234 research outputs found
Phenomenological analysis of quantum collapse as source of the seeds of cosmic structure
The standard inflationary version of the origin of the cosmic structure as
the result of the quantum fluctuations during the early universe is less than
fully satisfactory as has been argued in [A. Perez, H. Sahlmann, and D.
Sudarsky, Class. Quantum Grav., 23, 2317, (2006)]. A proposal is made there of
a way to address the shortcomings by invoking a process similar to the collapse
of the quantum mechanical wave function of the various modes of the inflaton
field. This in turn was inspired on the ideas of R. Penrose about the role that
quantum gravity might play in bringing about such breakdown of the standard
unitary evolution of quantum mechanics. In this paper we study in some detail
the two schemes of collapse considered in the original work together with an
alternative scheme, which can be considered as "more natural" than the former
two. The new scheme, assumes that the collapse follows the correlations
indicated in the Wigner functional of the initial state. We end with
considerations regarding the degree to which the various schemes can be
expected to produce a spectrum that resembles the observed one.Comment: 18 pages, 9 figure
Continuous distribution of frequencies and deformed dispersion relations
The possibilities that, in the realm of the detection of the so--called
deformed dispersion relation, a light source with a continuous distribution of
frequencies offers is discussed. It will be proved that the presence of finite
coherence length entails the emergence of a new term in the interference
pattern. This is a novel trait, which renders a new possibility in the quest
for bounds associated with these deformed dispersion relations.Comment: Accepted in Classical and Quantum Gravit
Exoplanet albedo spectra and colors as a function of planet phase, separation, and metallicity
First generation optical coronagraphic telescopes will obtain images of cool
gas and ice giant exoplanets around nearby stars. The albedo spectra of
exoplanets at planet-star separations larger than about 1 AU are dominated by
reflected light to beyond 1 {\mu}m and are punctuated by molecular absorption
features. We consider how exoplanet albedo spectra and colors vary as a
function of planet-star separation, metallicity, mass, and observed phase for
Jupiter and Neptune analogs from 0.35 to 1 {\mu}m. We model Jupiter analogs
with 1x and 3x the solar abundance of heavy elements, and Neptune analogs with
10x and 30x. Our model planets orbit a solar analog parent star at separations
of 0.8 AU, 2 AU, 5 AU, and 10 AU. We use a radiative-convective model to
compute temperature-pressure profiles. The giant exoplanets are cloud-free at
0.8 AU, have H2O clouds at 2 AU, and have both NH3 and H2O clouds at 5 AU and
10 AU. For each model planet we compute moderate resolution spectra as a
function of phase. The presence and structure of clouds strongly influence the
spectra. Since the planet images will be unresolved, their phase may not be
obvious, and multiple observations will be needed to discriminate between the
effects of planet-star separation, metallicity, and phase. We consider the
range of these combined effects on spectra and colors. For example, we find
that the spectral influence of clouds depends more on planet-star separation
and hence temperature than metallicity, and it is easier to discriminate
between cloudy 1x and 3x Jupiters than between 10x and 30x Neptunes. In
addition to alkalis and methane, our Jupiter models show H2O absorption
features near 0.94 {\mu}m. We also predict that giant exoplanets receiving
greater insolation than Jupiter will exhibit higher equator to pole temperature
gradients than are found on Jupiter and thus may have differing atmospheric
dynamics.Comment: 62 pages, 19 figures, 6 tables Accepted for publication in Ap
On the puzzle of Bremsstrahlung as described by coaccelerated observers
We consider anew some puzzling aspects of the equivalence of the quantum
field theoretical description of Bremsstrahlung from the inertial and
accelerated observer's perspectives. More concretely, we focus on the seemingly
paradoxical situation that arises when noting that the radiating source is in
thermal equilibrium with the thermal state of the quantum field in the wedge in
which it is located, and thus its presence does not change there the state of
the field, while it clearly does not affect the state of the field on the
opposite wedge. How then is the state of the quantum field on the future wedge
changed, as it must in order to account for the changed energy momentum tensor
there? This and related issues are carefully discussed.Comment: 29 pages, 1 figure; Revtex, minor changes, PACS correcte
A window to quantum gravity phenomena in the emergence of the seeds of cosmic structure
Inflationary cosmology has, in the last few years,received a strong dose of
support from observations. The fact that the fluctuation spectrum can be
extracted from the inflationary scenario through an analysis that involves
quantum field theory in curved space-time, and that it coincides with the
observational data has lead to a certain complacency in the community, which
prevents the critical analysis of the obscure spots in the derivation. We argue
here briefly, as we have discussed in more detail elsewhere, that there is
something important missing in our understanding of the origin of the seeds of
Cosmic Structure, as is evidenced by the fact that in the standard accounts the
inhomogeneity and anisotropy of our universe seems to emerge from an exactly
homogeneous andisotropic initial state through processes that do not break
those symmetries. This article gives a very brief recount of the problems faced
by the arguments based on established physics. The conclusion is that we need
some new physics to be able to fully address the problem. The article then
exposes one avenue that has been used to address the central issue and
elaborates on the degree to which, the new approach makes different predictions
from the standard analyses. The approach is inspired on Penrose's proposals
that Quantum Gravity might lead to a real, dynamical collapse of the wave
function, a process that we argued has the properties needed to extract us from
the theoretical impasse described above.Comment: 13 pages, 3 figures. To appear in DICE 2008 conference proceeding
Stability analysis of cosmological models through Liapunov's method
We investigate the general asymptotic behaviour of Friedman-Robertson-Walker
(FRW) models with an inflaton field, scalar-tensor FRW cosmological models and
diagonal Bianchi-IX models by means of Liapunov's method. This method provides
information not only about the asymptotic stability of a given equilibrium
point but also about its basin of attraction. This cannot be obtained by the
usual methods found in the literature, such as linear stability analysis or
first order perturbation techniques. Moreover, Liapunov's method is also
applicable to non-autonomous systems. We use this advantadge to investigate the
mechanism of reheating for the inflaton field in FRW models.Comment: Latex file, 8 pages, no figures, accepted for publication in Class. &
Quant. Gra
Stability of self-gravitating magnetic monopoles
The stability of a spherically symmetric self-gravitating magnetic monopole
is examined in the thin wall approximation: modeling the interior false vacuum
as a region of de Sitter space; the exterior as an asymptotically flat region
of the Reissner-Nordstr\"om geometry; and the boundary separating the two as a
charged domain wall. There remains only to determine how the wall gets embedded
in these two geometries. In this approximation, the ratio of the false
vacuum to surface energy densities is a measure of the symmetry breaking scale
. Solutions are characterized by this ratio, the charge on the wall ,
and the value of the conserved total energy . We find that for each fixed
and up to some critical value, there exists a unique globally static
solution, with ; any stable radial excitation has bounded
above by , the value assumed in an extremal Reissner-Nordstr\"om geometry
and these are the only solutions with . As is raised above a black
hole forms in the exterior: (i) for low or , the wall is crushed; (ii)
for higher values, it oscillates inside the black hole. If the mass is not too
high these `collapsing' solutions co-exist with an inflating bounce; (iii) for
, or outside the above regimes, there is a unique inflating
solution. In case (i) the course of the bounce lies within a single
asymptotically flat region (AFR) and it resembles closely the bounce exhibited
by a false vacuum bubble (with Q=0). In cases (ii) and (iii) the course of the
bounce spans two consecutive AFRs.Comment: 19 pages, RevTex two cols., 11 eps figs. Submitted to Phys. Rev.
A comparison of spectroscopic methods for detecting starlight scattered by transiting hot Jupiters, with application to Subaru data for HD 209458b and HD 189733b
The measurement of the light scattered from extrasolar planets informs
atmospheric and formation models. With the discovery of many hot Jupiter
planets orbiting nearby stars, this motivates the development of robust methods
of characterisation from follow up observations. In this paper we discuss two
methods for determining the planetary albedo in transiting systems. First, the
most widely used method for measuring the light scattered by hot Jupiters
(Collier Cameron et al.) is investigated for application for typical echelle
spectra of a transiting planet system, showing that detection requires high
signal-to-noise ratio data of bright planets. Secondly a new Fourier analysis
method is also presented, which is model-independent and utilises the benefits
of the reduced number of unknown parameters in transiting systems. This
approach involves solving for the planet and stellar spectra in Fourier space
by least-squares. The sensitivities of the methods are determined via Monte
Carlo simulations for a range of planet-to-star fluxes. We find the Fourier
analysis method to be better suited to the ideal case of typical observations
of a well constrained transiting system than the Collier Cameron et al. method.
We apply the Fourier analysis method for extracting the light scattered by
transiting hot Jupiters from high resolution spectra to echelle spectra of HD
209458 and HD 189733. Unfortunately we are unable to improve on the previous
upper limit of the planet-to-star flux for HD 209458b set by space-based
observations. A 1{\sigma}upper limit on the planet-to-star flux of HD 189733b
is measured in the wavelength range of 558.83-599.56 nm yielding {\epsilon} <
4.5 \times 10-4. Improvement in the measurement of the upper limit of the
planet-to-star flux of this system, with ground-based capabilities, requires
data with a higher signal-to-noise ratio, and increased stability of the
telescope.Comment: 15 pages, 8 figures, 2 tables. Monthly Notices of the Royal
Astronomical Society, in press. Accepted 2011 March 17. Received 2011 March
17; in original form 2010 June 2
Modeling Multi-Wavelength Stellar Astrometry. III. Determination of the Absolute Masses of Exoplanets and Their Host Stars
Astrometric measurements of stellar systems are becoming significantly more
precise and common, with many ground and space-based instruments and missions
approaching 1 microarcsecond precision. We examine the multi-wavelength
astrometric orbits of exoplanetary systems via both analytical formulae and
numerical modeling. Exoplanets have a combination of reflected and thermally
emitted light that cause the photocenter of the system to shift increasingly
farther away from the host star with increasing wavelength. We find that, if
observed at long enough wavelengths, the planet can dominate the astrometric
motion of the system, and thus it is possible to directly measure the orbits of
both the planet and star, and thus directly determine the physical masses of
the star and planet, using multi-wavelength astrometry. In general, this
technique works best for, though is certainly not limited to, systems that have
large, high-mass stars and large, low-mass planets, which is a unique parameter
space not covered by other exoplanet characterization techniques. Exoplanets
that happen to transit their host star present unique cases where the physical
radii of the planet and star can be directly determined via astrometry alone.
Planetary albedos and day-night contrast ratios may also be probed via this
technique due to the unique signature they impart on the observed astrometric
orbits. We develop a tool to examine the prospects for near-term detection of
this effect, and give examples of some exoplanets that appear to be good
targets for detection in the K to N infrared observing bands, if the required
precision can be achieved.Comment: Accepted to the Astrophysical Journal. 9 pages, 6 figures, 1 table in
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