5,565 research outputs found
Numerical study of curvature perturbations in a brane-world inflation at high-energies
We study the evolution of scalar curvature perturbations in a brane-world
inflation model in a 5D Anti-de Sitter spacetime. The inflaton perturbations
are confined to a 4D brane but they are coupled to the 5D bulk metric
perturbations. We numerically solve full coupled equations for the inflaton
perturbations and the 5D metric perturbations using Hawkins-Lidsey inflationary
model. At an initial time, we assume that the bulk is unperturbed. We find that
the inflaton perturbations at high energies are strongly coupled to the bulk
metric perturbations even on subhorizon scales, leading to the suppression of
the amplitude of the comoving curvature perturbations at a horizon crossing.
This indicates that the linear perturbations of the inflaton field does not
obey the usual 4D Klein-Gordon equation due to the coupling to 5D gravitational
field on small scales and it is required to quantise the coupled brane-bulk
system in a consistent way in order to calculate the spectrum of the scalar
perturbations in a brane-world inflation.Comment: 16 pages, 5 figure
Universality of massive scalar field late-time tails in black-hole spacetimes
The late-time tails of a massive scalar field in the spacetime of black holes
are studied numerically. Previous analytical results for a Schwarzschild black
hole are confirmed: The late-time behavior of the field as recorded by a static
observer is given by , where
depends weakly on time. This result is carried over to the case of
a Kerr black hole. In particular, it is found that the power-law index of -5/6
depends on neither the multipole mode nor on the spin rate of the black
hole . In all black hole spacetimes, massive scalar fields have the same
late-time behavior irrespective of their initial data (i.e., angular
distribution). Their late-time behavior is universal.Comment: 11 pages, 14 figures, published versio
Slow-roll corrections to inflaton fluctuations on a brane
Quantum fluctuations of an inflaton field, slow-rolling during inflation are
coupled to metric fluctuations. In conventional four dimensional cosmology one
can calculate the effect of scalar metric perturbations as slow-roll
corrections to the evolution of a massless free field in de Sitter spacetime.
This gives the well-known first-order corrections to the field perturbations
after horizon-exit. If inflaton fluctuations on a four dimensional brane
embedded in a five dimensional bulk spacetime are studied to first-order in
slow-roll then we recover the usual conserved curvature perturbation on
super-horizon scales. But on small scales, at high energies, we find that the
coupling to the bulk metric perturbations cannot be neglected, leading to a
modified amplitude of vacuum oscillations on small scales. This is a large
effect which casts doubt on the reliability of the usual calculation of
inflaton fluctuations on the brane neglecting their gravitational coupling.Comment: 18 pages, 4 figure
Slow-roll corrections to inflaton fluctuations on a brane
Quantum fluctuations of an inflaton field, slow-rolling during inflation are
coupled to metric fluctuations. In conventional four dimensional cosmology one
can calculate the effect of scalar metric perturbations as slow-roll
corrections to the evolution of a massless free field in de Sitter spacetime.
This gives the well-known first-order corrections to the field perturbations
after horizon-exit. If inflaton fluctuations on a four dimensional brane
embedded in a five dimensional bulk spacetime are studied to first-order in
slow-roll then we recover the usual conserved curvature perturbation on
super-horizon scales. But on small scales, at high energies, we find that the
coupling to the bulk metric perturbations cannot be neglected, leading to a
modified amplitude of vacuum oscillations on small scales. This is a large
effect which casts doubt on the reliability of the usual calculation of
inflaton fluctuations on the brane neglecting their gravitational coupling.Comment: 18 pages, 4 figure
Ghosts in the self-accelerating universe
The self-accelerating universe realizes the accelerated expansion of the
universe at late times by large-distance modification of general relativity
without a cosmological constant. The Dvali-Gabadadze-Porrati (DGP) braneworld
model provides an explicit example of the self-accelerating universe. Recently,
the DGP model becomes very popular to study the observational consequences of
the modified gravity models as an alternative to dark energy models in GR.
However, it has been shown that the self-accelerating universe in the DGP model
contains a ghost at the linearized level. The ghost carries negative energy
densities and it leads to the instability of the spacetime. In this article, we
review the origin of the ghost in the self-accelerating universe and explore
the physical implication of the existence of the ghost.Comment: Invited topical review for Classical and Quantum Gravity, 20 pages, 4
figure
Primordial perturbations from slow-roll inflation on a brane
In this paper we quantise scalar perturbations in a Randall-Sundrum-type
model of inflation where the inflaton field is confined to a single brane
embedded in five-dimensional anti-de Sitter space-time. In the high energy
regime, small-scale inflaton fluctuations are strongly coupled to metric
perturbations in the bulk and gravitational back-reaction has a dramatic effect
on the behaviour of inflaton perturbations on sub-horizon scales. This is in
contrast to the standard four-dimensional result where gravitational
back-reaction can be neglected on small scales. Nevertheless, this does not
give rise to significant particle production, and the correction to the power
spectrum of the curvature perturbations on super-horizon scales is shown to be
suppressed by a slow-roll parameter. We calculate the complete first order
slow-roll corrections to the spectrum of primordial curvature perturbations.Comment: 23 pages, 10 figure
Discovery of K-Shell Emission Lines of Neutral Atoms in the Galactic Center Region
The K-shell emission line of neutral irons from the Galactic center (GC)
region is one of the key for the structure and activity of the GC. The origin
is still open question, but possibly due either to X-ray radiation or to
electron bombarding to neutral atoms. To address this issue, we analyzed the
Suzaku X-ray spectrum from the GC region of intense neutral iron line emission,
and report on the discovery of Kalpha lines of neutral argon, calcium, chrome,
and manganese atoms. The equivalent widths of these Kalpha lines indicate that
the metal abundances in the GC region should be ~1.6 and ~4 of solar value,
depending on the X-ray and the electron origins, respectively. On the other
hand, the metal abundances in the hot plasma in the GC region are found to be
~1-2 solar. These results favor that the origin of the neutral Kalpha lines are
due to X-ray irradiation.Comment: 7 pages, 5 figures, accepted for publication in PASJ (Vol.62, No.2,
pp.423--429
Discovery of a superluminal Fe K echo at the Galactic Center: The glorious past of Sgr A* preserved by molecular clouds
We present the result of a study of the X-ray emission from the Galactic
Centre (GC) Molecular Clouds (MC) within 15 arcmin from Sgr A*. We use
XMM-Newton data (about 1.2 Ms of observation time) spanning about 8 years. The
MC spectra show all the features characteristic of reflection: i) intense Fe
Kalpha, with EW of about 0.7-1 keV, and the associated Kbeta line; ii) flat
power law continuum and iii) a significant Fe K edge (tau~0.1-0.3). The diffuse
low ionisation Fe K emission follows the MC distribution, nevertheless not all
MC are Fe K emitters. The long baseline monitoring allows the characterisation
of the temporal evolution of the MC emission. A complex pattern of variations
is shown by the different MC, with some having constant Fe K emission, some
increasing and some decreasing. In particular, we observe an apparent
super-luminal motion of a light front illuminating a Molecular nebula. This
might be due to a source outside the MC (such as Sgr A* or a bright and long
outburst of a X-ray binary), while it cannot be due to low energy cosmic rays
or a source located inside the cloud. We also observe a decrease of the X-ray
emission from G0.11-0.11, behaviour similar to the one of Sgr B2. The line
intensities, clouds dimensions, columns densities and positions with respect to
Sgr A*, are consistent with being produced by the same Sgr A* flare. The
required high luminosity (about 1.5~10^39 erg/s) can hardly be produced by a
binary system, while it is in agreement with a flare of Sgr A* fading about 100
years ago. The low intensity of the Fe K emission coming from the 50 and the 20
km/s MC places an upper limit of 10^36 erg/s to the mean luminosity of Sgr A*
in the last 60-90 years. The Fe K emission and variations from these MC might
have been produced by a single flare of Sgr A*.Comment: ApJ in press 17 pages, 14 Figures, 3 table
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