129 research outputs found
Gravitational instantons and internal dimensions
We Study instanton solutions in general relativity with a scalar field. The
metric ansatz we use is composed of a particular warp product of general
Einstein metrics, such as those found in a number of cosmological settings,
including string cosmology, supergravity compactifications and general Kaluza
Klein reductions. Using the Hartle-Hawking prescription the instantons we
obtain determine whether metrics involving extra compact dimensions of this
type are favoured as initial conditions for the universe. Specifically, we find
that these product metric instantons, viewed as constrained instantons, do have
a local minima in the action. These minima are then compared with the higher
dimensional version of the Hawking-Turok instantons, and we argue that the
latter always have lower action than those associated with these product
metrics.Comment: 10 pages, 5 figure
Dynamics of Q-Balls in an expanding universe
We analyse the evolution of light Q-balls in a cosmological background, and
find a number of interesting features. For Q-balls formed with a size
comparable to the Hubble radius, we demonstrate that there is no charge
radiation, and that the Q-ball maintains a constant physical radius. Large
expansion rates cause charge migration to the surface of the Q-ball,
corresponding to a non-homogeneous internal rotation frequency. We argue that
this is an important phenomenon as it leads to a large surface charge and
possible fragmentation of the Q-ball. We also explore the deviation of the
Q-ball profile function from the static case. By introducing a parameter
, which is the ratio of the Hubble parameter to the frequency of
oscillation of the Q-ball field, and using solutions to an analytically
approximated equation for the profile function, we determine the dependence of
the new features on the expansion rate. This allows us to gain an understanding
of when they should be considered and when they can be neglected, thereby
placing restrictions on the existence of homogeneous Q-balls in expanding
backgrounds.Comment: 5 pages, 4 figure
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
Dark energy after GW170817, revisited
We revisit the status of scalar-tensor theories with applications to dark
energy in the aftermath of the gravitational wave signal GW170817 and its
optical counterpart GRB170817A. At the level of the cosmological background, we
identify a class of theories, previously declared unviable in this context,
whose anomalous gravitational wave speed is proportional to the scalar equation
of motion. As long as the scalar field is assumed not to couple directly to
matter, this raises the possibility of compatibility with the gravitational
wave data, for any cosmological sources, thanks to the scalar dynamics. This
newly "rescued" class of theories includes examples of generalised quintic
galileons from Horndeski theories. Despite the promise of this leading order
result, we show that the loophole ultimately fails when we include the effect
of large scale inhomogeneities.Comment: Updated with corrections to the gravitational wave propagation coming
from higher order terms in the presence of large scale inhomogeneities. These
close off any remaining loopholes. References adde
Decay of an inhomogeneous state via resonant tunnelling
We recently investigated the nature of resonant tunnelling in standard scalar
Quantum Field Theory, uncovering the conditions required for resonance. It was
shown that whereas the homogeneous false vacuum may decay via bubble
nucleation, it may not decay in a resonant fashion. The no-go theorem given
there is circumvented in this study by considering an initial state other than
the homogeneous false vacuum, and we confirm our mechanism by showing in an
explicit model how resonant tunnelling occurs. Using this model we demonstrate
how the tunnelling rate depends on the energy of specially constructed initial
states, with these states corresponding to contracting spherical bubbles of
some vacuum that evolve to a minimum radius and then tunnel to another vacuum,
instead of the classical motion where the bubble would just start to expand.Comment: 19 pages, 22 figure
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
A Class of Nonperturbative Configurations in Abelian-Higgs Models: Complexity from Dynamical Symmetry Breaking
We present a numerical investigation of the dynamics of symmetry breaking in
both Abelian and non-Abelian Higgs models in three spatial
dimensions. We find a class of time-dependent, long-lived nonperturbative field
configurations within the range of parameters corresponding to type-1
superconductors, that is, with vector masses () larger than scalar masses
(). We argue that these emergent nontopological configurations are related
to oscillons found previously in other contexts. For the Abelian-Higgs model,
our lattice implementation allows us to map the range of parameter space -- the
values of -- where such configurations exist and to
follow them for times t \sim \O(10^5) m^{-1}. An investigation of their
properties for -symmetric models reveals an enormously rich structure
of resonances and mode-mode oscillations reminiscent of excited atomic states.
For the SU(2) case, we present preliminary results indicating the presence of
similar oscillonic configurations.Comment: 21 pages, 19 figures, prd, revte
The dynamics of coset dimensional reduction
The evolution of multiple scalar fields in cosmology has been much studied,
particularly when the potential is formed from a series of exponentials. For a
certain subclass of such systems it is possible to get `assisted` behaviour,
where the presence of multiple terms in the potential effectively makes it
shallower than the individual terms indicate. It is also known that when
compactifying on coset spaces one can achieve a consistent truncation to an
effective theory which contains many exponential terms, however, if there are
too many exponentials then exact scaling solutions do not exist. In this paper
we study the potentials arising from such compactifications of eleven
dimensional supergravity and analyse the regions of parameter space which could
lead to scaling behaviour.Comment: 27 pages, 4 figures; added citation
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