215 research outputs found
Domain Walls in SU(5)
We consider the Grand Unified SU(5) model with a small or vanishing cubic
term in the adjoint scalar field in the potential. This gives the model an
approximate or exact Z symmetry whose breaking leads to domain walls. The
simplest domain wall has the structure of a kink across which the Higgs field
changes sign () and inside which the full SU(5) is restored.
The kink is shown to be perturbatively unstable for all parameters. We then
construct a domain wall solution that is lighter than the kink and show it to
be perturbatively stable for a range of parameters. The symmetry in the core of
this domain wall is smaller than that outside. The interactions of the domain
wall with magnetic monopole is discussed and it is shown that magnetic
monopoles with certain internal space orientations relative to the wall pass
through the domain wall. Magnetic monopoles in other relative internal space
orientations are likely to be swept away on collision with the domain walls,
suggesting a scenario where the domain walls might act like optical
polarization filters, allowing certain monopole ``polarizations'' to pass
through but not others. As SU(5) domain walls will also be formed at small
values of the cubic coupling, this leads to a very complicated picture of the
evolution of defects after the Grand Unified phase transition.Comment: 6 pages, 1 figure. Animations can be viewed at
http://theory4.phys.cwru.edu/~levon/figures.htm
Dynamics of the infinitely-thin kink
We consider the dynamics of the domain-wall kink soliton, in particular we
study the zero mode of translation. In the infinitely-thin kink limit, we show
that the zero mode is almost completely frozen out, the only remnant being a
dynamically constrained four-dimensional mode of a single but arbitrary
frequency. In relation to this result, we show that the usual mode expansion
for dealing with zero modes -- implicit collective coordinates -- is not in
fact a completely general expansion, and that one must use instead a
traditional generalised Fourier analysis.Comment: 13 pages; v2: added references, to appear in Phys Lett
Scaling configurations of cosmic superstring networks and their cosmological implications
We study the cosmic microwave background temperature and polarisation spectra
sourced by multi-tension cosmic superstring networks. First we obtain solutions
for the characteristic length scales and velocities associated with the
evolution of a network of F-D strings, allowing for the formation of junctions
between strings of different tensions. We find two distinct regimes describing
the resulting scaling distributions for the relative densities of the different
types of strings, depending on the magnitude of the fundamental string coupling
g_s. In one of them, corresponding to the value of the coupling being of order
unity, the network's stress-energy power spectrum is dominated by populous
light F and D strings, while the other regime, at smaller values of g_s, has
the spectrum dominated by rare heavy D strings. These regimes are seen in the
CMB anisotropies associated with the network. We focus on the dependence of the
shape of the B-mode polarisation spectrum on g_s and show that measuring the
peak position of the B-mode spectrum can point to a particular value of the
string coupling. Finally, we assess how this result, along with pulsar bounds
on the production of gravitational waves from strings, can be used to constrain
a combination of g_s and the fundamental string tension mu_F. Since CMB and
pulsar bounds constrain different combinations of the string tensions and
densities, they result in distinct shapes of bounding contours in the (mu_F,
g_s) parameter plane, thus providing complementary constraints on the
properties of cosmic superstrings.Comment: 23 pages, 8 figures, 3 tables; V2: matches published version (PRD
A Note on the evolution of cosmic string/superstring networks
In the context of brane world scenario, cosmic superstrings can be formed in
D-brane annihilation at the end of the brane inflationary era. The cosmic
superstring network has a scaling solution and the characteristic scale of the
network is proportional to the square root of the reconnection probability.Comment: 13 pages, 12 figures (revised version
A class of kinks in SU(N)\times Z_2
In a classical, quartic field theory with symmetry, a
class of kink solutions can be found analytically for one special choice of
parameters. We construct these solutions and determine their energies. In the
limit , the energy of the kink is equal to that of a kink in a
model with the same mass parameter and quartic coupling (coefficient of
). We prove the stability of the solutions to small
perturbations but global stability remains unproven. We then argue that the
continuum of choices for the boundary conditions leads to a whole space of kink
solutions. The kinks in this space occur in classes that are determined by the
chosen boundary conditions. Each class is described by the coset space
where is the unbroken symmetry group and is the symmetry group that
leaves the kink solution invariant.Comment: 7 pages; included discussion of gauge fields and other improvement
Anthropic predictions for vacuum energy and neutrino masses
It is argued that the observed vacuum energy density and the small values of
the neutrino masses could be due to anthropic selection effects. Until now,
these two quantities have been treated separately from each other and, in
particular, anthropic predictions for the vacuum energy were made under the
assumption of zero neutrino masses. Here we consider two cases. In the first,
we calculate predictions for the vacuum energy for a fixed (generally non-zero)
value of the neutrino mass. In the second we allow both quantities to vary from
one part of the universe to another. We find that the anthropic predictions for
the vacuum energy density are in a better agreement with observations when one
allows for non-zero neutrino masses. We also find that the individual
distributions for the vacuum energy and the neutrino masses are reasonably
robust and do not change drastically when one adds the other variable.Comment: 9 pages, 4 figure
Cosmic strings and Natural Inflation
In the present work we discuss cosmic strings in natural inflation. Our
analysis is based entirely on the CMB quadrupole temperature anisotropy and on
the existing upper bound on the cosmic string tension. Our results show that
the allowed range for both parameters of the inflationary model is very
different from the range obtained recently if cosmic strings are formed at the
same time with inflation, while if strings are formed after inflation we find
that the parameters of the inflationary model are similar to the ones obtained
recently.Comment: 12 pages, 0 tables, 4 figures, accepted for publication in JHE
An Index Theorem for Domain Walls in Supersymmetric Gauge Theories
The supersymmetric abelian Higgs model with N scalar fields admits multiple
domain wall solutions. We perform a Callias-type index calculation to determine
the number of zero modes of this soliton. We confirm that the most general
domain wall has 2(N-1) zero modes, which can be interpreted as the positions
and phases of (N-1) constituent domain walls. This implies the existence of
moduli for a D-string interpolating between N D5-branes in IIB string theory.Comment: 9 pages, REVTeX4; v2: reference adde
Observational constraints on cosmic strings: Bayesian analysis in a three dimensional parameter space
Current data exclude cosmic strings as the primary source of primordial
density fluctuations. However, in a wide class of inflationary models, strings
can form at later stages of inflation and have potentially detectable
observational signatures. We study the constraints from WMAP and SDSS data on
the fraction of primordial fluctuations sourced by local cosmic strings. The
Bayesian analysis presented in this brief report is restricted to the minimal
number of parameters. Yet it is useful for two reasons. It confirms the results
of Pogosian et al (2003) using an alternative statistical method. Secondly, it
justifies the more costly multi-parameter analysis. Already, varying only three
parameters -- the spectral index and the amplitudes of the adiabatic and string
contributions -- we find that the upper bound on the cosmic string contribution
is of order 10%. We expect that the full multi-parameter study, currently
underway, will likely loosen this bound.Comment: v3: 4 pages, 5 figures, slight modifications to match published
versio
Multiple universes, cosmic coincidences, and other dark matters
Even when completely and consistently formulated, a fundamental theory of
physics and cosmological boundary conditions may not give unambiguous and
unique predictions for the universe we observe; indeed inflation, string/M
theory, and quantum cosmology all arguably suggest that we can observe only one
member of an ensemble with diverse properties. How, then, can such theories be
tested? It has been variously asserted that in a future measurement we should
observe the a priori most probable set of predicted properties (the
``bottom-up'' approach), or the most probable set compatible with all current
observations (the ``top-down'' approach), or the most probable set consistent
with the existence of observers (the ``anthropic'' approach). These inhabit a
spectrum of levels of conditionalization and can lead to qualitatively
different predictions. For example, in a context in which the densities of
various species of dark matter vary among members of an ensemble of otherwise
similar regions, from the top-down or anthropic viewpoints -- but not the
bottom-up -- it would be natural for us to observe multiple types of dark
matter with similar contributions to the observed dark matter density. In the
anthropic approach it is also possible in principle to strengthen this argument
and the limit the number of likely dark matter sub-components. In both cases
the argument may be extendible to dark energy or primordial density
perturbations. This implies that the anthropic approach to cosmology,
introduced in part to explain "coincidences" between unrelated constituents of
our universe, predicts that more, as-yet-unobserved coincidences should come to
light.Comment: 18 JCAP-style pages, accepted by JCAP. Revised version adds
references and some clarification
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