613 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
Duality Invariance of Cosmological Perturbation Spectra
I show that cosmological perturbation spectra produced from quantum
fluctuations in massless or self-interacting scalar fields during an
inflationary era remain invariant under a two parameter family of
transformations of the homogeneous background fields. This relates slow-roll
inflation models to solutions which may be far from the usual slow-roll limit.
For example, a scale-invariant spectrum of perturbations in a minimally
coupled, massless field can be produced by an exponential expansion with
, or by a collapsing universe with .Comment: 5 pages, Latex with Revtex. Hamiltonian formulation added and
discussion expanded. Version to appear in Phys Rev
Instantons from Low Energy String Actions
We look for instanton solutions in a class of two scalar field gravity
models, which includes the low energy string action in four dimensions. In
models where the matter field has a potential with a false vacuum, we find that
non-singular instantons exist as long as the Dilaton field found in string
theory has a potential with a minimum, and provide an example of such an
instanton. The class of singular instanton solutions are also examined, and we
find that depending on the parameter values, the volume factor of the Euclidean
region does not always vanish fast enough at the singularity to make the action
finite.Comment: revtex 6 pages with 3 figures. Minor numerical correction mad
Scaling in a SU(2)/Z_3 model of cosmic superstring networks
Motivated by recent developments in superstring theory in the cosmological
context, we examine a field theory which contains string networks with 3-way
junctions. We perform numerical simulations of this model, identify the length
scales of the network that forms, and provide evidence that the length scales
tend towards a scaling regime, growing in proportion to time. We infer that the
presence of junctions does not in itself cause a superstring network to
dominate the energy density of the early Universe.Comment: 12pp, 3 fig
Modified gravity with negative and positive powers of the curvature: unification of the inflation and of the cosmic acceleration
The modified gravity, which eliminates the need for dark energy and which
seems to be stable, is considered. The terms with positive powers of the
curvature support the inflationary epoch while the terms with negative powers
of the curvature serve as effective dark energy, supporting current cosmic
acceleration. The equivalent scalar-tensor gravity may be compatible with the
simplest solar system experiments.Comment: 23 pages, 3 figures, discussion is extended, references added,
version to appear in PR
Vortex-Antivortex Pair Production in a First Order Phase Transition
We carry out numerical simulation of a first order phase transition in 2+1
dimensions by randomly nucleating bubbles, and study the formation of global
U(1) vortices. Bubbles grow and coalesce and vortices are formed at junctions
of bubbles via standard Kibble mechanism as well as due to a new mechanism,
recently proposed by us, where defect-antidefect pairs are produced due to
field oscillations. We make a comparative study of the contribution of both of
these mechanisms for vortex production. We find that, for high nucleation rate
of bubbles, vortex-antivortex pairs produced via the new mechanism have
overlapping configurations, and annihilate quickly; so only those vortices
survive till late which are produced via the Kibble mechanism. However, for low
nucleation rates, bubble collisions are energetic enough to lead to many well
separated vortex-antivortex pairs being produced via the new mechanism. For
example, in a simulation involving nucleation of 20 bubbles, a total of 14
non-overlapping vortices and antivortices formed via this new mechanism of pair
creation (6 of them being very well separated), as compared to 6 vortices and
antivortices produced via the Kibble mechanism. Our results show the
possibility that in extremely energetic bubble collisions, such as those in the
inflationary models of the early Universe, this new mechanism may drastically
affect the defect production scenario.Comment: 8 pages, Revtex, 14 figures. Figs.1a,b and 5a,d are included, rest
are availaible on reques
Reconstructing the Equation of State of Tachyon
Recent progress in theoretical physics suggests that the dark energy in the
universe might be resulted from the rolling tachyon field of string theory.
Measurements to SNe Ia can be helpful to reconstruct the equation of state of
the rolling tachyon which is a possible candidate of dark energy. We present a
numerical analysis for the evolution of the equation of state of the rolling
tachyon and derive the reconstruction equations for the equation of state as
well as the potential.Comment: 6 pages, 3 figures, to appear Phys. Rev.
Braneworld Dynamics of Inflationary Cosmologies with Exponential Potentials
In this work we consider Randall-Sundrum braneworld type scenarios, in which
the spacetime is described by a five-dimensional manifold with matter fields
confined in a domain wall or three-brane. We present the results of a
systematic analysis, using dynamical systems techniques, of the qualitative
behaviour of Friedmann-Lemaitre-Robertson-Walker type models, whose matter is
described by a scalar field with an exponential potential. We construct the
state spaces for these models and discuss how their structure changes with
respect to the general-relativistic case, in particular, what new critical
points appear and their nature and the occurrence of bifurcation.Comment: 15 pages, 9 figures, RevTex 4. Submitted to Physical Review
Search for the most stable massive state in superstring theory
In ten dimensional type II superstring, all perturbative massive states are
unstable, typically with a short lifetime compared to the string scale. We find
that the lifetime of the average string state of mass M has the asymptotic form
T < const.1/(g^2 M). The most stable string state seems to be a certain state
with high angular momentum which can be classically viewed as a circular string
rotating in several planes ("the rotating ring"), predominantly decaying by
radiating soft massless NS-NS particles, with a lifetime T = c_0 M^5/g^2.
Remarkably, the dominant channel is the decay into a similar rotating ring
state of smaller mass. The total lifetime to shrink to zero size is ~ M^7. In
the presence of D branes, decay channels involving open strings in the final
state are exponentially suppressed, so the lifetime is still proportional to
M^5, except for a D brane at a special angle or flux. For large mass, the
spectrum for massless emission exhibits qualitative features typical of a
thermal spectrum, such as a maximum and an exponential tail. We also discuss
the decay properties of rotating rings in the case of compact dimensions.Comment: 24 pages, 1 figure. Correction on lifetime of average stat
Growth of Inflaton Perturbations and the Post-Inflation Era in Supersymmetric Hybrid Inflation Models
It has been shown that hybrid inflation may end with the formation of
non-topological solitons of inflaton field. As a first step towards a fully
realistic picture of the post-inflation era and reheating in supersymmetric
hybrid inflation models, we study the classical scalar field equations of a
supersymmetric hybrid inflation model using a semi-analytical ansatz for the
spatial dependence of the fields. Using the minimal D-term inflation model as
an example, the inflaton field is evolved using the full 1-loop effective
potential from the slow-rolling era to the U(1)_{FI} symmetry-breaking phase
transition. Spatial perturbations of the inflaton corresponding to quantum
fluctuations are introduced for the case where there is spatially coherent
U(1)_{FI} symmetry breaking. The maximal growth of the dominant perturbation is
found to depend only on the ratio of superpotential coupling \lambda to the
gauge coupling g. The inflaton condensate fragments to non-topological solitons
for \lambda/g > 0.09. Possible consequences of non-topological soliton
formation in fully realistic SUSY hybrid inflation models are discussed.Comment: 27 pages LaTeX, 8 figures. Additional references and discussio
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