261 research outputs found
On formation of domain wall lattices
We study the formation of domain walls in a phase transition in which an
S_5\times Z_2 symmetry is spontaneously broken to S_3\times S_2. In one compact
spatial dimension we observe the formation of a stable domain wall lattice. In
two spatial dimensions we find that the walls form a network with junctions,
there being six walls to every junction. The network of domain walls evolves so
that junctions annihilate anti-junctions. The final state of the evolution
depends on the relative dimensions of the simulation domain. In particular we
never observe the formation of a stable lattice of domain walls for the case of
a square domain but we do observe a lattice if one dimension is somewhat
smaller than the other. During the evolution, the total wall length in the
network decays with time as t^{-0.71}, as opposed to the usual t^{-1} scaling
typical of regular Z_2 networks.Comment: 7 pages, 4 figures. Minor changes, final version accepted for
publication in Phys. Rev.
Symmetries within domain walls
The comparison of symmetries in the interior and the exterior of a domain
wall is relevant when discussing the correspondence between domain walls and
branes, and also when studying the interaction of walls and magnetic monopoles.
I discuss the symmetries in the context of an SU(N) times Z_2 model (for odd N)
with a single adjoint scalar field. Situations in which the wall interior has
less symmetry than the vacuum are easy to construct while the reverse situation
requires significant engineering of the scalar potential.Comment: 5 pages. Added reference
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
Kink interactions in
There are classes of kink solutions in . We show how
interactions between various kinks depend on the classes of individual kinks as
well as on their orientations with respect to each other in the internal space.
In particular, we find that the attractive or repulsive nature of the
interaction depends on the trace of the product of charges of the two kinks. We
calculate the interaction potential for all combinations of kinks and antikinks
in and study their collisions. The outcome of kink-antikink
collisions, as expected from previous studies, is sensitive to their initial
relative velocity. We find that heavier kinks tend to break up into lighter
ones, while interactions between the lightest kinks and antikinks in this model
can be repulsive as well as attractive.Comment: 11 pages, 9 figures. Two insignificant sign errors corrected in the
revised versio
Still flat after all these years
The Universe could be spatially flat, positively curved or negatively curved.
Each option has been popular at various times, partly affected by an
understanding that models tend to evolve away from flatness. The curvature of
the Universe is amenable to measurement, through tests such as the
determination of the angles of sufficiently large triangles. The angle
subtended by the characteristic scale on the Cosmic Microwave sky provides a
direct test, which has now been realised through a combination of exquisite
results from a number of CMB experiments.
After a long and detailed investigation, with many false clues, it seems that
the mystery of the curvature of the Universe is now solved. It's an open and
shut case: the Universe is flat.Comment: 7 pages, 2 figures, submitted to the Gravity Research Foundation
Essay Competition for 200
Observational signatures of f(R) dark energy models that satisfy cosmological and local gravity constraints
We discuss observational consequences of f(R) dark energy scenarios that
satisfy local gravity constraints (LGC) as well as conditions of the
cosmological viability. The model we study is given by m(r)=C(-r-1)^p (C>0,
p>1) with m=Rf_{,RR}/f_{,R} and r=-Rf_{,R}/f, which cover viable f(R) models
proposed so far in a high-curvature region designed to be compatible with LGC.
The equation of state of dark energy exhibits a divergence at a redshift z_c
that can be as close as a few while satisfying sound horizon constraints of
Cosmic Microwave Background (CMB). We study the evolution of matter density
perturbations in details and place constraints on model parameters from the
difference of spectral indices of power spectra between CMB and galaxy
clustering. The models with p>5 can be consistent with those observational
constraints as well as LGC. We also discuss the evolution of perturbations in
the Ricci scalar R and show that an oscillating mode (scalaron) can easily
dominate over a matter-induced mode as we go back to the past. This violates
the stability of cosmological solutions, thus posing a problem about how the
over-production of scalarons should be avoided in the early universe.Comment: 13 pages, 7 figures, version to appear in Physical Review
Space of kink solutions in SU(N)\times Z_2
We find distinct classes (``generations'') of kink solutions in an
field theory. The classes are labeled by an integer . The
members of one class of kinks will be globally stable while those of the other
classes may be locally stable or unstable. The kink solutions in the
class have a continuous degeneracy given by the manifold ,
where is the unbroken symmetry group and is the group under which the
kink solution remains invariant. The space is found to contain
incontractable two spheres for some values of , indicating the possible
existence of certain incontractable spherical structures in three dimensions.
We explicitly construct the three classes of kinks in an SU(5) model with
quartic potential and discuss the extension of these ideas to magnetic monopole
solutions in the model.Comment: 11 pages, 3 figures. Several minor changes made. Matches the version
accepted to PR
A Rotating Charged Black Hole Solution in f(R) Gravity
In the context of f(R) theories of gravity, we address the problem of finding
a rotating charged black hole solution in the case of constant curvature. The
new metric is obtained by solving the field equations and we show that the
behavior of it is typical of a rotating charged source. In addition, we analyze
the thermodynamics of the new black hole. The results ensures that the
thermodynamical properties in f(R) gravities are qualitatively similar to those
of standard General Relativity.Comment: 9 pages, no figure
The circular loop equation of a cosmic string with time-varying tension
The equation of circular loops of cosmic string with time-dependent tension
is studied in the Minkowski spacetime and Robertson-Walker universe. We find
that, in the case where the tension depends on some power of the cosmic time,
cosmic string loops with time-varying tension should not collapse to form a
black hole if the power is lower than a critical value.Comment: 8 pages, 3 figures. accepted by Modern Physics Letters
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
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