4,318 research outputs found
Probing Brownstein-Moffat Gravity via Numerical Simulations
In the standard scenario of the Newtonian gravity, a late-type galaxy (i.e.,
a spiral galaxy) is well described by a disk and a bulge embedded in a halo
mainly composed by dark matter. In Brownstein-Moffat gravity, there is a claim
that late-type galaxy systems would not need to have halos, avoiding as a
result the dark matter problem, i.e., a modified gravity (non-Newtonian) would
account for the galactic structure with no need of dark matter. In the present
paper, we probe this claim via numerical simulations. Instead of using a
"static galaxy," where the centrifugal equilibrium is usually adopted, we probe
the Brownstein-Moffat gravity dynamically via numerical -body simulations.Comment: 33 pages and 14 figures - To appear in The Astrophysical Journa
Noncommutative Einstein-AdS Gravity in three Dimensions
We present a Lorentzian version of three-dimensional noncommutative
Einstein-AdS gravity by making use of the Chern-Simons formulation of pure
gravity in 2+1 dimensions. The deformed action contains a real, symmetric
metric and a real, antisymmetric tensor that vanishes in the commutative limit.
These fields are coupled to two abelian gauge fields. We find that this theory
of gravity is invariant under a class of transformations that reduce to
standard diffeomorphisms once the noncommutativity parameter is set to zero.Comment: 11 pages, LaTeX, minor errors corrected, references adde
Applying MOG to lensing: Einstein rings, Abell 520 and the Bullet Cluster
We investigate gravitational lensing in the context of the MOG modified
theory of gravity. Using a formulation of the theory with no adjustable or
fitted parameters, we present the MOG equations of motion for slow,
nonrelativistic test particles and for ultrarelativistic test particles, such
as rays of light. We demonstrate how the MOG prediction for the bending of
light can be applied to astronomical observations. Our investigation first
focuses on a small set of strong lensing observations where the properties of
the lensing objects are found to be consistent with the predictions of the
theory. We also present an analysis of the colliding clusters 1E0657-558 (known
also as the Bullet Cluster) and Abell 520; in both cases, the predictions of
the MOG theory are in good agreement with observation.Comment: 14 pages, 2 figures; final proof before publicatio
Bimetric Gravity Theory, Varying Speed of Light and the Dimming of Supernovae
In the bimetric scalar-tensor gravitational theory there are two frames
associated with the two metrics {\hat g}_{\mu\nu} and g_{\mu\nu}, which are
linked by the gradients of a scalar field \phi. The choice of a comoving frame
for the metric {\hat g}_{\mu\nu} or g_{\mu\nu} has fundamental consequences for
local observers in either metric spacetimes, while maintaining diffeomorphism
invariance. When the metric g_{\mu\nu} is chosen to be associated with comoving
coordinates, then the speed of light varies in the frame with the metric {\hat
g}_{\mu\nu}. Observers in this frame see the dimming of supernovae because of
the increase of the luminosity distance versus red shift, due to an increasing
speed of light in the early universe. Moreover, in this frame the scalar field
\phi describes a dark energy component in the Friedmann equation for the cosmic
scale without acceleration. If we choose {\hat g}_{\mu\nu} to be associated
with comoving coordinates, then an observer in the g_{\mu\nu} metric frame will
observe the universe to be accelerating and the supernovae will appear to be
farther away. The theory predicts that the gravitational constant G can vary in
spacetime, while the fine-structure constant \alpha=e^2/\hbar c does not vary.
The problem of cosmological horizons as viewed in the two frames is discussed.Comment: 22 pages, Latex file. No figures. Corrected typos. Added reference.
Further references added. Further corrections. To be published in Int. J.
Mod. Phys. D, 200
Full-coverage film cooling: 3-dimensional measurements of turbulence structure and prediction of recovery region hydrodynamics
Hydrodynamic measurements were made with a triaxial hot-wire in the full-coverage region and the recovery region following an array of injection holes inclined downstream, at 30 degrees to the surface. The data were taken under isothermal conditions at ambient temperature and pressure for two blowing ratios: M = 0.9 and M = 0.4. Profiles of the three main velocity components and the six Reynolds stresses were obtained at several spanwise positions at each of the five locations down the test plate. A one-equation model of turbulence (using turbulent kinetic energy with an algebraic mixing length) was used in a two-dimensional computer program to predict the mean velocity and turbulent kinetic energy profiles in the recovery region. A new real-time hotwire scheme was developed to make measurements in the three-dimensional turbulent boundary layer over the full-coverage surface
A 10-hour period revealed in optical spectra of the highly variable WN8 Wolf-Rayet star WR 123
Aims. What is the origin of the large-amplitude variability in Wolf-Rayet WN8
stars in general and WR123 in particular? A dedicated spectroscopic campaign
targets the ten-hour period previously found in the high-precision photometric
data obtained by the MOST satellite. Methods. In June-August 2003 we obtained a
series of high signal-to-noise, mid-resolution spectra from several sites in
the {\lambda}{\lambda} 4000 - 6940 A^{\circ} domain. We also followed the star
with occasional broadband (Johnson V) photometry. The acquired spectroscopy
allowed a detailed study of spectral variability on timescales from \sim 5
minutes to months. Results. We find that all observed spectral lines of a given
chemical element tend to show similar variations and that there is a good
correlation between the lines of different elements, without any significant
time delays, save the strong absorption components of the Hei lines, which tend
to vary differently from the emission parts. We find a single sustained
periodicity, P \sim 9.8 h, which is likely related to the relatively stable
pulsations found in MOST photometry obtained one year later. In addition,
seemingly stochastic, large-amplitude variations are also seen in all spectral
lines on timescales of several hours to several days.Comment: 6 pages, 4 figures, 2 tables, data available on-line, accepted in A&A
Research Note
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