913 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
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
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
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
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
Direct probing of band-structure Berry phase in diluted magnetic semiconductors
We report on experimental evidence of the Berry phase accumulated by the
charge carrier wave function in single-domain nanowires made from a
(Ga,Mn)(As,P) diluted ferromagnetic semiconductor layer. Its signature on the
mesoscopic transport measurements is revealed as unusual patterns in the
magnetoconductance, that are clearly distinguished from the universal
conductance fluctuations. We show that these patterns appear in a magnetic
field region where the magnetization rotates coherently and are related to a
change in the band-structure Berry phase as the magnetization direction
changes. They should be thus considered as a band structure Berry phase
fingerprint of the effective magnetic monopoles in the momentum space. We argue
that this is an efficient method to vary the band structure in a controlled way
and to probe it directly. Hence, (Ga,Mn)As appears to be a very interesting
test bench for new concepts based on this geometrical phase.Comment: 7 pages, 6 figure
CMB anisotropies seen by an off-center observer in a spherically symmetric inhomogeneous universe
The current authors have previously shown that inhomogeneous, but spherically
symmetric universe models containing only matter can yield a very good fit to
the SNIa data and the position of the first CMB peak. In this work we examine
how far away from the center of inhomogeneity the observer can be located in
these models and still fit the data well. Furthermore, we investigate whether
such an off-center location can explain the observed alignment of the lowest
multipoles of the CMB map. We find that the observer has to be located within a
radius of 15 Mpc from the center for the induced dipole to be less than that
observed by the COBE satellite. But for such small displacements from the
center, the induced quadru- and octopoles turn out to be insufficiently large
to explain the alignment.Comment: 8 pages (REVTeX4), 7 figures; v2: minor changes, matches published
versio
Expansion-Free Evolving Spheres Must Have Inhomogeneous Energy Density Distributions
In a recent paper a systematic study on shearing expansion-free spherically
symmetric distributions was presented. As a particular case of such systems,
the Skripkin model was mentioned, which corresponds to a nondissipative perfect
fluid with a constant energy density. Here we show that such a model is
inconsistent with junction conditions. It is shown that in general for any
nondissipative fluid distribution, the expansion-free condition requires the
energy density to be inhomogeneous. As an example we consider the case of dust,
which allows for a complete integration.Comment: 8 pages, Latex. To appear in Phys. Rev.D. Typos correcte
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