132 research outputs found

### Spectrum of radiation from axion strings

In the wide variety of axion cosmologies in which axion strings form, their
radiative decay is the dominant mechanism for the production of axions,
imposing a tight constraint on the axion mass. Here, we focus on the mechanism
by which axions are produced in this scenario and, in particular, the key issue
of the axion spectrum emitted by an evolving network of strings.Comment: to be published in the proceedings of the 5th IFT Workshop on Axion

### Radiation constraints from cosmic strings

We show that it is possible to evolve a network of global strings numerically
including the effects of radiative backreaction, using the renormalised
equations for the Kalb-Ramond action. We calculate radiative corrections to the
equations of motion and deduce the effect on a network of global strings. We
also discuss the implications of this work for the cosmological axion density.Comment: 4 Pages, UUencoded postscript file, to appear in 'Trends in
Astro-Particle Physics - Nuclear Physics B, Proceedings Supplement

### Hybrid Inflation Exit through Tunneling

For hybrid inflationary potentials, we derive the tunneling rate from field
configurations along the flat direction towards the waterfall regime. This
process competes with the classically rolling evolution of the scalar fields
and needs to be strongly subdominant for phenomenologically viable models.
Tunneling may exclude models with a mass scale below 10^12 GeV, but can be
suppressed by small values of the coupling constants. We find that tunneling is
negligible for those models, which do not require fine tuning in order to
cancel radiative corrections, in particular for GUT-scale SUSY inflation. In
contrast, electroweak scale hybrid inflation is not viable, unless the
inflaton-waterfall field coupling is smaller than approximately 10^-11.Comment: 17 pages, 2 figure

### Gravitational Perturbations of Relativistic Membranes and Strings

We consider gravitationally induced perturbations of relativistic
Dirac--Goto--Nambu membranes and strings (or {\it p}-branes). The dynamics are
described by the first and second fundamental tensors, and related curvature
tensors in an {\it n}-dimensional spacetime. We show how equations of motion
can be derived for the perturbations within a general gauge and then discuss
how various simple gauge choices can be used to simplify the equations of
motion for specific applications. We also show how the same equations of motion
can be derived from an effective action by a variational principle. Finally, we
compare these equations of motion to those using more familiar notation for
brane dynamics, which involves the induced metric on the worldsheet. This work
sets up a general formalism for understanding the effects of backreaction on
brane dynamics and the background curvature.Comment: 11 Pages, Plain TEX, to appear Phys. Lett.

### Poly-essential and general Hyperelastic World (brane) models

This article provides a unified treatment of an extensive category of
non-linear classical field models whereby the universe is represented (perhaps
as a brane in a higher dimensional background) in terms of a structure of a
mathematically convenient type describable as hyperelastic, for which a
complete set of equations of motion is provided just by the energy-momentum
conservation law. Particular cases include those of a perfect fluid in
quintessential backgrounds of various kinds, as well as models of the elastic
solid kind that has been proposed to account for cosmic acceleration. It is
shown how an appropriately generalised Hadamard operator can be used to
construct a symplectic structure that controles the evolution of small
perturbations, and that provides a characteristic equation governing the
propagation of weak discontinuities of diverse (extrinsic and extrinsic) kinds.
The special case of a poly-essential model - the k-essential analogue of an
ordinary polytropic fluid - is examined and shown to be well behaved (like the
fluid) only if the pressure to density ratio $w$ is positive.Comment: 16 pages Latex, Contrib. to 10th Peyresq Pysics Meeting, June 2005:
Micro and Macro Structures of Spacetim

### Searching for Signatures of Cosmic Superstrings in the CMB

Because cosmic superstrings generically form junctions and gauge theoretic
strings typically do not, junctions may provide a signature to distinguish
between cosmic superstrings and gauge theoretic cosmic strings. In cosmic
microwave background anisotropy maps, cosmic strings lead to distinctive line
discontinuities. String junctions lead to junctions in these line
discontinuities. In turn, edge detection algorithms such as the Canny algorithm
can be used to search for signatures of strings in anisotropy maps. We apply
the Canny algorithm to simulated maps which contain the effects of cosmic
strings with and without string junctions. The Canny algorithm produces edge
maps. To distinguish between edge maps from string simulations with and without
junctions, we examine the density distribution of edges and pixels crossed by
edges. We find that in string simulations without Gaussian noise (such as
produced by the dominant inflationary fluctuations) our analysis of the output
data from the Canny algorithm can clearly distinguish between simulations with
and without string junctions. In the presence of Gaussian noise at the level
expected from the current bounds on the contribution of cosmic strings to the
total power spectrum of density fluctuations, the distinction between models
with and without junctions is more difficult. However, by carefully analyzing
the data the models can still be differentiated.Comment: 15 page

### The 21 cm Signature of Shock Heated and Diffuse Cosmic String Wakes

The analysis of the 21 cm signature of cosmic string wakes is extended in
several ways. First we consider the constraints on $G\mu$ from the absorption
signal of shock heated wakes laid down much later than matter radiation
equality. Secondly we analyze the signal of diffuse wake, that is those wakes
in which there is a baryon overdensity but which have not shock heated. Finally
we compare the size of these signals compared to the expected thermal noise per
pixel which dominates over the background cosmic gas brightness temperature and
find that the cosmic string signal will exceed the thermal noise of an
individual pixel in the Square Kilometre Array for string tensions $G\mu > 2.5
\times 10^{-8}$.Comment: 10 pages, 4 figures, Appendix added, version published in JCA

### On the gravitational, dilatonic and axionic radiative damping of cosmic strings

We study the radiation reaction on cosmic strings due to the emission of
dilatonic, gravitational and axionic waves. After verifying the (on average)
conservative nature of the time-symmetric self-interactions, we concentrate on
the finite radiation damping force associated with the half-retarded minus
half-advanced ``reactive'' fields. We revisit a recent proposal of using a
``local back reaction approximation'' for the reactive fields. Using
dimensional continuation as convenient technical tool, we find, contrary to
previous claims, that this proposal leads to antidamping in the case of the
axionic field, and to zero (integrated) damping in the case of the
gravitational field. One gets normal positive damping only in the case of the
dilatonic field. We propose to use a suitably modified version of the local
dilatonic radiation reaction as a substitute for the exact (non-local)
gravitational radiation reaction. The incorporation of such a local
approximation to gravitational radiation reaction should allow one to complete,
in a computationally non-intensive way, string network simulations and to give
better estimates of the amount and spectrum of gravitational radiation emitted
by a cosmologically evolving network of massive strings.Comment: 48 pages, RevTex, epsfig, 1 figure; clarification of the domain of
validity of the perturbative derivation of the string equations of motion,
and of their renormalizabilit

### Instanton vibrations of the 3-Skyrmion

The Atiyah-Drinfeld-Hitchin-Manin matrix corresponding to a tetrahedrally
symmetric 3-instanton is calculated. Some small variations of the matrix
correspond to vibrations of the instanton-generated 3-Skyrmion. These
vibrations are decomposed under tetrahedral symmetry and this decomposition is
compared to previous knowledge of the 3-Skyrmion vibration spectrum.Comment: 10 pages, LaTeX, no figures, PRD version with longer introduction and
minor change

### Symmetries of generalized soliton models and submodels on target space $S^2$

Some physically relevant non-linear models with solitons, which have target
space $S^2$, are known to have submodels with infinitly many conservation laws
defined by the eikonal equation. Here we calculate all the symmetries of these
models and their submodels by the prolongation method. We find that for some
models, like the Baby Skyrme model, the submodels have additional symmetries,
whereas for others, like the Faddeev--Niemi model, they do not.Comment: 18 pages, one Latex fil

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