428 research outputs found
Magnetic monopoles and vortices in the standard model of electroweak interactions
These lectures start with an elementary introduction to the subject of
magnetic monopoles which should be accesible from any physics background. In
the Weinberg-Salam model of electroweak interactions, magnetic monopoles appear
at the ends of a type of non-topological vortices called electroweak strings.
These will also be discussed, as well as recent simulations of their formation
during a phase transition which indicate that, in the (unphysical) range of
parameters in which the strings are classically stable, they can form with a
density comparable to topological vortices.Comment: 19 pages, Les Houches lectures, NATO-ASI on Topological defects and
the non-equilibrium dynamics of symmetry breaking phase transitions, Feb. 9
Relating black holes in two and three dimensions
The three dimensional black hole solutions of Ba\~nados, Teitelboim and
Zanelli (BTZ) are dimensionally reduced in various different ways. Solutions
are obtained to the Jackiw-Teitelboim theory of two dimensional gravity for
spinless BTZ black holes, and to a simple extension with a non-zero dilaton
potential for black holes of fixed spin. Similar reductions are given for
charged black holes. The resulting two dimensional solutions are themselves
black holes, and are appropriate for investigating exact ``S-wave'' scattering
in the BTZ metrics. Using a different dimensional reduction to the string
inspired model of two dimensional gravity, the BTZ solutions are related to the
familiar two dimensional black hole and the linear dilaton vacuum.Comment: 12 pages, CTP #2181, January 199
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
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
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
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
Robust predictions for an oscillatory bispectrum in Planck 2015 data from transient reductions in the speed of sound of the inflaton
We update the search for features in the Cosmic Microwave Background (CMB)
power spectrum due to transient reductions in the speed of sound, using Planck
2015 CMB temperature and polarisation data. We enlarge the parameter space to
much higher oscillatory frequencies of the feature, and define a robust prior
independent of the ansatz for the reduction, guaranteed to reproduce the
assumptions of the theoretical model and exhaustive in the regime in which the
feature is easily distinguishable from the baseline cosmology. We find a fit to
the -- minus/plus structure in Planck TT power spectrum, as
well as features spanning along the higher 's (--).
For the last ones, we compute the correlated features that we expect to find in
the CMB bispectrum, and asses their signal-to-noise and correlation to the
ISW-lensing secondary bispectrum. We compare our findings to the shape-agnostic
oscillatory template tested in Planck 2015, and we comment on some tantalising
coincidences with some of the traits described in Planck's 2015 bispectrum
data.Comment: 19 pages - matches published versio
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