28 research outputs found
Gravitational Waves in Bianchi Type-I Universes I: The Classical Theory
The propagation of classical gravitational waves in Bianchi Type-I universes
is studied. We find that gravitational waves in Bianchi Type-I universes are
not equivalent to two minimally coupled massless scalar fields as it is for the
Robertson-Walker universe. Due to its tensorial nature, the gravitational wave
is much more sensitive to the anisotropy of the spacetime than the scalar field
is and it gains an effective mass term. Moreover, we find a coupling between
the two polarization states of the gravitational wave which is also not present
in the Robertson-Walker universe.Comment: 34 papers, written in ReVTeX, submitted to Physical Review
Doubly Enhanced Skyrmions in Bilayer Quantum Hall States
By tilting the samples in the magnetic field, we measured and compared the
Skyrmion excitations in the bilayer quantum Hall (QH) state at the Landau-level
filling factor and in the monolayer QH state at . The observed
number of flipped spins is in the bilayer system with a large
tunneling gap, and in the bilayer system with a small tunneling gap,
while it is in the monolayer system. The difference is interpreted due
to the interlayer exchange interaction. Moreover, we have observed seemingly
preferred numbers for the flipped spins by tilting bilayer
samples.Comment: 4 pages, 4 figure
Observation of Quantized Hall Drag in a Strongly Correlated Bilayer Electron System
The frictional drag between parallel two-dimensional electron systems has
been measured in a regime of strong interlayer correlations. When the bilayer
system enters the excitonic quantized Hall state at total Landau level filling
factor \nu_T=1 the longitudinal component of the drag vanishes but a strong
Hall component develops. The Hall drag resistance is observed to be accurately
quantized at h/e^2.Comment: 4 pages, 3 figures. Version accepted for publication in Physical
Review Letters. Improved discussion of experimental and theoretical issues,
added references, correction to figure
Quantum-Hall Quantum-Bits
Bilayer quantum Hall systems can form collective states in which electrons
exhibit spontaneous interlayer phase coherence. We discuss the possibility of
using bilayer quantum dot many-electron states with this property to create
two-level systems that have potential advantages as quantum bits.Comment: 4 pages, 4 figures included, version to appear in Phys. Rev. B (Rapid
Communications
Chaos, decoherence and quantum cosmology
In this topical review we discuss the connections between chaos, decoherence
and quantum cosmology. We understand chaos as classical chaos in systems with a
finite number of degrees of freedom, decoherence as environment induced
decoherence and quantum cosmology as the theory of the Wheeler - DeWitt
equation or else the consistent history formulation thereof, first in mini
super spaces and later through its extension to midi super spaces. The overall
conclusion is that consideration of decoherence is necessary (and probably
sufficient) to sustain an interpretation of quantum cosmology based on the Wave
function of the Universe adopting a Wentzel - Kramers - Brillouin form for
large Universes, but a definitive account of the semiclassical transition in
classically chaotic cosmological models is not available in the literature yet.Comment: 40 page
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom