158 research outputs found
Interactions of cosmological gravitational waves and magnetic fields
The energy momentum tensor of a magnetic field always contains a spin-2
component in its anisotropic stress and therefore generates gravitational
waves. It has been argued in the literature (Caprini & Durrer \cite{CD}) that
this gravitational wave production can be very strong and that back-reaction
cannot be neglected. On the other hand, a gravitational wave background does
affect the evolution of magnetic fields. It has also been argued (Tsagas et al.
\cite{Tsagas:2001ak},\cite{Tsagas:2005ki}) that this can lead to very strong
amplification of a primordial magnetic field. In this paper we revisit these
claims and study back reaction to second order.Comment: Added references, accepted for publication in PR
Testing Superstring Theories with Gravitational Waves
We provide a simple transfer function that determines the effect of an early
matter dominated era on the gravitational wave background and show that a large
class of compactifications of superstring theory might be tested by
observations of the gravitational wave background from inflation. For large
enough reheating temperatures > 10^9 \GeV the test applies to all models
containing at least one scalar with mass < 10^{12}\GeV that acquires a large
initial oscillation amplitude after inflation and has only gravitational
interaction strength, i.e., a field with the typical properties of a modulus.Comment: 5 pages 2 figures, v2: changes in presentation, refs revised, matches
version in print in PR
Gravitational waves from stochastic relativistic sources: primordial turbulence and magnetic fields
The power spectrum of a homogeneous and isotropic stochastic variable,
characterized by a finite correlation length, does in general not vanish on
scales larger than the correlation scale. If the variable is a divergence free
vector field, we demonstrate that its power spectrum is blue on large scales.
Accounting for this fact, we compute the gravitational waves induced by an
incompressible turbulent fluid and by a causal magnetic field present in the
early universe. The gravitational wave power spectra show common features: they
are both blue on large scales, and peak at the correlation scale. However, the
magnetic field can be treated as a coherent source and it is active for a long
time. This results in a very effective conversion of magnetic energy in
gravitational wave energy at horizon crossing. Turbulence instead acts as a
source for gravitational waves over a time interval much shorter than a Hubble
time, and the conversion into gravitational wave energy is much less effective.
We also derive a strong constraint on the amplitude of a primordial magnetic
field when the correlation length is much smaller than the horizon.Comment: Replaced with revised version accepted for publication in Phys Rev
General Properties of the Gravitational Wave Spectrum from Phase Transitions
In this paper we discuss some general aspects of the gravitational wave
background arising from post-inflationary short-lasting cosmological events
such as phase transitions. We concentrate on the physics which determines the
shape and the peak frequency of the gravitational wave spectrum. We then apply
our general findings to the case of bubble collisions during a first order
phase transition and compare different results in the recent literature.Comment: 11 pages, 8 figures; v2: minor clarifications, published versio
Direct Determinations of the Redshift Behavior of the Pressure, Energy Density, and Equation of State of the Dark Energy and the Acceleration of the Universe
One of the goals of current cosmological studies is the determination of the
expansion and acceleration rates of the universe as functions of redshift, and
the determination of the properties of the dark energy that can explain these
observations. Here the expansion and acceleration rates are determined directly
from the data, without the need for the specification of a theory of gravity,
and without adopting an a priori parameterization of the form or redshift
evolution of the dark energy. We use the latest set of distances to SN standard
candles from Riess et al. (2004), supplemented by data on radio galaxy standard
ruler sizes, as described by Daly and Djorgovski (2003, 2004). We find that the
universe transitions from acceleration to deceleration at a redshift of about
0.4. The standard "concordance model" provides a reasonably good fit to the
dimensionless expansion rate as a function of redshift, though it fits the
dimensionless acceleration rate as a function of redshift less well. The
expansion and acceleration rates are then combined with a theory of gravity to
determine the pressure, energy density, and equation of state of the dark
energy as functions of redshift. Adopting General Relativity as the correct
theory of gravity, the redshift trends for the pressure, energy density, and
equation of state of the dark energy out to redshifts of about one are
determined, and are found to be generally consistent with the concordance
model.Comment: 8 pages, 5 figures. Invited presentation at Coral Gables 200
Quantum Fermion Hair
It is shown that the Dirac operator in the background of a magnetic
%Reissner-Nordstr\"om black hole and a Euclidean vortex possesses normalizable
zero modes in theories containing superconducting cosmic strings. One
consequence of these zero modes is the presence of a fermion condensate around
magnetically charged black holes which violates global quantum numbers.Comment: 16pp (harvmac (l)) and 2 figs.(not included
The Oscillating Universe: an Alternative to Inflation
The aim of this paper is to show, that the 'oscillating universe' is a viable
alternative to inflation. We remind that this model provides a natural solution
to the flatness or entropy and to the horizon problem of standard cosmology. We
study the evolution of density perturbations and determine the power spectrum
in a closed universe. The results lead to constraints of how a previous cycle
might have looked like. We argue that most of the radiation entropy of the
present universe may have originated from gravitational entropy produced in a
previous cycle.
We show that measurements of the power spectrum on very large scales could in
principle decide whether our universe is closed, flat or open.Comment: revised version for publication in Classical and Quantum Gravity, 23
pages, uuencoded compressed tarred Latex file with 7 eps figures included,
fig.8 upon reques
Dynamical Instabilities of the Randall-Sundrum Model
We derive dynamical equations to describe a single 3-brane containing fluid
matter and a scalar field coupling to the dilaton and the gravitational field
in a five dimensional bulk. First, we show that a scalar field or an arbitrary
fluid on the brane cannot evolve to cancel the cosmological constant in the
bulk. Then we show that the Randall-Sundrum model is unstable under small
deviations from the fine-tuning between the brane tension and the bulk
cosmological constant and even under homogeneous gravitational perturbations.
Implications for brane world cosmologies are discussed.Comment: 12 pages, 2 figure
Non-Perturbative Gravity and the Spin of the Lattice Graviton
The lattice formulation of quantum gravity provides a natural framework in
which non-perturbative properties of the ground state can be studied in detail.
In this paper we investigate how the lattice results relate to the continuum
semiclassical expansion about smooth manifolds. As an example we give an
explicit form for the lattice ground state wave functional for semiclassical
geometries. We then do a detailed comparison between the more recent
predictions from the lattice regularized theory, and results obtained in the
continuum for the non-trivial ultraviolet fixed point of quantum gravity found
using weak field and non-perturbative methods. In particular we focus on the
derivative of the beta function at the fixed point and the related universal
critical exponent for gravitation. Based on recently available lattice
and continuum results we assess the evidence for the presence of a massless
spin two particle in the continuum limit of the strongly coupled lattice
theory. Finally we compare the lattice prediction for the vacuum-polarization
induced weak scale dependence of the gravitational coupling with recent
calculations in the continuum, finding similar effects.Comment: 46 pages, one figur
A phase II dose‑escalation trial of perioperative desmopressin (1‑desamino‑8‑D‑arginine vasopressin) in breast cancer patients
Desmopressin (dDAVP) is a well-known peptide analog of the antidiuretic hormone vasopressin, used to prevent excessive bleeding during surgical procedures. dDAVP increases hemostatic mediators, such as the von Willebrand factor (vWF), recently considered a key element in resistance to metastasis. Studies in mouse models and veterinary trials in dogs with locally-advanced mammary tumors demonstrated that high doses of perioperative dDAVP inhibited lymph node and early blood-borne metastasis and significantly prolonged survival. We conducted a phase II dose-escalation trial in patients with breast cancer, administering a lyophilized formulation of dDAVP by intravenous infusion in saline, 30–60 min before and 24 h after surgical resection. Primary endpoints were safety and tolerability, as well as selection of the best dose for cancer surgery. Secondary endpoints included surgical bleeding, plasma levels of vWF, and circulating tumor cells (CTCs) as measured by quantitative PCR of cytokeratin-19 transcripts. Only 2 of a total of 20 patients experienced reversible adverse events, including hyponatremia (grade 4) and hypersensitivity reaction (grade 2). Reactions were adequately managed by slowing the infusion rate. A reduced intraoperative bleeding was noted with increasing doses of dDAVP. Treatment was associated with higher vWF plasma levels and a postoperative drop in CTC counts. At the highest dose level evaluated (2 μg/kg) dDAVP appeared safe when administered in two slow infusions of 1 μg/kg, before and after surgery. Clinical trials to establish the effectiveness of adjunctive perioperative dDAVP therapy are warranted. This trial is registered on www.clinicaltrials.gov (NCT01606072).Facultad de Ciencias Médica
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