8,948 research outputs found
Brane inflation without slow-roll
The scenario of brane inflation without using the conventional slow-roll
approximations has been investigated. Based on the mechanism of generating the
curvature perturbations at the end of inflation, a new brane inflation paradigm
was developed. The conditions for making a sufficiently large enough number of
e-foldings and for generating the curvature perturbations without producing
dangerous relics were also examined. Benefits of our scenario are subsequently
discussed in detail.Comment: 21 pages, 2 figures, added an appendix, accepted for publication in
JHE
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
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
Generating the curvature perturbation with instant preheating
A new mechanism for generating the curvature perturbation at the end of
inflaton has been investigated. The dominant contribution to the primordial
curvature perturbation may be generated during the period of instant
preheating. The mechanism converts isocurvature perturbation related to a light
field into curvature perturbation, where the ``light field'' is not the
inflaton field. This mechanism is important in inflationary models where
kinetic energy is significant at the end of inflaton. We show how one can apply
this mechanism to various brane inflationary models.Comment: 17 pages, 1 figure, To appear in JCA
Q ball inflation
We show that inflation can occur in the core of a Q-ball.Comment: 11 pages, latex2e, no figure, references added, final version to
appear in PR
Elliptic Inflation: Generating the curvature perturbation without slow-roll
There are many inflationary models in which inflaton field does not satisfy
the slow-roll condition. However, in such models, it is always difficult to
generate the curvature perturbation during inflation. Thus, to generate the
curvature perturbation, one must introduce another component to the theory. To
cite a case, curvatons may generate dominant part of the curvature perturbation
after inflation. However, we have a question whether it is unrealistic to
consider the generation of the curvature perturbation during inflation without
slow-roll. Assuming multi-field inflation, we encounter the generation of the
curvature perturbation during inflation without slow-roll. The potential along
equipotential surface is flat by definition and thus we do not have to worry
about symmetry. We also discuss about KKLT models, in which corrections lifting
the inflationary direction may not become a serious problem if there is a
symmetry enhancement at the tip (not at the moving brane) of the inflationary
throat.Comment: 27pages, 8figures, to appear in JCA
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
Curvaton paradigm can accommodate multiple low inflation scales
Recent arguments show that some curvaton field may generate the cosmological
curvature perturbation. As the curvaton is independent of the inflaton field,
there is a hope that the fine-tunings of inflation models can be cured by the
curvaton scenario. More recently, however, D.H.Lyth discussed that there is a
strong bound for the Hubble parameter during inflation even if one assumes the
curvaton scenario. Although the most serious constraint was evaded, the bound
seems rather crucial for many models of a low inflation scale. In this paper we
try to remove this constraint. We show that the bound is drastically modified
if there were multiple stages of inflation.Comment: 9pages, no figure, references added, final versio
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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