11,955 research outputs found
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
Remote Inflation: Hybrid-like inflation without hybrid-type potential
A new scenario of hybrid-like inflation is considered without using
hybrid-type potential. Radiation raised continuously by a dissipating inflaton
field keeps symmetry restoration in a remote sector, and the false-vacuum
energy of the remote sector dominates the energy density during inflation.
Remote inflation is terminated when the temperature reaches the critical
temperature, or when the slow-roll condition is violated. Without introducing a
complex form of couplings, inflaton field may either roll-in (like a standard
hybrid inflation) or roll-out (like an inverted-hybrid model or quintessential
inflation) on arbitrary inflaton potential. Significant signatures of remote
inflation can be observed in the spectrum caused by (1) the inhomogeneous phase
transition in the remote sector, or (2) a successive phase transition in the
remote sector. Remote inflation can predict strong amplification or suppression
of small-scale perturbations without introducing multiple inflation. Since the
inflaton may have a run-away potential, it is also possible to identify the
inflaton with quintessence, without introducing additional mechanisms. Even if
the false-vacuum energy is not dominated by the remote sector, the phase
transition in the remote sector is possible during warm inflation, which may
cause significant amplification/suppression of the curvature perturbations.Comment: 28 pages, 1 figure, fixed references, accepted for publication in
JCA
Evolution of the curvature perturbations during warm inflation
This paper considers warm inflation as an interesting application of
multi-field inflation. Delta-N formalism is used for the calculation of the
evolution of the curvature perturbations during warm inflation. Although the
perturbations considered in this paper are decaying after the horizon exit, the
corrections to the curvature perturbations sourced by these perturbations can
remain and dominate the curvature perturbations at large scales. In addition to
the typical evolution of the curvature perturbations, inhomogeneous diffusion
rate is considered for warm inflation, which may lead to significant
non-Gaussianity of the spectrum.Comment: 23 pages, 1 figure, fixed references, accepted for publication in
JCA
Running spectral index from shooting-star moduli
We construct an inflationary model that is consistent with both large
non-Gaussianity and a running spectral index. The scenario of modulated
inflation suggests that modulated perturbation can induce the curvature
perturbation with a large non-Gaussianity, even if the inflaton perturbation is
negligible. Using this idea, we consider a multi-field extension of the
modulated inflation scenario and examine the specific situation where different
moduli are responsible for the perturbation at different scales. We suppose
that the additional moduli (shooting-star moduli) is responsible for the
curvature perturbation at the earlier inflationary epoch and it generates the
fluctuation with n>1 spectral index at this scale. After a while, another
moduli (or inflaton) takes the place and generates the perturbation with n<1.
At the transition point the two fluctuations are comparable with each other. We
show how the spectral index is affected by the transition induced by the
shooting-star moduli.Comment: 14 pages, latex, 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
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
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
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
Replica analysis of partition-function zeros in spin-glass models
We study the partition-function zeros in mean-field spin-glass models. We
show that the replica method is useful to find the locations of zeros in a
complex parameter plane. For the random energy model, we obtain the phase
diagram in the plane and find that there are two types of distribution of
zeros: two-dimensional distribution within a phase and one-dimensional one on a
phase boundary. Phases with a two-dimensional distribution are characterized by
a novel order parameter defined in the present replica analysis. We also
discuss possible patterns of distributions by studying several systems.Comment: 23 pages, 12 figures; minor change
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