validity of our results to all orders in the strength of the corrections to
Einstein gravity. We then discuss the particular case of Gauss-Bonnet gravity,
for which the equations of motion are only of two-derivative order and the
value of can decrease below when treated in a nonperturbative
way. Our findings provide further evidence for the validity of the KSS bound
for theories that can be viewed as perturbative corrections to Einstein
Gravity.Comment: Sign error in the equations of motion corrected, leading to several
numerical changes. Clarifications added, references added. Main results and
cnclusions essentially unchanged. V3 published version. Clarifications added,
discussion of Gauss-Bonnet moved to main tex
Signatures of HyperCharge Axions in Colliders
If in addition to the standard model fields, a new pseudoscalar field that
couples to hypercharge topological number density, the hypercharge axion,
exists, it can be produced in colliders in association with photons or Z
bosons, and detected by looking for its decay into photons or Z's. For a range
of masses below a TeV and coupling above a fraction of 1/TeV, existing data
from LEP II and the Tevatron can already put interesting constraints, and in
future colliders accessible detection range is increased significantly. The
hypercharge axion can help in explaining the matter-antimatter asymmetry in the
universe.Comment: 11 pages, 8 figures, uses axodraw.st
Candidates for HyperCharge Axion in Extensions of the Standard Model
Many theoretically well-motivated extensions of the Standard Model contain
heavy pseudoscalars that couple to hypercharge topological density. The
cosmological dynamics of such hypercharge axions could, under certain
conditions, lead to generation of a net baryon number in a sufficient amount to
explain the observed baryon asymmetry in the universe. We examine the Minimal
Supersymmetric Standard Model and string/M-theory models and determine specific
conditions which heavy axion-like pseudoscalars must satisfy to successfully
drive baryogenesis. We find that all candidates in the Minimal Supersymmetric
Standard Model fail to obey some of the constraints, and that only in special
string/M-theory models some axions may be adequate.Comment: 18 pages, 1 figure, uses axodra
Anisotropic String Cosmology at Large Curvatures
We study the effect of the antisymmetric tensor field on the
large curvature phase of string cosmology. It is well-known that a
non-vanishing value of leads to an anisotropic expansion of the spatial
dimensions. Correspondingly, in the string phase of the model, including
corrections, we find anisotropic fixed points of the evolution,
which act as regularizing attractors of the lowest order solutions. The
attraction basin can also include isotropic initial conditions for the scale
factors. We present explicit examples at order for different values
of the number of spatial dimensions and for different ans\"{a}tze for .Comment: 16 pages, Latex, 2 figure
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