3,023 research outputs found
The Spectral Energy Distribution of Normal, Starburst and Active Galaxies
We present the results of an extensive literature search of multiwavelength
data for a sample of 59 galaxies, consisting of 26 Starbursts, 15 Seyfert 2's,
5 LINER's, 6 normal spirals and 7 normal elliptical galaxies. The data include
soft X-ray fluxes, ultraviolet and optical spectra, near, mid/far infrared
photometry and radio measurements, selected to match as closely as possible the
IUE aperture (10" X 20"). The galaxies are separated into 6 groups with similar
characteristics, namely, Ellipticals, Spirals, LINER's, Seyfert 2's, Starbursts
of Low and High reddening, for which we create average spectral energy
distributions (SED). The individual groups SED's are normalized to the
7000\AA flux and compared, looking for similarities and differences
among them.The bolometric fluxes of different types of galaxies were calculated
integrating their SED's. These values are compared with individual waveband
flux densities, in order to determine the wavebands which contribute most to
the bolometric flux. Linear regressions were performed between the bolometric
and individual band fluxes for each kind of galaxy. These fits can be used in
the calculation of the bolometric flux for other objects of similar activity
type, but with reduced waveband information. We have also collected
multiwavelength data for 4 HII regions, a thermal supernova remnant, and a
non-thermal supernova remnant (SNR), which are compared with the Starburst
SED's.Comment: 29 pages, 13 postscript figures and 10 tables. To appear in The
Astronomical Journa
Slow roll in simple non-canonical inflation
We consider inflation using a class of non-canonical Lagrangians for which
the modification to the kinetic term depends on the field, but not its
derivatives. We generalize the standard Hubble slow roll expansion to the
non-canonical case and derive expressions for observables in terms of the
generalized slow roll parameters. We apply the general results to the
illustrative case of ``Slinky'' inflation, which has a simple, exactly
solvable, non-canonical representation. However, when transformed into a
canonical basis, Slinky inflation consists of a field oscillating on a
multi-valued potential. We calculate the power spectrum of curvature
perturbations for Slinky inflation directly in the non-canonical basis, and
show that the spectrum is approximately a power law on large scales, with a
``blue'' power spectrum. On small scales, the power spectrum exhibits strong
oscillatory behavior. This is an example of a model in which the widely used
solution of Garriga and Mukhanov gives the wrong answer for the power spectrum.Comment: 9 pages, LaTeX, four figures. (V2: minor changes to text. Version
submitted to JCAP.
A Hamilton-Jacobi approach to non-slow-roll inflation
I describe a general approach to characterizing cosmological inflation
outside the standard slow-roll approximation, based on the Hamilton-Jacobi
formulation of scalar field dynamics. The basic idea is to view the equation of
state of the scalar field matter as the fundamental dynamical variable, as
opposed to the field value or the expansion rate. I discuss how to formulate
the equations of motion for scalar and tensor fluctuations in situations where
the assumption of slow roll is not valid. I apply the general results to the
simple case of inflation from an ``inverted'' polynomial potential, and to the
more complicated case of hybrid inflation.Comment: 21 pages, RevTeX (minor revisions to match published version
New Solutions of the Inflationary Flow Equations
The inflationary flow equations are a frequently used method of surveying the
space of inflationary models. In these applications the infinite hierarchy of
differential equations is truncated in a way which has been shown to be
equivalent to restricting the set of models considered to those characterized
by polynomial inflaton potentials. This paper explores a different method of
solving the flow equations, which does not truncate the hierarchy and in
consequence covers a much wider class of models while retaining the practical
usability of the standard approach.Comment: References added, and a couple of comment
The Lyth Bound and the End of Inflation
We derive an extended version of the well-known Lyth Bound on the total
variation of the inflaton field, incorporating higher order corrections in slow
roll. We connect the field variation to both the spectral index of
scalar perturbations and the amplitude of tensor modes. We then investigate the
implications of this bound for ``small field'' potentials, where the field
rolls off a local maximum of the potential. The total field variation during
inflation is {\em generically} of order , even for potentials with
a suppressed tensor/scalar ratio. Much of the total field excursion arises in
the last e-fold of inflation and in single field models this problem can only
be avoided via fine-tuning or the imposition of a symmetry. Finally, we discuss
the implications of this result for inflationary model building in string
theory and supergravity.Comment: 10 pages, RevTeX, 2 figures (V3: version accepted for publication by
JCAP
Cascade Failure in a Phase Model of Power Grids
We propose a phase model to study cascade failure in power grids composed of
generators and loads. If the power demand is below a critical value, the model
system of power grids maintains the standard frequency by feedback control. On
the other hand, if the power demand exceeds the critical value, an electric
failure occurs via step out (loss of synchronization) or voltage collapse. The
two failures are incorporated as two removal rules of generator nodes and load
nodes. We perform direct numerical simulation of the phase model on a
scale-free network and compare the results with a mean-field approximation.Comment: 7 pages, 2 figure
General conditions for scale-invariant perturbations in an expanding universe
We investigate the general properties of expanding cosmological models which
generate scale-invariant curvature perturbations in the presence of a variable
speed of sound. We show that in an expanding universe, generation of a
super-Hubble, nearly scale-invariant spectrum of perturbations over a range of
wavelengths consistent with observation requires at least one of three
conditions: (1) accelerating expansion, (2) a speed of sound faster than the
speed of light, or (3) super-Planckian energy density.Comment: 4 pages, RevTe
The scalar bi-spectrum during preheating in single field inflationary models
In single field inflationary models, preheating refers to the phase that
immediately follows inflation, but precedes the epoch of reheating. During this
phase, the inflaton typically oscillates at the bottom of its potential and
gradually transfers its energy to radiation. At the same time, the amplitude of
the fields coupled to the inflaton may undergo parametric resonance and, as a
consequence, explosive particle production can take place. A priori, these
phenomena could lead to an amplification of the super-Hubble scale curvature
perturbations which, in turn, would modify the standard inflationary
predictions. However, remarkably, it has been shown that, although the
Mukhanov-Sasaki variable does undergo narrow parametric instability during
preheating, the amplitude of the corresponding super-Hubble curvature
perturbations remain constant. Therefore, in single field models, metric
preheating does not affect the power spectrum of the large scale perturbations.
In this article, we investigate the corresponding effect on the scalar
bi-spectrum. Using the Maldacena's formalism, we analytically show that, for
modes of cosmological interest, the contributions to the scalar bi-spectrum as
the curvature perturbations evolve on super-Hubble scales during preheating is
completely negligible. Specifically, we illustrate that, certain terms in the
third order action governing the curvature perturbations which may naively be
expected to contribute significantly are exactly canceled by other
contributions to the bi-spectrum. We corroborate selected analytical results by
numerical investigations. We conclude with a brief discussion of the results we
have obtained.Comment: v1: 15 pages, 4 figures; v2: 15 pages, 4 figures, discussion and
references added, to appear in Phys. Rev.
Second Order General Slow-Roll Power Spectrum
Recent combined results from the Wilkinson Microwave Anisotropy Probe (WMAP)
and Sloan Digital Sky Survey (SDSS) provide a remarkable set of data which
requires more accurate and general investigation. Here we derive formulae for
the power spectrum P(k) of the density perturbations produced during inflation
in the general slow-roll approximation with second order corrections. Also,
using the result, we derive the power spectrum in the standard slow-roll
picture with previously unknown third order corrections.Comment: 11 pages, 1 figure ; A typo in Eq. (38) is fixed ; References
expanded and a note adde
Quantum Gravity and Inflation
Using the Ashtekar-Sen variables of loop quantum gravity, a new class of
exact solutions to the equations of quantum cosmology is found for gravity
coupled to a scalar field, that corresponds to inflating universes. The scalar
field, which has an arbitrary potential, is treated as a time variable,
reducing the hamiltonian constraint to a time-dependent Schroedinger equation.
When reduced to the homogeneous and isotropic case, this is solved exactly by a
set of solutions that extend the Kodama state, taking into account the time
dependence of the vacuum energy. Each quantum state corresponds to a classical
solution of the Hamiltonian-Jacobi equation. The study of the latter shows
evidence for an attractor, suggesting a universality in the phenomena of
inflation. Finally, wavepackets can be constructed by superposing solutions
with different ratios of kinetic to potential scalar field energy, resolving,
at least in this case, the issue of normalizability of the Kodama state.Comment: 18 Pages, 2 Figures; major corrections to equations but prior results
still hold, updated reference
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