1,059 research outputs found
Editorial note to "The beginning of the world from the point of view of quantum theory"
This is an editorial note to accompany reprinting as a Golden Oldie in the
Journal of General Relativity and Gravitation of the famous note by Georges
Lemaitre on the quantum birth of the universe, published in Nature in 1931. We
explain why this short (457 words) article can be considered to be the true
"Charter" of the modern Big Bang theory.Comment: This is an editorial comment to accompany reprinting of a classical
paper in the Journal of General Relativity and Gravitation. 16 pages, 2
figure
Velocity dominated singularities in the cheese slice universe
We investigate the properties of spacetimes resulting from matching together
exact solutions using the Darmois matching conditions. In particular we focus
on the asymptotically velocity term dominated property (AVTD). We propose a
criterion that can be used to test if a spacetime constructed from a matching
can be considered AVTD. Using the Cheese Slice universe as an example, we show
that a spacetime constructed from a such a matching can inherit the AVTD
property from the original spacetimes. Furthermore the singularity resulting
from this particular matching is an AVTD singularity.Comment: 11 pages, 3 figures, accepted for publication in the International
Journal of Modern Physics
Can a charged dust ball be sent through the Reissner--Nordstr\"{o}m wormhole?
In a previous paper we formulated a set of necessary conditions for the
spherically symmetric weakly charged dust to avoid Big Bang/Big Crunch, shell
crossing and permanent central singularities. However, we did not discuss the
properties of the energy density, some of which are surprising and seem not to
have been known up to now. A singularity of infinite energy density does exist
-- it is a point singularity situated on the world line of the center of
symmetry. The condition that no mass shell collapses to if it had initially thus turns out to be still insufficient for avoiding a
singularity. Moreover, at the singularity the energy density is
direction-dependent: when we approach the singular
point along a const hypersurface and when we
approach that point along the center of symmetry. The appearance of
negative-energy-density regions turns out to be inevitable. We discuss various
aspects of this property of our configuration. We also show that a permanently
pulsating configuration, with the period of pulsation independent of mass, is
possible only if there exists a permanent central singularity.Comment: 30 pages, 21 figures; several corrections after referee's comments, 4
figures modifie
Cavity polariton optomechanics: Polariton path to fully resonant dispersive coupling in optomechanical resonators
Resonant photoelastic coupling in semiconductor nanostructures opens new
perspectives for strongly enhanced light-sound interaction in optomechanical
resonators. One potential problem, however, is the reduction of the cavity
Q-factor induced by dissipation when the resonance is approached. We show in
this letter that cavity-polariton mediation in the light-matter process
overcomes this limitation allowing for a strongly enhanced photon-phonon
coupling without significant lifetime reduction in the strongly-coupled regime.
Huge optomechanical coupling factors in the PetaHz/nm range are envisaged,
three orders of magnitude larger than the backaction produced by the mechanical
displacement of the cavity mirrors.Comment: 6 pages, 4 figure
Is nonrelativistic gravity possible?
We study nonrelativistic gravity using the Hamiltonian formalism. For the
dynamics of general relativity (relativistic gravity) the formalism is well
known and called the Arnowitt-Deser-Misner (ADM) formalism. We show that if the
lapse function is constrained correctly, then nonrelativistic gravity is
described by a consistent Hamiltonian system. Surprisingly, nonrelativistic
gravity can have solutions identical to relativistic gravity ones. In
particular, (anti-)de Sitter black holes of Einstein gravity and IR limit of
Horava gravity are locally identical.Comment: 4 pages, v2, typos corrected, published in Physical Review
Reflection and Transmission at the Apparent Horizon during Gravitational Collapse
We examine the wave-functionals describing the collapse of a self-gravitating
dust ball in an exact quantization of the gravity-dust system. We show that
ingoing (collapsing) dust shell modes outside the apparent horizon must
necessarily be accompanied by outgoing modes inside the apparent horizon, whose
amplitude is suppressed by the square root of the Boltzmann factor at the
Hawking temperature. Likewise, ingoing modes in the interior must be
accompanied by outgoing modes in the exterior, again with an amplitude
suppressed by the same factor. A suitable superposition of the two solutions is
necessary to conserve the dust probability flux across the apparent horizon,
thus each region contains both ingoing and outgoing dust modes. If one
restricts oneself to considering only the modes outside the apparent horizon
then one should think of the apparent horizon as a partial reflector, the
probability for a shell to reflect being given by the Boltzmann factor at the
Hawking temperature determined by the mass contained within it. However, if one
considers the entire wave function, the outgoing wave in the exterior is seen
to be the transmission through the horizon of the interior outgoing wave that
accompanies the collapsing shells. This transmission could allow information
from the interior to be transferred to the exterior.Comment: 19 pages, no figures. To appear in Phys. Rev.
Symmetry analysis and exact solutions of modified Brans-Dicke cosmological equations
We perform a symmetry analysis of modified Brans-Dicke cosmological equations
and present exact solutions. We discuss how the solutions may help to build
models of cosmology where, for the early universe, the expansion is linear and
the equation of state just changes the expansion velocity but not the
linearity. For the late universe the expansion is exponential and the effect of
the equation of state on the rate of expansion is just to change the constant
Hubble parameter.Comment: LaTeX2e source file, 14 pages, 7 reference
Topological Quintessence
A global monopole (or other topological defect) formed during a recent phase
transition with core size comparable to the present Hubble scale, could induce
the observed accelerating expansion of the universe. In such a model,
topological considerations trap the scalar field close to a local maximum of
its potential in a cosmologically large region of space. We perform detailed
numerical simulations of such an inhomogeneous dark energy system (topological
quintessence) minimally coupled to gravity, in a flat background of initially
homogeneous matter. We find that when the energy density of the field in the
monopole core starts dominating the background density, the spacetime in the
core starts to accelerate its expansion in accordance to a \Lambda CDM model
with an effective inhomogeneous spherical dark energy density parameter
\Omega_\Lambda(r). The matter density profile is found to respond to the global
monopole profile via an anti-correlation (matter underdensity in the monopole
core). Away from the monopole core, the spacetime is effectively
Einstein-deSitter (\Omega_\Lambda(r_{out}) -> 0) while at the center
\Omega_\Lambda(r ~ 0) is maximum. We fit the numerically obtained expansion
rate at the monopole core to the Union2 data and show that the quality of fit
is almost identical to that of \Lambda CDM. Finally, we discuss potential
observational signatures of this class of inhomogeneous dark energy models.Comment: Accepted in Phys. Rev. D (to appear). Added observational bounds on
parameters. 10 pages (two column revtex), 6 figures. The Mathematica files
used to produce the figures of this study may be downloaded from
http://leandros.physics.uoi.gr/topquin
Macroscopic Discontinuous Shear Thickening vs Local Shear Jamming in Cornstarch
We study the emergence of discontinuous shear-thickening (DST) in cornstarch,
by combining macroscopic rheometry with local Magnetic Resonance Imaging (MRI)
measurements. We bring evidence that macroscopic DST is observed only when the
flow separates into a low-density flowing and a high-density jammed region. In
the shear-thickened steady state, the local rheology in the flowing region, is
not DST but, strikingly, is often shear-thinning. Our data thus show that the
stress jump measured during DST, in cornstach, does not capture a secondary,
high-viscosity branch of the local steady rheology, but results from the
existence of a shear jamming limit at volume fractions quite significantly
below random close packing.Comment: To be published in PR
- …