87 research outputs found
Inhomogeneous Fragmentation of the Rolling Tachyon
Dirac-Born-Infeld type effective actions reproduce many aspects of string
theory classical tachyon dynamics of unstable Dp-branes. The inhomogeneous
tachyon field rolling from the top of its potential forms topological defects
of lower codimensions. In between them, as we show, the tachyon energy density
fragments into a p-dimensional web-like high density network evolving with
time. We present an analytic asymptotic series solution of the non-linear
equations for the inhomogeneous tachyon and its stress energy. The generic
solution for a tachyon field with a runaway potential in arbitrary dimensions
is described by the free streaming of noninteracting massive particles whose
initial velocities are defined by the gradients of the initial tachyon profile.
Thus, relativistic particle mechanics is a dual picture of the tachyon field
effective action. Implications of this picture for inflationary models with a
decaying tachyon field are discussed.Comment: 10 pages, 1 figur
Preheating in Supersymmetric Theories
We examine the particle production via preheating at the end of inflation in
supersymmetric theories. The inflaton and matter scalars are now necessarily
complex fields, and their relevant interactions are restricted by holomorphy.
In general this leads to major changes both in the inflaton dynamics and in the
efficiency of the preheating process. In addition, supersymmetric models
generically contain multiple isolated vacua, raising the possibility of
non-thermal production of dangerous topological defects. Because of these
effects, the success of leptogenesis or WIMPZILLA production via preheating
depends much more sensitively on the detailed parameters in the inflaton sector
than previously thought.Comment: 24 pages, 3 figures; references adde
Beauty is Attractive: Moduli Trapping at Enhanced Symmetry Points
We study quantum effects on moduli dynamics arising from the production of
particles which are light at special points in moduli space. The resulting
forces trap the moduli at these points, which often exhibit enhanced symmetry.
Moduli trapping occurs in time-dependent quantum field theory, as well as in
systems of moving D-branes, where it leads the branes to combine into stacks.
Trapping also occurs in an expanding universe, though the range over which the
moduli can roll is limited by Hubble friction. We observe that a scalar field
trapped on a steep potential can induce a stage of acceleration of the
universe, which we call trapped inflation. Moduli trapping ameliorates the
cosmological moduli problem and may affect vacuum selection. In particular,
rolling moduli are most powerfully attracted to the points with the largest
number of light particles, which are often the points of greatest symmetry.
Given suitable assumptions about the dynamics of the very early universe, this
effect might help to explain why among the plethora of possible vacuum states
of string theory, we appear to live in one with a large number of light
particles and (spontaneously broken) symmetries. In other words, some of the
surprising properties of our world might arise not through pure chance or
miraculous cancellations, but through a natural selection mechanism during
dynamical evolution.Comment: 50 pages, 4 figures; v2: added references and an appendix describing
a related classical proces
D-Brane Effective Actions and Particle Production near the Beginning of the Tachyon Condensation
In this paper we will study the quantum field theory of fluctuation modes
around the classical solution that describes tachyon condensation on unstable
D-brane.We will calculate the number of particle produced near the beginning of
the rolling tachyon process. We will perform this calculation for different
tachyon effective actions and we will find that the rate of the particle
production strongly depends on the form of the effective action used for the
description of the early stage of the tachyon condensation.Comment: 21 page
No Go Theorem for Self Tuning Solutions With Gauss-Bonnet Terms
We consider self tuning solutions for a brane embedded in an anti de Sitter
spacetime. We include the higher derivative Gauss-Bonnet terms in the action
and study singularity free solutions with finite effective Newton's constant.
Using the methods of Csaki et al, we prove that such solutions, when exist,
always require a fine tuning among the brane parameters. We then present a new
method of analysis in which the qualitative features of the solutions can be
seen easily without obtaining the solutions explicitly. Also, the origin of the
fine tuning is transparent in this method.Comment: 17 pages, 3 figure
Particle Production in Tachyon Condensation
We study particle production in the tachyon condensation process as described
by different effective actions for the tachyon. By making use of invariant
operators, we are able to obtain exact results for the density of produced
particles, which is shown to depend strongly on the specific action. In
particular, the rate of particle production remains finite only for one of the
actions considered, hence confirming results previously appeared in the
literature.Comment: LaTeX, 6 pages, 3 figure
On the Extra Mode and Inconsistency of Horava Gravity
We address the consistency of Horava's proposal for a theory of quantum
gravity from the low-energy perspective. We uncover the additional scalar
degree of freedom arising from the explicit breaking of the general covariance
and study its properties. The analysis is performed both in the original
formulation of the theory and in the Stueckelberg picture. A peculiarity of the
new mode is that it satisfies an equation of motion that is of first order in
time derivatives. At linear level the mode is manifest only around spatially
inhomogeneous and time-dependent backgrounds. We find two serious problems
associated with this mode. First, the mode develops very fast exponential
instabilities at short distances. Second, it becomes strongly coupled at an
extremely low cutoff scale. We also discuss the "projectable" version of
Horava's proposal and argue that this version can be understood as a certain
limit of the ghost condensate model. The theory is still problematic since the
additional field generically forms caustics and, again, has a very low strong
coupling scale. We clarify some subtleties that arise in the application of the
Stueckelberg formalism to Horava's model due to its non-relativistic nature.Comment: Discussion expanded; a figure added; accepted to JHE
Braneworld dynamics with the BraneCode
We give a full nonlinear numerical treatment of time-dependent 5d braneworld
geometry, which is determined self-consistently by potentials for the scalar
field in the bulk and at two orbifold branes, supplemented by boundary
conditions at the branes. We describe the BraneCode, an algorithm which we
designed to solve the dynamical equations numerically. We applied the BraneCode
to braneworld models and found several novel phenomena of the brane dynamics.
Starting with static warped geometry with de Sitter branes, we found
numerically that this configuration is often unstable due to a tachyonic mass
of the radion during inflation. If the model admits other static configurations
with lower values of de Sitter curvature, this effect causes a violent
re-structuring towards them, flattening the branes, which appears as a lowering
of the 4d effective cosmological constant. Braneworld dynamics can often lead
to brane collisions. We found that in the presence of the bulk scalar field,
the 5d geometry between colliding branes approaches a universal, homogeneous,
anisotropic strong gravity Kasner-like asymptotic, irrespective of the
bulk/brane potentials. The Kasner indices of the brane directions are equal to
each other but different from that of the extra dimension.Comment: 38 pages, 10 figure
Inhomogeneous tachyon condensation
We investigate the spacetime-dependent condensation of the tachyon in
effective field theories. Previous work identified singularities in the field
which appear in finite time: infinite gradients at the kinks, and (in the
eikonal approximation) caustics near local minima. By performing a perturbation
analysis, and with numerical simulations, we demonstrate and explain key
features of the condensation process: perturbations generically freeze, and
minima develop singular second derivatives in finite time (caustics). This last
has previously been understood in terms of the eikonal approximation to the
dynamics. We show explicitly from the field equations how this approximation
emerges, and how the caustics develop, both in the DBI and BSFT effective
actions. We also investigate the equation of state parameter of tachyon matter
showing that it is small, but generically non-zero. The energy density tends to
infinity near field minima with a charateristic profile. A proposal to regulate
infinities by modifying the effective action is also studied. We find that
although the infinities at the kinks are successfully regularised in the
time-dependent case, caustics still present.Comment: 4 figures,19p
Tachyon cosmology with non-vanishing minimum potential: a unified model
We investigate the tachyon condensation process in the effective theory with
non-vanishing minimum potential and its implications to cosmology. It is shown
that the tachyon condensation on an unstable three-brane described by this
modified tachyon field theory leads to lower-dimensional branes (defects)
forming within a stable three-brane. Thus, in the cosmological background, we
can get well-behaved tachyon matter after tachyon inflation, (partially)
avoiding difficulties encountered in the original tachyon cosmological models.
This feature also implies that the tachyon inflated and reheated universe is
followed by a Chaplygin gas dark matter and dark energy universe. Hence, such
an unstable three-brane behaves quite like our universe, reproducing the key
features of the whole evolutionary history of the universe and providing a
unified description of inflaton, dark matter and dark energy in a very simple
single-scalar field model.Comment: 18 p
- …