16,005 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

### 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

### Aspects of warm-flat directions

Considering the mechanism of dissipative slow-roll that has been used in warm
inflation scenario, we show that dissipation may alter usual cosmological
scenarios associated with SUSY-flat directions. We mainly consider SUSY-flat
directions that have strong interactions with non-flat directions and may cause
strong dissipation both in thermal and non-thermal backgrounds. An example is
the Affleck-Dine mechanism in which dissipation may create significant (both
qualitative and quantitative) discrepancies between the conventional scenario
and the dissipative one. We also discuss several mechanisms of generating
curvature perturbations in which the dissipative field, which is distinguished
from the inflaton field, can be used as the source of cosmological
perturbations. Considering the Morikawa-Sasaki dissipative coefficient, the
damping caused by the dissipation may be significant for many MSSM flat
directions even if the dissipation is far from thermal equilibrium.Comment: 22 pages, accepted for publication in International Journal of Modern
Physics

### 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

### 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

### Hybrid Curvatons from Broken Symmetry

We present a new general mechanism for generating curvature perturbations
after inflation. Our model is based on the simple assumption that a field that
starts to oscillate after inflation has a potential characterized by an
underlying global symmetry that is slightly or badly broken. Inhomogeneous
preheating occurs due to the oscillation with the broken symmetry. Unlike the
traditional curvaton model, we will not identify the curvaton with the
oscillating field. The curvaton is identified with the preheat field that could
be either a scalar, vector, or fermionic field. We introduce an explicit mass
term for the curvaton, which is important for later evolution and the decay.
Our present model represents the simplest example of the hybrid of the
curvatons and inhomogeneous preheating.Comment: 21pages, 5 figures, accepted for publication in JHE

### 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 $f$
becomes a tiny fraction $f\sim 10^{-20}$, however it becomes $f \sim 1$ 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 $r$, 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

### 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

### Cosmological perturbations from inhomogeneous preheating and multi-field trapping

We consider inhomogeneous preheating in a multi-field trapping model. The
curvature perturbation is generated by inhomogeneous preheating which induces
multi-field trapping at the enhanced symmetric point (ESP), and results in
fluctuation in the number of e-foldings. Instead of considering simple
reheating after preheating, we consider a scenario of shoulder inflation
induced by the trapping. The fluctuation in the number of e-foldings is
generated during this weak inflationary period, when the additional light
scalar field is trapped at the local maximum of its potential. The situation
may look similar to locked or thermal inflation or even to hybrid inflation,
but we will show that the present mechanism of generating the curvature
perturbation is very different from these others. Unlike the conventional
trapped inflationary scenario, we do not make the assumption that an ESP
appears at some unstable point on the inflaton potential. This assumption is
crucial in the original scenario, but it is not important in the multi-field
model. We also discuss inhomogeneous preheating at late-time oscillation, in
which the magnitude of the curvature fluctuation can be enhanced to accommodate
low inflationary scale.Comment: 18pages, 2 figures, to appear 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

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