1,106 research outputs found
Gravitational energy as dark energy: Concordance of cosmological tests
We provide preliminary quantitative evidence that a new solution to averaging
the observed inhomogeneous structure of matter in the universe [gr-qc/0702082,
arxiv:0709.0732], may lead to an observationally viable cosmology without
exotic dark energy. We find parameters which simultaneously satisfy three
independent tests: the match to the angular scale of the sound horizon detected
in the cosmic microwave background anisotropy spectrum; the effective comoving
baryon acoustic oscillation scale detected in galaxy clustering statistics; and
type Ia supernova luminosity distances. Independently of the supernova data,
concordance is obtained for a value of the Hubble constant which agrees with
the measurement of the Hubble Key team of Sandage et al [astro-ph/0603647].
Best-fit parameters include a global average Hubble constant H_0 = 61.7
(+1.2/-1.1) km/s/Mpc, a present epoch void volume fraction of f_{v0} = 0.76
(+0.12/-0.09), and an age of the universe of 14.7 (+0.7/-0.5) billion years as
measured by observers in galaxies. The mass ratio of non-baryonic dark matter
to baryonic matter is 3.1 (+2.5/-2.4), computed with a baryon-to-photon ratio
that concords with primordial lithium abundances.Comment: 4 pages, 2 figures; v2 improved statistics, references added, to
appear in ApJ Letter
Average observational quantities in the timescape cosmology
We examine the properties of a recently proposed observationally viable
alternative to homogeneous cosmology with smooth dark energy, the timescape
cosmology. In the timescape model cosmic acceleration is realized as an
apparent effect related to the calibration of clocks and rods of observers in
bound systems relative to volume-average observers in an inhomogeneous geometry
in ordinary general relativity. The model is based on an exact solution to a
Buchert average of the Einstein equations with backreaction. The present paper
examines a number of observational tests which will enable the timescape model
to be distinguished from homogeneous cosmologies with a cosmological constant
or other smooth dark energy, in current and future generations of dark energy
experiments. Predictions are presented for: comoving distance measures; H(z);
the equivalent of the dark energy equation of state, w(z); the Om(z) measure of
Sahni, Shafieloo and Starobinsky; the Alcock-Paczynski test; the baryon
acoustic oscillation measure, D_v; the inhomogeneity test of Clarkson, Bassett
and Lu; and the time drift of cosmological redshifts. Where possible, the
predictions are compared to recent independent studies of similar measures in
homogeneous cosmologies with dark energy. Three separate tests with indications
of results in possible tension with the Lambda CDM model are found to be
consistent with the expectations of the timescape cosmology.Comment: 22 pages, 12 figures; v2 discussion, references added, matches
published versio
Imaging the Near Field
In an earlier paper we introduced the concept of the perfect lens which
focuses both near and far electromagnetic fields, hence attaining perfect
resolution. Here we consider refinements of the original prescription designed
to overcome the limitations of imperfect materials. In particular we show that
a multi-layer stack of positive and negative refractive media is less sensitive
to imperfections. It has the novel property of behaving like a fibre-optic
bundle but one that acts on the near field, not just the radiative component.
The effects of retardation are included and minimized by making the slabs
thinner. Absorption then dominates image resolution in the near-field. The
deleterious effects of absorption in the metal are reduced for thinner layers.Comment: RevTeX, (9 pages, 8 figures
Dynamical coherent states and physical solutions of quantum cosmological bounces
A new model is studied which describes the quantum behavior of transitions
through an isotropic quantum cosmological bounce in loop quantum cosmology
sourced by a free and massless scalar field. As an exactly solvable model even
at the quantum level, it illustrates properties of dynamical coherent states
and provides the basis for a systematic perturbation theory of loop quantum
gravity. The detailed analysis is remarkably different from what is known for
harmonic oscillator coherent states. Results are evaluated with regard to their
implications in cosmology, including a demonstration that in general quantum
fluctuations before and after the bounce are unrelated. Thus, even within this
solvable model the condition of classicality at late times does not imply
classicality at early times before the bounce without further assumptions.
Nevertheless, the quantum state does evolve deterministically through the
bounce.Comment: 30 pages, 3 figure
Spectral fingerprinting for specific algal groups on sediments in situ: a new sensor
Currently it is still extremely difficult to adequately sample populations of microalgae on sediments for large-scale biomass determination. We have now devised a prototype of a new benthic sensor (BenthoFluor) for the quantitative and qualitative assessment of microphytobenthos populations in situ. This sensor enables a high spatial and temporal resolution and a rapid evaluation of the community structure and distribution. These determinations are based on the concept that five spectral excitation ranges can be used to differentiate groups of microalgae, in situ, within a few seconds. In addition, because sediments contain a lot of yellow substances, which can affect the fluorescence and optical differentiation of the algae, the device was equipped with a UV-LED for yellow substances correction. The device was calibrated against HPLC with cultures and tested in the field. Our real-time approach can be used to monitor algal assemblage composition on sediments and is an ideal tool for investigations on the large-scale spatial and temporal variation of algal populations in sediments. Apart from the differentiation of algal populations, the BenthoFluor allows instantaneous monitoring of the chlorophyll concentrations and determination of which algae are responsible for this on the uppermost surface of sediments in the field and in experimental set-ups
Black Hole Area in Brans-Dicke Theory
We have shown that the dynamics of the scalar field
in Brans-Dicke theories of gravity makes the surface area of the black hole
horizon {\it oscillatory} during its dynamical evolution. It explicitly
explains why the area theorem does not hold in Brans-Dicke theory. However, we
show that there exists a certain non-decreasing quantity defined on the event
horizon which is proportional to the black hole entropy for the case of
stationary solutions in Brans-Dicke theory. Some numerical simulations have
been demonstrated for Oppenheimer-Snyder collapse in Brans-Dicke theory.Comment: 12 pages, latex, 5 figures, epsfig.sty, some statements clarified and
two references added, to appear in Phys. Rev.
Noise performance of magneto-inductive cables
Magneto-inductive (MI) waveguides are metamaterial structures based on periodic arrangements of inductively coupled resonant magnetic elements. They are of interest for power transfer, communications and sensing, and can be realised in a flexible cable format. Signal-to-noise ratio is extremely important in applications involving signals. Here, we present the first experimental measurements of the noise performance of metamaterial cables. We focus on an application involving radiofrequency signal transmission in internal magnetic resonance imaging (MRI), where the subdivision of the metamaterial cable provides intrinsic patient safety. We consider MI cables suitable for use at 300 MHz during 1H MRI at 7 T, and find noise figures of 2.3–2.8 dB/m, together with losses of 3.0–3.9 dB/m, in good agreement with model calculations. These values are high compared to conventional cables, but become acceptable when (as here) the environment precludes the use of continuous conductors. To understand this behaviour, we present arguments for the fundamental performance limitations of these cables
Cosmology, cohomology, and compactification
Ashtekar and Samuel have shown that Bianchi cosmological models with compact
spatial sections must be of Bianchi class A. Motivated by general results on
the symmetry reduction of variational principles, we show how to extend the
Ashtekar-Samuel results to the setting of weakly locally homogeneous spaces as
defined, e.g., by Singer and Thurston. In particular, it is shown that any
m-dimensional homogeneous space G/K admitting a G-invariant volume form will
allow a compact discrete quotient only if the Lie algebra cohomology of G
relative to K is non-vanishing at degree m.Comment: 6 pages, LaTe
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