507 research outputs found
Singular Instantons Made Regular
The singularity present in cosmological instantons of the Hawking-Turok type
is resolved by a conformal transformation, where the conformal factor has a
linear zero of codimension one. We show that if the underlying regular manifold
is taken to have the topology of , and the conformal factor is taken to
be a twisted field so that the zero is enforced, then one obtains a
one-parameter family of solutions of the classical field equations, where the
minimal action solution has the conformal zero located on a minimal volume
noncontractible submanifold. For instantons with two singularities, the
corresponding topology is that of a cylinder with D=4
analogues of `cross-caps' at each of the endpoints.Comment: 23 pages, compressed and RevTex file, including nine postscript
figure files. Submitted versio
Cosmic Texture from a Broken Global SU(3) Symmetry
We investigate the observable consequences of creating cosmic texture by
breaking a global SU(3) symmetry, rather than the SU(2) case which is generally
studied. To this end, we study the nonlinear sigma model for a totally broken
SU(3) symmetry, and develop a technique for numerically solving the classical
field equations. This technique is applied in a cosmological context: the
energy of the collapsing SU(3) texture field is used as a gravitational source
for the production of perturbations in the primordial fluids of the early
universe. From these calculations, we make predictions about the appearance of
the anisotropies in the cosmic microwave background radiation (CMBR) which
would be present if the large scale structure of the universe was
gravitationally seeded by the collapse of SU(3) textures.Comment: 28 pages, latex, 11 figures, submitted to Phys. Rev.
A Causal Source which Mimics Inflation
How unique are the inflationary predictions for the cosmic microwave
anisotropy pattern? In this paper, it is asked whether an arbitrary causal
source for perturbations in the standard hot big bang could effectively mimic
the predictions of the simplest inflationary models. A surprisingly simple
example of a `scaling' causal source is found to closely reproduce the
inflationary predictions. This letter extends the work of a previous paper
(ref. 6) to a full computation of the anisotropy pattern, including the Sachs
Wolfe integral. I speculate on the possible physics behind such a source.Comment: 4 pages, RevTex, 3 figure
A Semi-Analytical Analysis of Texture Collapse
This study presents a simplified approach to studying the dynamics of global
texture collapse. We derive equations of motion for a spherically symmetric
field configuration using a two parameter ansatz. Then we analyse the effective
potential for the resulting theory to understand possible trajectories of the
field configuration in the parameter space of the ansatz. Numerical results are
given for critical winding and collapse time in spatially flat non-expanding,
and flat expanding universes. In addition, the open non-expanding and
open-expanding cases are studied.Comment: 12 pages, figures available from author, BROWN-HET-895, uses phyzz
A smooth bouncing cosmology with scale invariant spectrum
We present a bouncing cosmology which evolves from the contracting to the
expanding phase in a smooth way, without developing instabilities or
pathologies and remaining in the regime of validity of 4d effective field
theory. A nearly scale invariant spectrum of perturbations is generated during
the contracting phase by an isocurvature scalar with a negative exponential
potential and then converted to adiabatic. The model predicts a slightly blue
spectrum, n_S >~ 1, no observable gravitational waves and a high (but model
dependent) level of non-Gaussianities with local shape. The model represents an
explicit and predictive alternative to inflation, although, at present, it is
clearly less compelling.Comment: 20 pages, 1 fig. v2: references added, JCAP published versio
The string wave function across a Kasner singularity
A collision of orbifold planes in eleven dimensions has been proposed as an
explanation of the hot big bang. When the two planes are close to each other,
the winding membranes become the lightest modes of the theory, and can be
effectively described in terms of fundamental strings in a ten dimensional
background. Near the brane collision, the eleven-dimensional metric is an
Euclidean space times a 1+1-dimensional Milne universe. However, one may expect
small perturbations to lead into a more general Kasner background. In this
paper we extend the previous classical analysis of winding membranes to Kasner
backgrounds, and using the Hamiltonian equations, solve for the wave function
of loops with circular symmetry. The evolution across the singularity is
regular, and explained in terms of the excitement of higher oscillation modes.
We also show there is finite particle production and unitarity is preserved.Comment: 28 pages, 10 figure
Collapse of topological texture
We study analytically the process of a topological texture collapse in the
approximation of a scaling ansatz in the nonlinear sigma-model. In this
approximation we show that in flat space-time topological texture eventually
collapses while in the case of spatially flat expanding universe its fate
depends on the rate of expansion. If the universe is inflationary, then there
is a possibility that texture will expand eternally; in the case of exponential
inflation the texture may also shrink or expand eternally to a finite limiting
size, although this behavior is degenerate. In the case of power law
noninflationary expansion topological texture eventually collapses. In a cold
matter dominated universe we find that texture which is formed comoving with
the universe expansion starts collapsing when its spatial size becomes
comparable to the Hubble size, which result is in agreement with the previous
considerations. In the nonlinear sigma-model approximation we consider also the
final stage of the collapsing ellipsoidal topological texture. We show that
during collapse of such a texture at least two of its principal dimensions
shrink to zero in a similar way, so that their ratio remains finite. The third
dimension may remain finite (collapse of cigar type), or it may also shrink to
zero similar to the other two dimensions (collapse of scaling type), or shrink
to zero similar to the product of the remaining two dimensions (collapse of
pancake type).Comment: 23 pages, LaTeX, to be published in Phys. Rev.
Beyond the Small-Angle Approximation For MBR Anisotropy from Seeds
In this paper we give a general expression for the energy shift of massless
particles travelling through the gravitational field of an arbitrary matter
distribution as calculated in the weak field limit in an asymptotically flat
space-time. It is {\it not} assumed that matter is non-relativistic. We
demonstrate the surprising result that if the matter is illuminated by a
uniform brightness background that the brightness pattern observed at a given
point in space-time (modulo a term dependent on the oberver's velocity) depends
only on the matter distribution on the observer's past light-cone. These
results apply directly to the cosmological MBR anisotropy pattern generated in
the immediate vicinity of of an object like a cosmic string or global texture.
We apply these results to cosmic strings, finding a correction to previously
published results for in the small-angle approximation. We also derive the
full-sky anisotropy pattern of a collapsing texture knot.Comment: 23 pages, FERMILAB-Pub-94/047-
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