4,915 research outputs found
On the Time-Dependent Analysis of Gamow Decay
Gamow's explanation of the exponential decay law uses complex "eigenvalues"
and exponentially growing "eigenfunctions". This raises the question, how
Gamow's description fits into the quantum mechanical description of nature,
which is based on real eigenvalues and square integrable wave functions.
Observing that the time evolution of any wave function is given by its
expansion in generalized eigenfunctions, we shall answer this question in the
most straightforward manner, which at the same time is accessible to graduate
students and specialists. Moreover the presentation can well be used in physics
lectures to students.Comment: 10 pages, 4 figures; heuristic argument simplified, different example
discussed, calculation of decay rate adde
Correspondence between Electro-Magnetic Field and other Dark Energies in Non-linear Electrodynamics
In this work, we have considered the flat FRW model of the universe filled
with electro-magnetic field. First, the Maxwell's electro-magnetic field in
linear form has been discussed and after that the modified Lagrangian in
non-linear form for accelerated universe has been considered. The corresponding
energy density and pressure for non-linear electro-magnetic field have been
calculated. We have found the condition such that the electro-magnetic field
generates dark energy. The correspondence between the electro-magnetic field
and the other dark energy candidates namely tachyonic field, DBI-essence,
Chaplygin gas, hessence dark energy, k-essenece and dilaton dark energy have
been investigated. We have also reconstructed the potential functions and the
scalar fields in this scenario.Comment: 11 pages, 7 figure
Inverting the Sachs-Wolfe Formula: an Inverse Problem Arising in Early-Universe Cosmology
The (ordinary) Sachs-Wolfe effect relates primordial matter perturbations to
the temperature variations in the cosmic microwave background
radiation; can be observed in all directions around us. A standard
but idealised model of this effect leads to an infinite set of moment-like
equations: the integral of with respect to k ()
is equal to a given constant, , for . Here, P is the
power spectrum of the primordial density variations, is a spherical
Bessel function and y is a positive constant. It is shown how to solve these
equations exactly for ~. The same solution can be recovered, in
principle, if the first ~m equations are discarded. Comparisons with classical
moment problems (where is replaced by ) are made.Comment: In Press Inverse Problems 1999, 15 pages, 0 figures, Late
On the quantum origin of the seeds of cosmic structure
The current understanding of the quantum origin of cosmic structure is
discussed critically. We point out that in the existing treatments a transition
from a symmetric quantum state to an (essentially classical) non-symmetric
state is implicitly assumed, but not specified or analyzed in any detail. In
facing the issue we are led to conclude that new physics is required to explain
the apparent predictive power of the usual schemes. Furthermore we show that
the novel way of looking at the relevant issues opens new windows from where
relevant information might be extracted regarding cosmological issues and
perhaps even clues about aspects of quantum gravity.Comment: replacement with final version to appear in Classical and Quantum
Gravit
Testing non-standard cosmological models with supernovae
In this work we study the magnitude-redshift relation of a non-standard
cosmological model. The model under consideration was firstly investigated
within a special case of metric-affine gravity (MAG) and was recently recovered
via different approaches by two other groups. Apart from the usual cosmological
parameters for pressure-less matter , cosmological
constant/dark energy , and radiation a new
density parameter emerges. The field equations of the model
reduce to a system which is effectively given by the usual Friedmann equations
of general relativity, supplied by a correction to the energy density and
pressure in form of , which is related to the non-Riemannian
structure of the underlying spacetime. We search for the best-fit parameters by
using recent SN Ia data sets and constrain the possible contribution of a new
dark-energy like component at low redshifts, thereby we put an upper limit on
the presence of non-Riemannian quantities in the late stages of the universe.
In addition the impact of placing the data in redshift bins of variable size is
studied. The numerical results of this work also apply to several anisotropic
cosmological models which, on the level of the field equations, exhibit a
similar scaling behavior of the density parameters like our non-Riemannian
model.Comment: 21 pages, 10 figures, uses IOP preprint style, submitted to Class.
Quantum Gra
Non-thermal right-handed sneutrino dark matter and the Omega_DM/Omega_b problem
We argue that the superpartner of the Dirac right-handed neutrino is a prime
candidate for dark matter created from a 'mattergenesis' mechanism. We show
that due to the smallness of the Yukawa couplings, a right-handed sneutrino
density created in the early Universe would not be erased by annihilations,
which remain out of thermal equilibrium. It would also not be drowned by a
later, additional production of right-handed sneutrinos, as the relic density
of the non-thermal right-handed sneutrinos is found to be generally negligible
compared to the observed dark matter density. Mild constraints on sneutrino
masses and trilinear SUSY-breaking couplings are obtained. Possible
mattergenesis scenarios are also mentionedComment: 11 pages, 2 figures, 2 typos added, 1 reference added, minor
corrections in section
Axino dark matter from thermal production
The axino is a promising candidate for dark matter in the Universe. It is
electrically and color neutral, very weakly interacting, and could be - as
assumed in this study - the lightest supersymmetric particle, which is stable
for unbroken R-parity. In supersymmetric extensions of the standard model, in
which the strong CP problem is solved via the Peccei-Quinn mechanism, the axino
arises naturally as the fermionic superpartner of the axion. We compute the
thermal production rate of axinos in supersymmetric QCD. Using hard thermal
loop resummation, we obtain a finite result in a gauge-invariant way, which
takes into account Debye screening in the hot quark-gluon-squark-gluino plasma.
The relic axino abundance from thermal scatterings after inflation is
evaluated. We find that thermally produced axinos could provide the dominant
part of cold dark matter, for example, for an axino mass of 100 keV and a
reheating temperature of 10^6 GeV.Comment: 33 pages, 7 figures, 1 table, erratum adde
Lysophosphatidic acid receptor 1 inhibition:a potential treatment target for pulmonary fibrosis
Lysophosphatidic acid (LPA)-mediated activation of LPA receptor 1 (LPAR1) contributes to the pathophysiology of fibrotic diseases such as idiopathic pulmonary fibrosis (IPF) and systemic sclerosis (SSc). These diseases are associated with high morbidity and mortality despite current treatment options. The LPA-producing enzyme autotaxin (ATX) and LPAR1 activation contribute to inflammation and mechanisms underlying fibrosis in preclinical fibrotic models. Additionally, elevated levels of LPA have been detected in bronchoalveolar lavage fluid from patients with IPF and in serum from patients with SSc. Thus, ATX and LPAR1 have gained considerable interest as pharmaceutical targets to combat fibrotic disease and inhibitors of these targets have been investigated in clinical trials for IPF and SSc. The goals of this review are to summarise the current literature on ATX and LPAR1 signalling in pulmonary fibrosis and to help differentiate the novel inhibitors in development. The mechanisms of action of ATX and LPAR1 inhibitors are described and preclinical studies and clinical trials of these agents are outlined. Because of their contribution to numerous physiologic events underlying fibrotic disease, ATX and LPAR1 inhibition presents a promising therapeutic strategy for IPF, SSc and other fibrotic diseases that may fulfil unmet needs of the current standard of care.</p
Multiple-field inflation and the CMB
In this paper, we investigate some consequences of multiple-field inflation
for the cosmic microwave background radiation (CMB). We derive expressions for
the amplitudes, the spectral indices and the derivatives of the indices of the
CMB power spectrum in the context of a very general multiple-field theory of
slow-roll inflation, where the field metric can be non-trivial. Both scalar
(adiabatic, isocurvature and mixing) and tensor perturbations are treated and
the differences with single-field inflation are discussed. From these
expressions, several relations are derived that can be used to determine the
importance of multiple-field effects observationally from the CMB. We also
study the evolution of the total entropy perturbation during radiation and
matter domination and the influence of this on the isocurvature spectral
quantities.Comment: 24 pages. References added, some very minor textual changes, matches
version to be published in CQ
Lysophosphatidic acid receptor 1 inhibition:a potential treatment target for pulmonary fibrosis
Lysophosphatidic acid (LPA)-mediated activation of LPA receptor 1 (LPAR1) contributes to the pathophysiology of fibrotic diseases such as idiopathic pulmonary fibrosis (IPF) and systemic sclerosis (SSc). These diseases are associated with high morbidity and mortality despite current treatment options. The LPA-producing enzyme autotaxin (ATX) and LPAR1 activation contribute to inflammation and mechanisms underlying fibrosis in preclinical fibrotic models. Additionally, elevated levels of LPA have been detected in bronchoalveolar lavage fluid from patients with IPF and in serum from patients with SSc. Thus, ATX and LPAR1 have gained considerable interest as pharmaceutical targets to combat fibrotic disease and inhibitors of these targets have been investigated in clinical trials for IPF and SSc. The goals of this review are to summarise the current literature on ATX and LPAR1 signalling in pulmonary fibrosis and to help differentiate the novel inhibitors in development. The mechanisms of action of ATX and LPAR1 inhibitors are described and preclinical studies and clinical trials of these agents are outlined. Because of their contribution to numerous physiologic events underlying fibrotic disease, ATX and LPAR1 inhibition presents a promising therapeutic strategy for IPF, SSc and other fibrotic diseases that may fulfil unmet needs of the current standard of care.</p
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