15,090 research outputs found
Galactic Dark Matter: a Dynamical Consequence of Cosmological Expansion
This work wants to show how standard General Relativity (GR) is able to
explain galactic rotation curves without the need for dark matter, this
starting from the idea that when Einstein's equations are applied to the
dynamics of a galaxy embedded in an expanding universe they do not reduce to
Poisson's equation but a generalisation of it taking cosmological expansion
into account. A non-linear scheme to perturb Einstein's field equations around
the Robertson-Walker (R-W) metric is devised in order to find their
non-relativistic limit without losing their characteristic non-linearities. The
resulting equation is used to numerically study the gravitational potential of
a cosmological perturbation and applied to a simple galactic model with an
exponentially decreasing baryonic matter distribution. The non-relativistic
limit of GR in a R-W space-time produces a generalised Poisson equation for the
gravitational potential which is non-linear, parabolic and heat-like. It is
shown how its non-linearities generate an effective "dark matter" distribution
caused by both cosmological expansion and the dynamics of the perturbation's
gravitational potential. It is also shown how this dynamical effect gets
completely lost during a linearisation of Einstein's equations. The equation is
then used to successfully fit real galactic rotation curves numerically using a
matter distribution following the shape of a simple S\'ersic luminosity
profile, common to most galaxies, thus without recourse to dark matter. A
relation for the dark to luminous matter ratio is found, explaining the
domination of dark matter in low-mass galaxies. A few rotation curves with a
faster than Newtonian decrease are also presented and successfully fitted,
opening the way to a new possible interpretation of these phenomena in terms of
an effective "anti-gravitational" dark matter distribution, purely geometrical
in origin.Comment: 8 pages, 18 figures, Research Pape
Joining of thirty three percent by weight random glass fibre reinforced polystyrene using variable frequency microwave
[Abstract]: This paper extends the range of applications for Variable Frequency Microwave (VFM) (2 – 18 GHz) facilities to joining thirty three percent by weight glass fibre reinforced polystyrene composite [PS/GF (33%)]. With a given power level, the composite was exposed to various exposure times to microwave irradiation. The primer or coupling agent used was 5-minute two-part adhesive containing 100% liquid epoxy and 8% amine, i.e. Araldite, which was more readily microwave reactive than the composite itself. Bond strengths of the joints were lap shear tested and results were compared with those obtained using fixed frequency (2.45 GHz) microwave processing. The VFMF was operated under software control, which provided automatic data logging facilities. The maximum lap shear bond strength of joint was 430 N/cm2 using variable frequency microwave facility while that obtained by fixed frequency microwave configuration was only 331 N/cm2. The former is nearly 30% stronger than the latter
Permittivity measurement of thermoplastic composites at elevated temperature
[Abstract]: The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity Epsilon = Epsilon' - jEpsilon'', and the loss tangent, tan Delta = Epsilon'/Epsilon''. This paper describes two convenient laboratory based methods to obtain Epsilon', Epsilon'' and hence tan Delta of fibre-reinforced thermoplastic (FRTP) composites. One method employs a microwave network analyser in conjunction with a waveguide transmission technique, chosen because it provides the widest possible frequency range with high accuracy. The values of the dielectric constant and dielectric loss of glass fibre reinforced (33%) low density polyethylene, LDPE/GF (33%), polystyrene, PS/GF (33%), and Nylon 66/GF (33%), were obtained. Results are compared with those obtained by another method using a high-temperature dielectric probe
The Progenitors of Recent Core-Collapse Supernovae
We present the results of our analysis of Hubble Space Telescope (HST) and deep ground-based images to isolate the massive progenitor stars of the two recent core-collapse supernovae 2008 bk and 2008 cn. The identification of the progenitors is facilitated in one of these two cases by high-precision astrometry based on our HST imaging of SNe at late times
Square-torsion gravity, dark matter halos and the baryonic Tully-Fisher relation
Square-torsion gravity is applied to the long standing dark matter problem.
In this context the theory reduces to General Relativity complemented by a dark
stress-energy tensor due to the torsion of spacetime and is studied under the
simplifying assumption of spherical symmetry. The dark stress-energy tensor is
found to satisfy an anisotropic structure equation. In vacuum this is shown to
be equivalent to a wave equation with sources. A natural class of exact
solutions is found which explicitly perturbs any seed spacetime metric by a
conformal factor satisfying a (1+1)-dimensional wave equation. This leads to
the concept of dark coating. The static solutions are then used to construct
structures that model dark matter halos surrounding baryonic bodies. In the
Newtonian r\'egime the baryonic mass and the flat rotation curve velocity
are found to be related by the baryonic Tully-Fisher relation . The present work proposes thus a possible theoretical motivation of
this hitherto purely empirical result. The example of a dark halo on the
Schwarzschild geometry is made as a toy model for a galaxy. All qualitative an
quantitative features of galactic rotation curves are recovered. A dark halo
surrounding a Schwarzschild black hole is found to possess a boundary of
staticity called torsion sphere placed between the photon sphere and the event
horizon. The phenomenon of dark radiation is briefly exposed. The way for
cosmological applications is then opened by showing how Hubble expansion is a
natural feature of the theory
Clustering of the Diffuse Infrared Light from the COBE DIRBE maps. III. Power spectrum analysis and excess isotropic component of fluctuations
The cosmic infrared background (CIB) radiation is the cosmic repository for
energy release throughout the history of the universe. Using the all-sky data
from the COBE DIRBE instrument at wavelengths 1.25 - 100 mic we attempt to
measure the CIB fluctuations. In the near-IR, foreground emission is dominated
by small scale structure due to stars in the Galaxy. There we find a strong
correlation between the amplitude of the fluctuations and Galactic latitude
after removing bright foreground stars. Using data outside the Galactic plane
() and away from the center () we extrapolate
the amplitude of the fluctuations to cosec. We find a positive intercept
of nW/m2/sr at 1.25, 2.2,3.5 and 4.9 mic
respectively, where the errors are the range of 92% confidence limits. For
color subtracted maps between band 1 and 2 we find the isotropic part of the
fluctuations at nW/m2/sr. Based on detailed numerical and
analytic models, this residual is not likely to originate from the Galaxy, our
clipping algorithm, or instrumental noise. We demonstrate that the residuals
from the fit used in the extrapolation are distributed isotropically and
suggest that this extra variance may result from structure in the CIB. For
2\deg< \theta < 15^\deg, a power-spectrum analysis yields firm upper limits
of (\theta/5^\deg) \times\delta F_{\rm rms} (\theta) < 6, 2.5, 0.8, 0.5
nW/m2/sr at 1.25, 2.2, 3.5 and 4.9 mic respectively. From 10-100 mic, the upper
limits <1 nW/m2/sr.Comment: Ap.J., in press. 69 pages including 24 fig
Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer
We have studied temperature dependences of electron transport in graphene and
its bilayer and found extremely low electron-phonon scattering rates that set
the fundamental limit on possible charge carrier mobilities at room
temperature. Our measurements have shown that mobilities significantly higher
than 200,000 cm2/Vs are achievable, if extrinsic disorder is eliminated. A
sharp (threshold-like) increase in resistivity observed above approximately
200K is unexpected but can qualitatively be understood within a model of a
rippled graphene sheet in which scattering occurs on intra-ripple flexural
phonons
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‘Awaken your incredible’: Love your body discourses and postfeminist contradictions
In this article we focus on a new yet under-examined cultural phenomenon: the turn to ‘Love your body’ (LYB) discourses. Taking a feminist critical standpoint, we move away from an affirmative reading of LYB discourses and instead understand them as a postfeminist articulation of sexism. Our analysis identifies the key motifs of LYB discourses and contextualizes their dramatic proliferation over the last decade. Situated at the historical convergence of neoliberal governmentality, emotional capitalism, the growth of social media and commodity feminism, we trace how LYB discourses have emerged within the advertising genre to quickly saturate media more broadly. The article concludes with a critical assessment of LYB discourses that seeks to flesh out its distinctive contradictions and its ideological workings. In so doing, we will argue that far from representing a liberation from harmful beauty standards, LYB discourses are implicated in a deeper and more pernicious regulation of women that has shifted from bodily to psychic regulation
Energy Distribution in 2d Stringy Black Hole Backgrounds
We utilize Moller's and Einstein's energy-momentum complexes in order to
explicitly evaluate the energy distributions associated with the
two-dimensional "Schwarzschild" and "Reissner-Nordstrom" black hole
backgrounds. While Moller's prescription provides meaningful physical results,
Einstein's prescription fails to do so in the aforementioned gravitational
backgrounds. These results hold for all two-dimensional static black hole
geometries. The results obtained within this context are exploited in order
Seifert's hypothesis to be investigated.Comment: 17 pages, LaTeX, v2: acknowledgements added, to appear in
Int.J.Mod.Phys.
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