13,699 research outputs found
Entropy production and curvature perturbation from dissipative curvatons
Considering the curvaton field that follows dissipative slow-roll equation,
we show that the field can lead to entropy production and generation of
curvature perturbation after reheating. Spectral index is calculated to
discriminate warm and thermal scenarios of dissipative curvatons from the
standard curvaton model. In contrast to the original curvaton model, quadratic
potential is not needed in the dissipative scenario, since the growth in the
oscillating period is not essential for the model.Comment: 29 pages, 2 figures, accepted for publication in JCA
Primordial black holes from cosmic necklaces
Cosmic necklaces are hybrid topological defects consisting of monopoles and
strings. We argue that primordial black holes(PBHs) may have formed from loops
of the necklaces, if there exist stable winding states, such as coils and
cycloops. Unlike the standard scenario of PBH formation from string loops, in
which the kinetic energy plays important role when strings collapse into black
holes, the PBH formation may occur in our scenario after necklaces have
dissipated their kinetic energy. Then, the significant difference appears in
the production ratio. In the standard scenario, the production ratio
becomes a tiny fraction , however it becomes in our
case. On the other hand, the typical mass of the PBHs is much smaller than the
standard scenario, if they are produced in the same epoch. As the two
mechanisms may work at the same time, the necklaces may have more than one
channel of the gravitational collapse. Although the result obtained in this
paper depends on the evolution of the dimensionless parameter , the
existence of the winding state could be a serious problem in some cases. Since
the existence of the winding state in brane models is due to the existence of a
non-tivial circle in the compactified space, the PBH formation can be used to
probe the structure of the compactified space. Black holes produced by this
mechanism may have peculiar properties.Comment: 22pages, 3 figures, added many comments, +1 figure, accepted for
publication in JHE
Dark matter production from cosmic necklaces
Cosmic strings have gained a great interest, since they are formed in a large
class of brane inflationary models. The most interesting story is that cosmic
strings in brane models are distinguished in future cosmological observations.
If the strings in brane models are branes or superstrings that can move along
compactified space, and also if there are degenerated vacua along the
compactified space, kinks interpolate between degenerated vacua become
``beads'' on the strings. In this case, strings turn into necklaces. In the
case that the compact manifold in not simply connected, a string loop that
winds around a nontrivial circle is stable due to the topological reason. Since
the existence of the (quasi-)degenerated vacua and the nontrivial circle is a
common feature of the brane models, it is important to study cosmological
constraints on the cosmic necklaces and the stable winding states. In this
paper, we consider dark matter production from loops of the cosmic necklaces.
Our result suggests that necklaces can put stringent bound on certain kinds of
brane models.Comment: 27 pages, 5 figures, added many comments and 3 figures, accepted for
publication in JCA
String production after angled brane inflation
We describe string production after angled brane inflation. First, we point
out that there was a discrepancy in previous discussions. The expected tension
of the cosmic string calculated from the four-dimensional effective Lagrangian
did not match the one obtained in the brane analysis. In the previous analysis,
the cosmic string is assumed to correspond to the lower-dimensional daughter
brane, which wraps the same compactified space as the original mother brane. In
this case, however, the tension of the daughter brane cannot depend on the
angle (\theta). On the other hand, from the analysis of the effective
Lagrangian for tachyon condensation, it is easy to see that the tension of the
cosmic string must be proportional to \theta, when \theta << 1. This is an
obvious discrepancy that must be explained by consideration of the explicit
brane dynamics. In this paper, we will solve this problem by introducing a
simple idea. We calculate the tension of the string in the two cases, which
matches precisely. The cosmological constraint for angled inflation is relaxed,
because the expected tension of the cosmic string becomes smaller than the one
obtained in previous arguments, by a factor of \theta.Comment: 13pages, 3 figures, typos correcte
Formation of monopoles and domain walls after brane inflation
We study cosmological defect formation after brane inflation. The
cosmological defects are corresponding to the branes that have less than three
spacial dimensions in the uncompactified spacetime. Contrary to the previous
arguments, production of monopoles and domain walls are not always negligible.
Monopoles and domain walls are formed by the branes extended between mother
branes.Comment: 27pages, 7 figures, many comments, footnotes and reviews are added,
to appear in JHE
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