1,003 research outputs found
Bubbles in the Self-Accelerating Universe
We revisit the issue of the stability in the Dvali-Gabadadze-Porrati model,
by considering the nucleation of bubbles of the conventional branch within the
self-accelerating branch. We construct an instanton describing this process in
the thin wall approximation. On one side of the bubble wall, the bulk consists
of the exterior of the brane while on the other side it is the interior. The
solution requires the presence of a 2-brane (the bubble wall) which induces the
transition. However, we show that this instanton cannot be realized as the thin
wall limit of any smooth solution. Once the bubble thickness is resolved, the
equations of motion do not allow O(4) symmetric solutions joining the two
branches. We conclude that the thin wall instanton is unphysical, and that one
cannot have processes connecting the two branches, unless negative tension
bubble walls are introduced. This also suggests that the self-accelerating
branch does not decay into the conventional branch nucleating bubbles. We
comment on other kinds of bubbles that could interpolate between the two
branches.Comment: 8 pages, 5 figure
On the full trispectrum in single field DBI-inflation
We compute the tree-level connected four-point function of the primordial
curvature perturbation for a fairly general minimally coupled single field
inflationary model, where the inflaton's Lagrangian is a general function of
the scalar field and its first derivatives. This model includes K-inflation and
DBI-inflation as particular cases. We show that, at the leading order in the
slow-roll expansion and in the small sound speed limit, there are two important
tree-level diagrams for the trispectrum. One is a diagram where a scalar mode
is exchanged and the other is a diagram where the interaction occurs at a
point, i.e. a contact interaction diagram. The scalar exchange contribution is
comparable to the contact interaction contribution. For the DBI-inflation
model, in the so-called equilateral configuration, the scalar exchange
trispectrum is maximized when the angles between the four momentum vectors are
equal and in this case the amplitude of the trispectrum from the scalar
exchange is one order of magnitude higher than the contact interaction
trispectrum.Comment: 16 pages, 6 figure
Lorentz boost and non-Gaussianity in multi-field DBI-inflation
We show that higher-order actions for cosmological perturbations in the
multi-field DBI-inflation model are obtained by a Lorentz boost from the rest
frame of the brane to the frame where the brane is moving. We confirm that this
simple method provides the same third- and fourth- order actions at leading
order in slow-roll and in the small sound speed limit as those obtained by the
usual ADM formalism. As an application, we compute the leading order connected
four-point function of the primordial curvature perturbation coming from the
intrinsic fourth-order contact interaction in the multi-field DBI-inflation
model. At the third order, the interaction Hamiltonian arises purely by the
boost from the second-order action in the rest frame of the brane. The boost
acts on the adiabatic and entropy modes in the same way thus there exists a
symmetry between the adiabatic and entropy modes. But at fourth order this
symmetry is broken due to the intrinsic fourth-order action in the rest frame
and the difference between the Lagrangian and the interaction Hamiltonian.
Therefore, contrary to the three-point function, the momentum dependence of the
purely adiabatic component and the components including the entropic
contributions are different in the four-point function. This suggests that the
trispectrum can distinguish the multi-field DBI-inflation model from the single
field DBI-inflation model.Comment: 11 pages, no figures, v2:references added. Accepted for publication
in PR
On the Rebrightenings of Classical Novae during the Early Phase
We report on the spectral evolution of 6 classical novae, V1186 Sco, V2540
Oph, V4745 Sgr, V5113 Sgr, V458 Vul, and V378 Ser, based on the low-resolution
spectra obtained at the Fujii-Bisei Observatory and the Bisei Astronomical
Observatory, Japan. In the light curves, these 6 novae show several
rebrightenings during the early phase lasting ~10 days after the first maximum
in fast novae, and ~100 days in slow novae. The early spectra of all of these
novae had emission lines with a P-Cygni profile at the maximum brightness. The
absorption component of the P-Cygni profiles then disappeared after the
maximum, and reappeared when the novae brightened to the next maximum. We
suggest that the re-appearance of the absorption component at the rebrightening
is attributable to re-expansion of the photosphere after it once shifts
sufficiently inside. From the light curves, we found that the time intervals of
the rebrightenings of these 6 novae show a similar systematic trend, which is
applicable to all types of novae: fast and slow, and Fe II type and hybrid
type. Moreover, we note the difference between the spectra at the
rebrightenings during the early phase and at the rebrightening in V2362 Cyg,
and at the oscillation during the transition phase in V1494 Aql, which means
difference of the physical mechanism of the rebrightening during the early
phase and the later oscillations.Comment: 11 pages, 15 figures, accepted for publication in PAS
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