72 research outputs found
Trapping Horizons in the Sultana-Dyer Space-Time
The Sultana-Dyer space-time is suggested as a model describing a black hole
embedded in an expanding universe. Recently, in \cite{0705.4012}, its global
structure is analyzed and the trapping horizons are shown. In the paper, by
directly calculating the expansions of the radial null vector fields normal to
the space-like two-spheres foliating the trapping horizons, we find that the
trapping horizon outside the event horizon in the Sultana-Dyer space-time is a
past trapping horizon. Further, we find that the past trapping horizon is an
outer, instantaneously degenerate or inner trapping horizon accordingly when
the radial coordinate is less than, equal to or greater than some value.Comment: no figures, 5 pages; PCAS and key words are adde
Magnetospectroscopy of epitaxial few-layer graphene
The inter-Landau level transitions observed in far-infrared transmission
experiments on few-layer graphene samples show a behaviour characteristic of
the linear dispersion expected in graphene. This behaviour persists in
relatively thick samples, and is qualitatively different from that of thin
samples of bulk graphite.Comment: Invited short review to appear in a special issue of Solid State
Communication
The Nursing Worklife Model: Extending and Refining a New Theory
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75205/1/j.1365-2834.2007.00670.x.pd
Anisotropy in the Hubble constant as observed in the HST Extragalactic Distance Scale Key Project results
Based on general relativity, it can be argued that deviations from a uniform
Hubble flow should be thought of as variations in the Universe's expansion
velocity field, rather than being thought of as peculiar velocities with
respect to a uniformly expanding space. The aim of this paper is to use the
observed motions of galaxies to map out variations in the Universe's expansion,
and more importantly, to investigate whether real variations in the Hubble
expansion are detectable given the observational uncertainties. All-sky maps of
the observed variation in the expansion are produced using measurements
obtained along specific lines-of-sight and smearing them across the sky using a
Gaussian profile. A map is produced for the final results of the HST
Extragalactic Distance Scale Key Project for the Hubble constant, a comparison
map is produced from a set of essentially independent data, and Monte Carlo
techniques are used to analyse the statistical significance of the variation in
the maps. A statistically significant difference in expansion rate of 9
km/s/Mpc is found to occur across the sky. Comparing maps of the sky at
different distances appears to indicate two distinct sets of extrema with even
stronger statistically significant variations. Within our supercluster,
variations tend to occur near the supergalactic plane, and beyond our
supercluster, variations tend to occur away from the supergalactic plane.
Comparison with bulk flow studies shows some concordance, yet also suggests the
bulk flow studies may suffer confusion, failing to discern the influence of
multiple perturbations.Comment: 23 pages, 5 figures, to be published in New Astronom
Do precocial mammals develop at a faster rate? A comparison of rates of skull development in Sigmodon fulviventer and Mus musculus domesticus
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72993/1/j.1420-9101.2003.00568.x.pd
Is the evidence for dark energy secure?
Several kinds of astronomical observations, interpreted in the framework of
the standard Friedmann-Robertson-Walker cosmology, have indicated that our
universe is dominated by a Cosmological Constant. The dimming of distant Type
Ia supernovae suggests that the expansion rate is accelerating, as if driven by
vacuum energy, and this has been indirectly substantiated through studies of
angular anisotropies in the cosmic microwave background (CMB) and of spatial
correlations in the large-scale structure (LSS) of galaxies. However there is
no compelling direct evidence yet for (the dynamical effects of) dark energy.
The precision CMB data can be equally well fitted without dark energy if the
spectrum of primordial density fluctuations is not quite scale-free and if the
Hubble constant is lower globally than its locally measured value. The LSS data
can also be satisfactorily fitted if there is a small component of hot dark
matter, as would be provided by neutrinos of mass 0.5 eV. Although such an
Einstein-de Sitter model cannot explain the SNe Ia Hubble diagram or the
position of the `baryon acoustic oscillation' peak in the autocorrelation
function of galaxies, it may be possible to do so e.g. in an inhomogeneous
Lemaitre-Tolman-Bondi cosmology where we are located in a void which is
expanding faster than the average. Such alternatives may seem contrived but
this must be weighed against our lack of any fundamental understanding of the
inferred tiny energy scale of the dark energy. It may well be an artifact of an
oversimplified cosmological model, rather than having physical reality.Comment: 12 pages, 5 figures; to appear in a special issue of General
Relativity and Gravitation, eds. G.F.R. Ellis et al; Changes: references
reformatted in journal style - text unchange
Dark energy as a mirage
Motivated by the observed cosmic matter distribution, we present the
following conjecture: due to the formation of voids and opaque structures, the
average matter density on the path of the light from the well-observed objects
changes from Omega_M ~ 1 in the homogeneous early universe to Omega_M ~ 0 in
the clumpy late universe, so that the average expansion rate increases along
our line of sight from EdS expansion Ht ~ 2/3 at high redshifts to free
expansion Ht ~ 1 at low redshifts. To calculate the modified observable
distance-redshift relations, we introduce a generalized Dyer-Roeder method that
allows for two crucial physical properties of the universe: inhomogeneities in
the expansion rate and the growth of the nonlinear structures. By treating the
transition redshift to the void-dominated era as a free parameter, we find a
phenomenological fit to the observations from the CMB anisotropy, the position
of the baryon oscillation peak, the magnitude-redshift relations of type Ia
supernovae, the local Hubble flow and the nucleosynthesis, resulting in a
concordant model of the universe with 90% dark matter, 10% baryons, no dark
energy, 15 Gyr as the age of the universe and a natural value for the
transition redshift z_0=0.35. Unlike a large local void, the model respects the
cosmological principle, further offering an explanation for the late onset of
the perceived acceleration as a consequence of the forming nonlinear
structures. Additional tests, such as quantitative predictions for angular
deviations due to an anisotropic void distribution and a theoretical derivation
of the model, can vindicate or falsify the interpretation that light
propagation in voids is responsible for the perceived acceleration.Comment: 33 pages, 2 figs; v2: minor clarifications, results unchanged; v3:
matches the version published in General Relativity and Gravitatio
The Lemaitre Model and the Generalisation of the Cosmic Mass
We consider the spherically symmetric metric with a comoving perfect fluid
and non-zero pressure -- the Lemaitre metric -- and present it in the form of a
calculational algorithm. We use it to review the definition of mass, and to
look at the apparent horizon relations on the observer's past null cone. We
show that the introduction of pressure makes it difficult to separate the mass
from other physical parameters in an invariant way. Under the usual mass
definition, the apparent horizon relation, that relates the diameter distance
to the cosmic mass, remains the same as in the Lemaitre-Tolman case.Comment: latex, 16 pages, Revision has minor changes due to referee's
comments
Imitating accelerated expansion of the Universe by matter inhomogeneities - corrections of some misunderstandings
A number of misunderstandings about modeling the apparent accelerated
expansion of the Universe, and about the `weak singularity' are clarified: 1.
Of the five definitions of the deceleration parameter given by Hirata and
Seljak (HS), only is a correct invariant measure of
acceleration/deceleration of expansion. The and are unrelated to
acceleration in an inhomogeneous model. 2. The averaging over directions
involved in the definition of does not correspond to what is done in
observational astronomy. 3. HS's equation (38) connecting to the flow
invariants gives self-contradictory results when applied at the centre of
symmetry of the Lema\^{\i}tre-Tolman (L-T) model. The intermediate equation
(31) that determines is correct, but approximate, so it cannot be used
for determining the sign of the deceleration parameter. Even so, at the centre
of symmetry of the L-T model, it puts no limitation on the sign of .
4. The `weak singularity' of Vanderveld {\it et al.} is a conical profile of
mass density at the centre - a perfectly acceptable configuration. 5. The
so-called `critical point' in the equations of the `inverse problem' for a
central observer in an L-T model is a manifestation of the apparent horizon - a
common property of the past light cones in zero-lambda L-T models, perfectly
manageable if the equations are correctly integrated.Comment: 15 pages. Completely rewritten to match the published version. We
added discussion of 2 key papers cited by VFW and identified more clearly the
assumptions, approximations and mistakes that led to certain misconceptions
Abstract sentence representations in 3-year-olds: Evidence from language production and comprehension
We use syntactic priming to test the abstractness of the sentence representations of young 3-year-olds (35-42 months). In describing pictures with inanimate participants, 18 children primed with passives produced more passives (11 with a strict scoring scheme, 16 with lax scoring) than did 18 children primed with actives (2 on either scheme) or 12 children who received no priming (0). Priming was comparable to that reported for older children and adults. Comprehension of reversible passives with animate participants before and after priming was above chance but did not improve as a result of priming. Young 3-year-olds represent sentences abstractly, have syntactic representations for noun, verb, "surface subject", and "surface object", have semantic representations for "agent" and "patient", and flexibly map the relation between syntax and semantics. Taken together with research on syntactic categories in 2-year-olds, our results provide empirical support for continuity in language acquisition
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