35 research outputs found
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
Extra-dimensional cosmology with domain-wall branes
We show how to define a consistent braneworld cosmology in a model in which
the brane is constructed as a field-theoretic domain wall of finite thickness.
The Friedmann, Robertson-Walker metric is recovered in the region of the brane,
but, remarkably, with scale factor that depends on particle energy and on
particle species, constituting a breakdown of the weak equivalence principle on
sufficiently small scales. This unusual effect comes from the extended nature
of particles confined to a domain-wall brane, and the fact that they feel an
"average" of the bulk spacetime. We demonstrate how to recover the standard
results of brane cosmology in the infinitely-thin brane limit, and comment on
how our results have the potential to place bounds on parameters such as the
thickness of domain-wall braneworlds.Comment: 23 pages; v2 has additional references and reflects journal versio
M-theory on seven-dimensional manifolds with SU(3) structure
In this paper we study M-theory compactifications on seven-dimensional manifolds with SU(3) structure. As such manifolds naturally pick out a specific direction, the resulting effective theory can be cast into a form which is similar to type IIA compactifications to four dimensions. We derive the gravitino mass matrix in four dimensions and show that for different internal manifolds (torsion classes) the vacuum preserves either no supersymmetry, or N=2 supersymmetry or, through spontaneous partial supersymmetry breaking, N=1 supersymmetry. For the latter case we derive the effective N=1 theory and give explicit examples where all the moduli are stabilised without the need of non-perturbative effects
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
On the evolution of cosmic-superstring networks
We model the behaviour of a network of interacting (p,q) strings from IIB
string theory by considering a field theory containing multiple species of
string, allowing us to study the effect of non-intercommuting events due to two
different species crossing each other. This then has the potential for a string
dominated Universe with the network becoming so tangled that it freezes. We
give numerical evidence, explained by a one-scale model, that such freezing
does not take place, with the network reaching a scaling limit where its
density relative to the background increases with N, the number of string
types.Comment: Extra references added showing constraints on cosmic superstrings, 7
pages, 7 figure
Oscillons in dilaton-scalar theories
It is shown by both analytical methods and numerical simulations that
extremely long living spherically symmetric oscillons appear in virtually any
real scalar field theory coupled to a massless dilaton (DS theories). In fact
such "dilatonic" oscillons are already present in the simplest non-trivial DS
theory -- a free massive scalar field coupled to the dilaton. It is shown that
in analogy to the previously considered cases with a single nonlinear scalar
field, in DS theories there are also time periodic quasibreathers (QB)
associated to small amplitude oscillons. Exploiting the QB picture the
radiation law of the small amplitude dilatonic oscillons is determined
analytically.Comment: extended discussion on stability, to appear in JHEP, 29 pages, 7
figure
Inflaton Fragmentation and Oscillon Formation in Three Dimensions
Analytical arguments suggest that a large class of scalar field potentials
permit the existence of oscillons -- pseudo-stable, non-topological solitons --
in three spatial dimensions. In this paper we numerically explore oscillon
solutions in three dimensions. We confirm the existence of these field
configurations as solutions to the Klein-Gorden equation in an expanding
background, and verify the predictions of Amin and Shirokoff for the
characteristics of individual oscillons for their model. Further, we
demonstrate that significant numbers of oscillons can be generated via
fragmentation of the inflaton condensate, consistent with the analysis of Amin.
These emergent oscillons can easily dominate the post-inflationary universe.
Finally, both analytic and numerical results suggest that oscillons are stable
on timescales longer than the post-inflationary Hubble time. Consequently, the
post-inflationary universe can contain an effective matter-dominated phase,
during which it is dominated by localized concentrations of scalar field
matter.Comment: See http://easther.physics.yale.edu/downloads.html for numerical
codes. Visualizations available at http://www.mit.edu/~mamin/oscillons.html
and http://easther.physics.yale.edu/fields.html V2 Minor fixes to reference
lis
Adiabatic and isocurvature perturbation projections in multi-field inflation
Current data are in good agreement with the predictions of single field inflation. However, the hemispherical asymmetry seen in the cosmic microwave background data, may hint at a potential problem. Generalizing to multi-field models may provide one possible explanation. A useful way of modeling perturbations in multi-field inflation is to investigate the projection of the perturbation along and perpendicular to the background fields' trajectory. These correspond to the adiabatic and isocurvature perturbations. However, it is important to note that in general there are no corresponding adiabatic and isocurvature fields. The purpose of this article is to highlight the distinction between a field redefinition and a perturbation projection. We provide a detailed derivation of the evolution of the isocurvature perturbation to show that no assumption of an adiabatic or isocurvature field is needed. We also show how this evolution equation is consistent with the field covariant evolution equations for the isocurvature perturbation in the flat field space limit
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
Curvature perturbation in multi-field inflation with non-minimal coupling
In this paper we discuss a multi-field model of inflation in which generally
all fields are non-minimally coupled to the Ricci scalar and have non-canonical
kinetic terms. The background evolution and first-order perturbations for the
model are evaluated in both the Jordan and Einstein frames, and the respective
curvature perturbations compared. We confirm that they are indeed not the same
- unlike in the single-field case - and also that the difference is a direct
consequence of the isocurvature perturbations inherent to multi-field models.
This result leads us to conclude that the notion of adiabaticity is not
invariant under conformal transformations. Using a two-field example we show
that even if in one frame the evolution is adiabatic, meaning that the
curvature perturbation is conserved on super-horizon scales, in general in the
other frame isocurvature perturbations continue to source the curvature
perturbation. We also find that it is possible to realise a particular model in
which curvature perturbations in both frames are conserved but with each being
of different magnitude. These examples highlight that the curvature
perturbation itself, despite being gauge-invariant, does not correspond
directly to an observable. The non-equivalence of the two curvature
perturbations would also be important when considering the addition of Standard
Model matter into the system.Comment: 21 pages, 2 figures, references added, typos corrected, version to
appear in JCA