518 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

### 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

### 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

### Searching for Signatures of Cosmic Superstrings in the CMB

Because cosmic superstrings generically form junctions and gauge theoretic
strings typically do not, junctions may provide a signature to distinguish
between cosmic superstrings and gauge theoretic cosmic strings. In cosmic
microwave background anisotropy maps, cosmic strings lead to distinctive line
discontinuities. String junctions lead to junctions in these line
discontinuities. In turn, edge detection algorithms such as the Canny algorithm
can be used to search for signatures of strings in anisotropy maps. We apply
the Canny algorithm to simulated maps which contain the effects of cosmic
strings with and without string junctions. The Canny algorithm produces edge
maps. To distinguish between edge maps from string simulations with and without
junctions, we examine the density distribution of edges and pixels crossed by
edges. We find that in string simulations without Gaussian noise (such as
produced by the dominant inflationary fluctuations) our analysis of the output
data from the Canny algorithm can clearly distinguish between simulations with
and without string junctions. In the presence of Gaussian noise at the level
expected from the current bounds on the contribution of cosmic strings to the
total power spectrum of density fluctuations, the distinction between models
with and without junctions is more difficult. However, by carefully analyzing
the data the models can still be differentiated.Comment: 15 page

### The bispectrum of matter perturbations from cosmic strings

We present the first calculation of the bispectrum of the matter perturbations induced by cosmic strings. The calculation is performed in two different ways: the first uses the unequal time correlators (UETCs) of the string network - computed using a Gaussian model previously employed for cosmic string power spectra. The second approach uses the wake model, where string density perturbations are concentrated in sheet-like structures whose surface density grows with time. The qualitative and quantitative agreement of the two gives confidence to the results. An essential ingredient in the UETC approach is the inclusion of compensation factors in the integration with the Green's function of the matter and radiation fluids, and we show that these compensation factors must be included in the wake model also. We also present a comparison of the UETCs computed in the Gaussian model, and those computed in the unconnected segment model (USM) used by the standard cosmic string perturbation package CMBACT. We compare numerical estimates for the bispectrum of cosmic strings to those produced by perturbations from an inflationary era, and discover that, despite the intrinsically non-Gaussian nature of string-induced perturbations, the matter bispectrum is unlikely to produce competitive constraints on a population of cosmic strings

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