284 research outputs found
High speed collision and reconnection of Abelian Higgs strings in the deep type-II regime
We study high speed collision and reconnection of cosmic strings in the
type-II regime (scalar-to-gauge mass ratios larger than one) of the Abelian
Higgs model. New phenomena such as multiple reconnections and clustering of
small scale structure have been observed and reported in a previous paper, as
well as the fact that the previously observed loop that mediates the second
intercommutation is only a loop for sufficiently large beta =
m_scalar^2/m_gauge^2. Here we give a more detailed account of our study,
involving 3D numerical simulations with beta in the range 1 to 64, the largest
value simulated to date, as well as 2D simulations of vortex-antivortex (v-av)
collisions to understand the possible relation to the new 3D phenomena. Our
simulations give further support to the idea that Abelian Higgs strings never
pass through each other, unless this is the result of a double reconnection;
and that the critical velocity (v_c) for double reconnection goes down with
increasing mass ratio, but energy conservation suggests a lower bound around
0.77c. We discuss the qualitative change in the intermediate state observed for
large mass ratios. We relate it to a similar change in the outcome of 2D v-av
collisions in the form of radiating bound states. In the deep type-II regime
the angular dependence of v_c for double reconnection does not seem to conform
to semi-analytic predictions based on the Nambu-Goto approximation. We model
the high angle collisions reasonably well by incorporating the effect of core
interactions, and the torque they produce on the approaching strings, into the
Nambu-Goto description of the collision. An interesting, counterintuitive
aspect is that the effective collision angle is smaller because of the torque.
Our results suggest differences in network evolution and radiation output with
respect to the predictions based on Nambu-Goto or beta = 1 Abelian Higgs
dynamics.Comment: 13 pages, 7 figures Send For Publication in Physics Review
The string swampland constraints require multi-field inflation
An important unsolved problem that affects practically all attempts to
connect string theory to cosmology and phenomenology is how to distinguish
effective field theories belonging to the string landscape from those that are
not consistent with a quantum theory of gravity at high energies (the "string
swampland"). It was recently proposed that potentials of the string landscape
must satisfy at least two conditions, the "swampland criteria", that severely
restrict the types of cosmological dynamics they can sustain. The first
criterion states that the (multi-field) effective field theory description is
only valid over a field displacement (in units where the Planck mass is 1), measured as a distance in the
target space geometry. A second, more recent, criterion asserts that, whenever
the potential is positive, its slope must be bounded from below, and
suggests . A recent analysis
concluded that these two conditions taken together practically rule out
slow-roll models of inflation. In this note we show that the two conditions
rule out inflationary backgrounds that follow geodesic trajectories in field
space, but not those following curved, non-geodesic, trajectories (which are
parametrized by a non-vanishing bending rate of the multi-field
trajectory). We derive a universal lower bound on (relative to the
Hubble parameter ) as a function of and the number of efolds
, assumed to be at least of order 60. If later studies confirm and
to be strictly , the bound implies strong turns with
. Slow-roll inflation in the landscape is not
ruled out, but it is strongly multi-field.Comment: v1: 15 pages; v2: 16 pages, references added, improved discussions,
version accepted for publication in JCA
Lineal gravity from planar gravity
We show how to obtain the two-dimensional black hole action by dimensional
reduction of the three-dimensional Einstein action with a non-zero cosmological
constant. Starting from the Chern-Simons formulation of 2+1 gravity, we obtain
the 1+1 dimensional gauge formulation given by Verlinde. Remarkably, the
proposed reduction shares the relevant features of the formulation of Cangemi
and Jackiw, without the need for a central charge in the algebra. We show how
the Lagrange multipliersin these formulations appear naturally as the remnants
of the three dimensional connection associated to symmetries that have been
lostin the dimensional reduction. The proposed dimensional reduction involves a
shift in the three dimensional connection whose effect is to make the length of
the extra dimension infinite.Comment: 13 pages, plain Te
Comment on ``Absence of abelian Higgs hair for extremal black holes''
We examine the claim of Chamblin et. al. that extreme black holes cannot
support abelian Higgs hair. We provide evidence that contradicts this claim and
discuss reasons for this discrepancy.Comment: 1 page 2 figures, revised titl
Reply to "Comment on 'Gravitating Magnetic Monopole in the Global Monopole Spacetime' "
In this Reply I present some arguments in favor of the stability of the
topological defect composed by global and magnetic monopoles.Comment: 1 page, no figures. Revised version improves the theoretical analysis
about electrostatic self-interaction in the global monopole spacetim
Exotic composites: the decay of deficit angles in global-local monopoles
We study static, spherically symmetric, composite global-local monopoles with
a direct interaction term between the two sectors in the regime where the
interaction potential is large. At some critical coupling the global defect
disappears and with it the deficit angle of the space-time. We find new
solutions which represent local monopoles in an Anti-de-Sitter spacetime. In
another parameter range the magnetic monopole, or even both, disappear. The
decay of the magnetic monopole is accompanied by a dynamical transition from
the higgsed phase to the gauge-symmetric phase. We comment on the applications
to cosmology, topological inflation and braneworlds.Comment: 17 pages, 11 figures; Minor corrections, matches published versio
Cumulative effects in inflation with ultra-light entropy modes
In multi-field inflation one or more non-adiabatic modes may become light,
potentially inducing large levels of isocurvature perturbations in the cosmic
microwave background. If in addition these light modes are coupled to the
adiabatic mode, they influence its evolution on super horizon scales. Here we
consider the case in which a non-adiabatic mode becomes approximately massless
("ultralight") while still coupled to the adiabatic mode, a typical situation
that arises with pseudo-Nambu-Goldstone bosons or moduli. This ultralight mode
freezes on super-horizon scales and acts as a constant source for the curvature
perturbation, making it grow linearly in time and effectively suppressing the
isocurvature component. We identify a Stuckelberg-like emergent shift symmetry
that underlies this behavior. As inflation lasts for many e-folds, the
integrated effect of this source enhances the power spectrum of the adiabatic
mode, while keeping the non-adiabatic spectrum approximately untouched. In this
case, towards the end of inflation all the fluctuations, adiabatic and
non-adiabatic, are dominated by a single degree of freedom.Comment: 27 pages, 1 figure; v2: improved discussions, version published in
JCA
Effective non-intercommutation of local cosmic strings at high collision speeds
We present evidence that Abrikosov-Nielsen-Olesen (ANO) strings pass through
each other for very high speeds of approach due to a double intercommutation.
In near-perpendicular collisions numerical simulations give threshold speeds
bounded above by for type I, and by for deep type
II strings. The second intercommutation occurs because at ultra high collision
speeds, the connecting segments formed by the first intercommutation are nearly
static and almost antiparallel, which gives them time to interact and
annihilate. A simple model explains the rough features of the threshold
velocity dependence with the incidence angle. For deep type II strings and
large incidence angles a second effect becomes dominant, the formation of a
loop that catches up with the interpolating segments. The loop is related to
the observed vortex - antivortex reemergence in two-dimensions. In this case
the critical value for double intercommutation can become much lower.Comment: 5 pages, 4 figures. data points added, plots for deep type II regime
showing lower critical velocities, some minor changes in tex
Nielsen-Olesen strings in Supersymmetric models
We investigate the behaviour of a model with two oppositely charged scalar
fields. In the Bogomol'nyi limit this may be seen as the scalar sector of N=1
supersymmetric QED, and it has been shown that cosmic strings form. We examine
numerically the model out of the Bogomol'nyi limit, and show that this remains
the case. We then add supersymmetry-breaking mass terms to the supersymmetric
model, and show that strings still survive.
Finally we consider the extension to N=2 supersymmetry with
supersymmetry-breaking mass terms, and show that this leads to the formation of
stable cosmic strings, unlike in the unbroken case.Comment: 7 pages, 2 figues, uses revtex4; minor typos corrected; references
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