2,555 research outputs found
5D gravitational waves from complexified black rings
In this paper we construct and briefly study the 5D time-dependent solutions
of general relativity obtained via double analytic continuation of the black
hole (Myers-Perry) and of the black ring solutions with a double
(Pomeransky-Senkov) and a single rotation (Emparan-Reall). The new solutions
take the form of a generalized Einstein-Rosen cosmology representing
gravitational waves propagating in a closed universe. In this context the
rotation parameters of the rings can be interpreted as the extra wave
polarizations, while it is interesting to state that the waves obtained from
Myers-Perry Black holes exhibit an extra boost-rotational symmetry in higher
dimensions which signals their better behavior at null infinity. The analogue
to the C-energy is analyzed.Comment: 18 pages, 4 figures. References added, introduction and conclusions
are amended, some issues related to singularity structure and symmetries are
discussed. Matches the print version to appear in JHE
Constants of motion for vacuum general relativity
The 3+1 Hamiltonian Einstein equations, reduced by imposing two commuting
spacelike Killing vector fields, may be written as the equations of the
principal chiral model with certain `source' terms. Using this
formulation, we give a procedure for generating an infinite number of non-local
constants of motion for this sector of the Einstein equations. The constants of
motion arise as explicit functionals on the phase space of Einstein gravity,
and are labelled by sl(2,R) indices.Comment: 10 pages, latex, version to appear in Phys. Rev. D
Plane waves in quantum gravity: breakdown of the classical spacetime
Starting with the Hamiltonian formulation for spacetimes with two commuting
spacelike Killing vectors, we construct a midisuperspace model for linearly
polarized plane waves in vacuum gravity. This model has no constraints and its
degrees of freedom can be interpreted as an infinite and continuous set of
annihilation and creation like variables. We also consider a simplified version
of the model, in which the number of modes is restricted to a discrete set. In
both cases, the quantization is achieved by introducing a Fock representation.
We find regularized operators to represent the metric and discuss whether the
coherent states of the quantum theory are peaked around classical spacetimes.
It is shown that, although the expectation value of the metric on Killing
orbits coincides with a classical solution, its relative fluctuations become
significant when one approaches a region where null geodesics are focused. In
that region, the spacetimes described by coherent states fail to admit an
approximate classical description. This result applies as well to the vacuum of
the theory.Comment: 11 pages, no figures, version accepted for publication in Phys. Rev.
Time and "angular" dependent backgrounds from stationary axisymmetric solutions
Backgrounds depending on time and on "angular" variable, namely polarized and
unpolarized Gowdy models, are generated as the sector inside
the horizons of the manifold corresponding to axisymmetric solutions. As is
known, an analytical continuation of ordinary -branes, -branes allows
one to find -brane solutions. Simple models have been constructed by means
of analytic continuation of the Schwarzchild and the Kerr metrics. The
possibility of studying the -Gowdy models obtained here is outlined with an
eye toward seeing if they could represent some kind of generalized -branes
depending not only on time but also on an ``angular'' variable.Comment: 24 pages, 5 figures, corrected typos, references adde
The Geroch group in the Ashtekar formulation
We study the Geroch group in the framework of the Ashtekar formulation. In
the case of the one-Killing-vector reduction, it turns out that the third
column of the Ashtekar connection is essentially the gradient of the Ernst
potential, which implies that the both quantities are based on the ``same''
complexification. In the two-Killing-vector reduction, we demonstrate Ehlers'
and Matzner-Misner's SL(2,R) symmetries, respectively, by constructing two sets
of canonical variables that realize either of the symmetries canonically, in
terms of the Ashtekar variables. The conserved charges associated with these
symmetries are explicitly obtained. We show that the gl(2,R) loop algebra
constructed previously in the loop representation is not the Lie algebra of the
Geroch group itself. We also point out that the recent argument on the
equivalence to a chiral model is based on a gauge-choice which cannot be
achieved generically.Comment: 40 pages, revte
Mirror Images of String Cosmologies
A discrete symmetry of the four-dimensional string effective action is
employed to derive spatially homogeneous and inhomogeneous string cosmologies
from vacuum solutions of general relativity that admit two commuting spacelike
Killing vectors. In particular, a tilted Bianchi type V cosmology is generated
from a vacuum type VI_h solution and a plane wave solution with a bounded and
oscillating dilaton field is found from a type model. Further
applications are briefly discussed.Comment: 10 pages plain late
Inhomogeneous Einstein-Rosen String Cosmology
Families of anisotropic and inhomogeneous string cosmologies containing
non-trivial dilaton and axion fields are derived by applying the global
symmetries of the string effective action to a generalized Einstein-Rosen
metric. The models exhibit a two-dimensional group of Abelian isometries. In
particular, two classes of exact solutions are found that represent
inhomogeneous generalizations of the Bianchi type VI_h cosmology. The
asymptotic behaviour of the solutions is investigated and further applications
are briefly discussed.Comment: Minor extension of concluding section; 18 pages, to appear in
Phys.Rev.
Ozone-derived Oxysterols Affect Liver X Receptor (LXR) Signaling: A POTENTIAL ROLE FOR LIPID-PROTEIN ADDUCTS
When inhaled, ozone (O3) interacts with cholesterols of airway epithelial cell membranes or the lung-lining fluid, generating chemically reactive oxysterols. The mechanism by which O3-derived oxysterols affect molecular function is unknown. Our data show that in vitro exposure of human bronchial epithelial cells to O3 results in the formation of oxysterols, epoxycholesterol-α and -β and secosterol A and B (Seco A and Seco B), in cell lysates and apical washes. Similarly, bronchoalveolar lavage fluid obtained from human volunteers exposed to O3 contained elevated levels of these oxysterol species. As expected, O3-derived oxysterols have a pro-inflammatory effect and increase NF-κB activity. Interestingly, expression of the cholesterol efflux pump ATP-binding cassette transporter 1 (ABCA1), which is regulated by activation of the liver X receptor (LXR), was suppressed in epithelial cells exposed to O3. Additionally, exposure of LXR knock-out mice to O3 enhanced pro-inflammatory cytokine production in the lung, suggesting LXR inhibits O3-induced inflammation. Using alkynyl surrogates of O3-derived oxysterols, our data demonstrate adduction of LXR with Seco A. Similarly, supplementation of epithelial cells with alkynyl-tagged cholesterol followed by O3 exposure causes observable lipid-LXR adduct formation. Experiments using Seco A and the LXR agonist T0901317 (T09) showed reduced expression of ABCA1 as compared with stimulation with T0901317 alone, indicating that Seco A-LXR protein adduct formation inhibits LXR activation by traditional agonists. Overall, these data demonstrate that O3-derived oxysterols have pro-inflammatory functions and form lipid-protein adducts with LXR, thus leading to suppressed cholesterol regulatory gene expression and providing a biochemical mechanism mediating O3-derived formation of oxidized lipids in the airways and subsequent adverse health effects
Quantization of pure gravitational plane waves
Pure gravitational plane waves are considered as a special case of spacetimes
with two commuting spacelike Killing vector fields. Starting with a
midisuperspace that describes this kind of spacetimes, we introduce
gauge-fixing and symmetry conditions that remove all non-physical degrees of
freedom and ensure that the classical solutions are plane waves. In this way,
we arrive at a reduced model with no constraints and whose only degrees of
freedom are given by two fields. In a suitable coordinate system, the reduced
Hamiltonian that generates the time evolution of this model turns out to
vanish, so that all relevant information is contained in the symplectic
structure. We calculate this symplectic structure and particularize our
discussion to the case of linearly polarized plane waves. The reduced phase
space can then be described by an infinite set of annihilation and creation
like variables. We finally quantize the linearly polarized model by introducing
a Fock representation for these variables.Comment: 11 pages, Revtex, no figure
Einstein's equations and the chiral model
The vacuum Einstein equations for spacetimes with two commuting spacelike
Killing field symmetries are studied using the Ashtekar variables. The case of
compact spacelike hypersurfaces which are three-tori is considered, and the
determinant of the Killing two-torus metric is chosen as the time gauge. The
Hamiltonian evolution equations in this gauge may be rewritten as those of a
modified SL(2) principal chiral model with a time dependent `coupling
constant', or equivalently, with time dependent SL(2) structure constants. The
evolution equations have a generalized zero-curvature formulation. Using this
form, the explicit time dependence of an infinite number of
spatial-diffeomorphism invariant phase space functionals is extracted, and it
is shown that these are observables in the sense that they Poisson commute with
the reduced Hamiltonian. An infinite set of observables that have SL(2) indices
are also found. This determination of the explicit time dependence of an
infinite set of spatial-diffeomorphism invariant observables amounts to the
solutions of the Hamiltonian Einstein equations for these observables.Comment: 22 pages, RevTeX, to appear in Phys. Rev.
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