114 research outputs found
The covariance of multi-field perturbations, pseudo-susy and f_NL
We reconsider cosmological perturbation theory for multi-component scalars,
enforcing covariance in field-space, and ensuring that phyical observations are
independent of field re-definitions. We use the formalism to clarify some
issues in the literature, and use pseudo-supersymmetry to derive exact
expressions for terms of interest.Comment: 22 pages, 2 figures. Replaced with Journal versio
The fermion spectrum in braneworld collisions
In braneworld collisions fermions originally localised on one brane can be
transferred to another brane, or to a space-time boundary. By modelling branes
as scalar field kinks we bounce them off boundaries and study resulting effects
according to a braneworld observer. Extending on our previous work, we
numerically compute the spectrum of excitations of fermion modes localised on
the brane and boundary, in terms of the momentum along the brane
dimensions. We find that the spectrum depends strongly on collision velocity
and fermion-scalar coupling. Also, high-momentum modes tend to ``fall off'' the
kinks and become delocalised radiation.Comment: 13 pages, 9 figure
Oscillons and quasi-breathers in D+1 dimensions
We study oscillons in D+1 space-time dimensions using a spherically symmetric
ansatz. From Gaussian initial conditions, these evolve by emitting radiation,
approaching ``quasi-breathers'', near-periodic solutions to the equations of
motion. Using a truncated mode expansion, we numerically determine these
quasi-breather solutions in 2<D<6 and the energy dependence on the oscillation
frequency. In particular, this energy has a minimum, which in turn depends on
the number of spatial dimensions. We study the time evolution and lifetimes of
the resulting quasi-breathers, and show how generic oscillons decay into these
before disappearing altogether. We comment on the apparent absence of oscillons
for D>5 and the possibility of stable solutions for D<2.Comment: 18 pages and 18 figure
Particle transfer in braneworld collisions
We study the behaviour of fermions localized on moving kinks as these collide
with either antikinks or spacetime boundaries. We numerically solve for the
evolution of the scalar kinks and the bound (i.e. localized) fermion modes, and
calculate the number of fermions transfered to the antikink and boundary in
terms of Bogoliubov coefficients. Interpreting the boundary as the brane on
which we live, this models the ability of fermions on branes incoming from the
bulk to ``stick'' on the world brane, even when the incoming branes bounce back
into the bulk.Comment: 17 pages, 15 figures. New version has clearer discussion of boundary
conditions, and corrects a typ
The evolution of conifolds
We simulate the gravitational dynamics of the conifold geometries (resolved
and deformed) involved in the description of certain compact spacetimes. As the
cycles of the conifold collapse towards a singular geometry we find that a
horizon develops, shielding the external spacetime from the curvature
singularity of the newly formed black hole. The structure of the black hole is
examined for a range of initial conditions, and we find a candidate black-hole
solution for the final state of the collapse.Comment: 22 pages, 6 figure
Colliding branes and big crunches
We examine the global structure of colliding domain walls in AdS spacetime
and come to the conclusion that singularities forming from such collisions are
of the big-crunch type rather than that of a black brane.Comment: 5 pages, 6 figure
Multi-galileons, solitons and Derrick's theorem
The field theory Galilean symmetry, which was introduced in the context of
modified gravity, gives a neat way to construct Lorentz-covariant theories of a
scalar field, such that the equations of motion contain at most second-order
derivatives. Here we extend the analysis to an arbitrary number of scalars, and
examine the restrictions imposed by an internal symmetry, focussing in
particular on SU(N) and SO(N). This therefore extends the possible gradient
terms that may be used to stabilise topological objects such as sigma model
lumps.Comment: 7 pages, 1 figure. Minor change to order of reference
Self-tuning and the derivation of the Fab Four
We have recently proposed a special class of scalar tensor theories known as
the Fab Four. These arose from attempts to analyse the cosmological constant
problem within the context of Horndeski's most general scalar tensor theory.
The Fab Four together give rise to a model of self-tuning, with the relevant
solutions evading Weinberg's no-go theorem by relaxing the condition of
Poincare invariance in the scalar sector. The Fab Four are made up of four
geometric terms in the action with each term containing a free potential
function of the scalar field. In this paper we rigorously derive this model
from the general model of Horndeski, proving that the Fab Four represents the
only classical scalar tensor theory of this type that has any hope of tackling
the cosmological constant problem. We present the full equations of motion for
this theory, and give an heuristic argument to suggest that one might be able
to keep radiative corrections under control. We also give the Fab Four in terms
of the potentials presented in Deffayet et al's version of Horndeski.Comment: 25 pages, 1 figur
Visualising quantum effective action calculations in zero dimensions
© 2019 IOP Publishing Ltd. We present an explicit treatment of the two-particle-irreducible (2PI) effective action for a zero-dimensional quantum field theory. The advantage of this simple playground is that we are required to deal only with functions rather than functionals, making complete analytic approximations accessible and full numerical evaluation of the exact result possible. Moreover, it permits us to plot intuitive graphical representations of the behaviour of the effective action, as well as the objects out of which it is built. We illustrate the subtleties of the behaviour of the sources and their convex-conjugate variables, and their relation to the various saddle points of the path integral. With this understood, we describe the convexity of the 2PI effective action and provide a comprehensive explanation of how the Maxwell construction arises in the case of multiple, classically stable saddle points, finding results that are consistent with previous studies of the one-particle-irreducible (1PI) effective action
Renormalization group flows from the Hessian geometry of quantum effective actions
We explore a geometric perspective on quantum field theory by considering the configuration space, where all field configurations reside. Employing -particle irreducible effective actions constructed via Legendre transforms of the Schwinger functional, this configuration space can be associated with a Hessian manifold. This allows for various properties and uses of the -particle irreducible effective actions to be re-cast in geometrical terms. In particular, interpreting the two-point source as a regulator, this approach can be readily connected to the functional renormalization group. Renormalization group flows are then understood in terms of geodesics on this Hessian manifold
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