2,544 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
Impact of string and monopole-type junctions on domain wall dynamics: implications for dark energy
We investigate the potential role of string and monopole-type junctions in
the frustration of domain wall networks using a velocity-dependent one-scale
model for the characteristic velocity, , and the characteristic length, ,
of the network. We show that, except for very special network configurations,
v^2 \lsim (HL)^2 \lsim (\rho_\sigma + \rho_\mu)/\rho_m where is the
Hubble parameter and , and are the average
density of domain walls, strings and monopole-type junctions. We further show
that if domain walls are to provide a significant contribution to the dark
energy without generating exceedingly large CMB temperature fluctuations then,
at the present time, the network must have a characteristic length L_0 \lsim
10 \Omega_{\sigma 0}^{-2/3} {\rm kpc} and a characteristic velocity v_0 \lsim
10^{-5} \Omega_{\sigma 0}^{-2/3} where and is the critical density. In order to satisfy these
constraints with , would have to be at
least 10 orders of magnitude larger than , which would be in
complete disagreement with observations. This result provides very strong
additional support for the conjecture that no natural frustration mechanism,
which could lead to a significant contribution of domain walls to the dark
energy budget, exists.Comment: 4 pages, 1 figur
Q-ball Dynamics
We investigate the dynamics of Q-balls in one, two and three space
dimensions, using numerical simulations of the full nonlinear equations of
motion. We find that the dynamics of Q-balls is extremely complex, involving
processes such as charge transfer and Q-ball fission. We present results of
simulations which illustrate the salient features of 2-Q-ball interactions and
give qualitative arguments to explain them in terms of the evolution of the
time-dependent phases.Comment: 37 pages, including figure
Scattering of Skyrmions
In this paper, we present a detailed study of Skyrmion-Skyrmion scattering for two B=1 Skyrmions in the attractive channel where we observe two different scattering regimes. For large separation, the scattering can be approximated as interacting dipoles. We give a qualitative estimate when this approximation breaks down. For small separations we observe an additional short-range repulsion which is qualitatively similar to monopole scattering. We also observe the interesting effect of "rotation without rotating" whereby two Skyrmions, whose orientations remain constant while well-separated, change their orientation after scattering. We can explain this effect by following preimages through the scattering process, thereby measuring which part of an in-coming Skyrmion forms part of an out-going Skyrmion. This leads to a new way of visualising Skyrmions. Furthermore, we consider spinning Skyrmions and find interesting trajectories
Testing CMB polarization data using position angles
We consider a novel null test for contamination which can be applied to CMB
polarization data that involves analysis of the statistics of the polarization
position angles. Specifically, we will concentrate on using histograms of the
measured position angles to illustrate the idea. Such a test has been used to
identify systematics in the NVSS point source catalogue with an amplitude well
below the noise level. We explore the statistical properties of polarization
angles in CMB maps. If the polarization angle is not correlated between pixels,
then the errors follow a simple law. However this is typically
not the case for CMB maps since these have correlations which result in an
increase in the variance since the effective number of independent pixels is
reduced. Then we illustrate how certain classes of systematic errors can result
in very obvious patterns in these histograms, and thus that these errors could
possibly be identified using this method. We discuss how this idea might be
applied in a realistic context, and make a preliminary analysis of the WMAP7
data, finding evidence of a systematic error in the Q and W band data,
consistent with a constant offset in Q and U.Comment: Accepted by MNRA
Regularisation of classical self interaction in strings
A general method of regularisation of classical self interaction in strings
is extended from the electromagnetic case (for which it was originally
developed) to the gravitation case, for which the result can also be
represented as a renormalisation.Comment: 4 pages, Latex. Contribution to proceedings of 1997 Cargese A.S.I.
``Strings, branes, and dualities'', ed. L. Baulieu, P. Winde
A Skyrme lattice with hexagonal symmetry
Recently it has been found that the structure of Skyrmions has a close
analogy to that of fullerene shells in carbon chemistry. In this letter we show
that this analogy continues further, by presenting a Skyrme field that
describes a lattice of Skyrmions with hexagonal symmetry. This configuration, a
novel `domain wall' in the Skyrme model, has low energy per baryon (about 6%
above the Faddeev-Bogomolny bound) and in many ways is analogous to graphite.
By comparison to the energy per baryon of other known Skyrmions and also the
Skyrme crystal, we discuss the possibility of finding Skyrmion shells of higher
charge.Comment: 12 pages, 1 figure. To appear in Phys. Lett.
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