181 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
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
Shortcomings in the Understanding of Why Cosmological Perturbations Look Classical
There is a persistent state of confusion regarding the account of the quantum
origin of the seeds of cosmological structure during inflation. In fact, a
recent article (C. Kiefer & D. Polarski, ArXiv: 0810.0087 [astro-ph]) addresses
the question "Why do the Cosmological Perturbations look Classical?" and offers
an answer based on unitary quantum mechanics (i.e., without reference to the
projection postulate) relying on the decoherence type of analysis. The argument
is, thus, implicitly assuming that decoherence offers a satisfactory solution
to the measurement problem in quantum mechanics. We will review here, why do
we, together with various other researchers in the field, consider that this is
not the case, in general, and particularly not at all in the situation at hand.
In fact, as has been previously discussed (A. Perez, H. Sahlmann, and D.
Sudarsky, CQG 23, 2317, (2006);[arXiv: gr-qc/0508100]), we will argue that the
cosmological situation is one where the measurement problem of quantum
mechanics appears in a particular exacerbated form, and that, it is this, even
sharper conondrum, the one that should be addressed when dealing with the
inflationary account of the origin of the seeds of cosmic structure in the
early universe.Comment: New version: In press in International Journal of Modern Physics
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
The collapse of the wave function in the joint metric-matter quantization for inflation
It has been argued that the standard inflationary scenario suffers from a
serious deficiency as a model for the origin of the seeds of cosmic structure:
it can not truly account for the transition from an early homogeneous and
isotropic stage to another one lacking such symmetries. The issue has often
been thought as a standard instance of the "quantum measurement problem", but
as has been recently argued by some of us the situation reaches a critical
level in the cosmological context of interest here. This has lead to a proposal
in which the standard paradigm is supplemented by a hypothesis concerning the
self-induced dynamical collapse of the wave function, as representing the
physical mechanism through which such change of symmetry is brought forth. This
proposal was formulated within the context of semiclassical gravity. Here we
investigate an alternative realization of such idea implemented directly within
the standard analysis in terms of a quantum field jointly describing the
inflaton and metric perturbations, the so called Mukhanov-Sasaki variable. We
show that even though the prescription is quite different, the theoretical
predictions include some deviations from the standard ones, which are indeed
very similar to those found in the early studies. We briefly discuss the
differences between the two at both, the conceptual and practical levels.Comment: 31 pages, 6 figures. Replaced to match the published versio
An L Band Spectrum of the Coldest Brown Dwarf
The coldest brown dwarf, WISE 0855, is the closest known planetary-mass,
free-floating object and has a temperature nearly as cold as the solar system
gas giants. Like Jupiter, it is predicted to have an atmosphere rich in
methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint
at near-infrared wavelengths and emits almost all its energy in the
mid-infrared. Skemer et al. 2016 presented a spectrum of WISE 0855 from 4.5-5.1
micron (M band), revealing water vapor features. Here, we present a spectrum of
WISE 0855 in L band, from 3.4-4.14 micron. We present a set of atmosphere
models that include a range of compositions (metallicities and C/O ratios) and
water ice clouds. Methane absorption is clearly present in the spectrum. The
mid-infrared color can be better matched with a methane abundance that is
depleted relative to solar abundance. We find that there is evidence for water
ice clouds in the M band spectrum, and we find a lack of phosphine spectral
features in both the L and M band spectra. We suggest that a deep continuum
opacity source may be obscuring the near-infrared flux, possibly a deep
phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE
0855 provide critical constraints for cold planetary atmospheres, bridging the
temperature range between the long-studied solar system planets and accessible
exoplanets. JWST will soon revolutionize our understanding of cold brown dwarfs
with high-precision spectroscopy across the infrared, allowing us to study
their compositions and cloud properties, and to infer their atmospheric
dynamics and formation processes.Comment: 19 pages, 21 figures. Accepted for publication in Ap
Quantum Gravity Phenomenology without Lorentz Invariance Violation: a detailed proposal
We describe a scheme for the exploration of quantum gravity phenomenology
focussing on effects that could be thought as arising from a fundamental
granularity of space-time. In contrast with the simplest assumptions, such
granularity is assumed to respect Lorentz Invariance but is otherwise left
unspecified. The proposal is fully observer covariant, it involves non-trivial
couplings of curvature to matter fields and leads to a well defined
phenomenology. We present the effective Hamiltonian which could be used to
analyze concrete experimental situations, some of which are briefly described,
and we shortly discuss the degree to which the present proposal is in line with
the fundamental ideas behind the equivalence principle.Comment: LaTeX, 24 pages. To be published in Classical and Quantum Gravit
Comparison of QG-Induced Dispersion with Standard Physics Effects
One of the predictions of quantum gravity phenomenology is that, in
situations where Planck-scale physics and the notion of a quantum spacetime are
relevant, field propagation will be described by a modified set of laws.
Descriptions of the underlying mechanism differ from model to model, but a
general feature is that electromagnetic waves will have non-trivial dispersion
relations. A physical phenomenon that offers the possibility of experimentally
testing these ideas in the foreseeable future is the propagation of high-energy
gamma rays from GRB's at cosmological distances. With the observation of
non-standard dispersion relations within experimental reach, it is thus
important to find out whether there are competing effects that could either
mask or be mistaken for this one. In this letter, we consider possible effects
from standard physics, due to electromagnetic interactions, classical as well
as quantum, and coupling to classical geometry. Our results indicate that, for
currently observed gamma-ray energies and estimates of cosmological parameter
values, those effects are much smaller than the quantum gravity one if the
latter is first-order in the energy; some corrections are comparable in
magnitude with the second-order quantum gravity ones, but they have a very
different energy dependence.Comment: 8 pages; Version to be published in CQG as a letter; Includes some
new comments and references, but no changes in the result
- …