3,905 research outputs found
A Modified Gravity and its Consequences for the Solar System, Astrophysics and Cosmology
A relativistic modified gravity (MOG) theory leads to a self-consistent,
stable gravity theory that can describe the solar system, galaxy and clusters
of galaxies data and cosmology.Comment: 16 pages. Latex file. Talk given at the International Workshop "From
Quantum to Cosmos: Fundamental Physics in Space", 22-24 May, 2006, Warrenton,
Virginia, USA. To be published in Int. J. Mod. Phys D. Equation correcte
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
Quantum Gravity Momentum Representation and Maximum Invariant Energy
We use the idea of the symmetry between the spacetime coordinates x^\mu and
the energy-momentum p^\mu in quantum theory to construct a momentum space
quantum gravity geometry with a metric s_{\mu\nu} and a curvature
P^\lambda_{\mu\nu\rho}. For a closed maximally symmetric momentum space with a
constant 3-curvature, the volume of the p-space admits a cutoff with an
invariant maximum momentum a. A Wheeler-DeWitt-type wave equation is obtained
in the momentum space representation. The vacuum energy density and the
self-energy of a charged particle are shown to be finite, and modifications of
the electromagnetic radiation density and the entropy density of a system of
particles occur for high frequencies.Comment: 16 pages, LateX file, no figure
Solutions to the Quasi-flatness and Quasi-lambda Problems
Big Bang models of the Universe predict rapid domination by curvature, a
paradox known as the flatness problem. Solutions to this problem usually leave
the Universe exactly flat for every practical purpose. Explaining a nearly but
not exactly flat current Universe is a new problem, which we label the
quasi-flatness problem. We show how theories incorporating time-varying
coupling constants could drive the Universe to a late-time near-flat attractor.
A similar problem may be posed with regards to the cosmological constant
, the quasi-lambda problem, and we exhibit a solution to this problem
as well.Comment: 9 pages, no figures. Minor changes corresponding to version to be
published in Physics Letters
Non-Locality and Ellipticity in a Gauge-Invariant Quantization
The quantum theory of a free particle in two dimensions with non-local
boundary conditions on a circle is known to lead to surface and bulk states.
Such a scheme is here generalized to the quantized Maxwell field, subject to
mixed boundary conditions. If the Robin sector is modified by the addition of a
pseudo-differential boundary operator, gauge-invariant boundary conditions are
obtained at the price of dealing with gauge-field and ghost operators which
become pseudo-differential. A good elliptic theory is then obtained if the
kernel occurring in the boundary operator obeys certain summability conditions,
and it leads to a peculiar form of the asymptotic expansion of the symbol. The
cases of ghost operator of negative and positive order are studied within this
framework.Comment: 17 pages, plain Te
Gravitational Waves in the Nonsymmetric Gravitational Theory
We prove that the flux of gravitational radiation from an isolated source in
the Nonsymmetric Gravitational Theory is identical to that found in Einstein's
General Theory of Relativity.Comment: 10 Page
The spin rates and spin evolution of the O components in WR+O binaries
Despite 50 years of extensive binary research we have to conclude that the
Roche lobe overflow/mass transfer process that governs close binary evolution
is still poorly understood. It is the scope of the present paper to lift a tip
of the veil by studying the spin-up and spin-down processes of the O-type
components of WR+O binaries. We critically analyze the available observational
data of rotation speeds of the O-type components in WR+O binaries. By combining
a binary evolutionary code and a formalism that describes the effects of tides
in massive stars with an envelope in radiative equilibrium, we compute the
corresponding rotational velocities during the Roche lobe overflow of the
progenitor binaries. In all the studied WR+O binaries, we find that the O-type
stars were affected by accretion of matter during the RLOF of the progenitor.
This means that common envelope evolution which excludes any accretion onto the
secondary O-star, has not played an important role to explain the WR+O
binaries. Moreover, although it is very likely that the O-type star progenitors
were spun-up by the mass transfer, many ended the RLOF/mass transfer phase with
a rotational velocity that is significantly smaller than the critical rotation
speed. This may indicate that during the mass transfer phase there is a
spin-down process which is of the same order as, although significantly less
than that of the spin-up process. We propose a Spruit-Tayler type dynamo
spin-down suggested in the past to explain the rotation speeds of the mass
gainers in long-period Algols.Comment: 6 pages, accepted for publication in A&A; accepted versio
Solving the Flatness and Quasi-flatness Problems in Brans-Dicke Cosmologies with a Varying Light Speed
We define the flatness and quasi-flatness problems in cosmological models. We
seek solutions to both problems in homogeneous and isotropic Brans-Dicke
cosmologies with varying speed of light. We formulate this theory and find
perturbative, non-perturbative, and asymptotic solutions using both numerical
and analytical methods. For a particular range of variations of the speed of
light the flatness problem can be solved. Under other conditions there exists a
late-time attractor with a constant value of \Omega that is smaller than, but
of order, unity. Thus these theories may solve the quasi-flatness problem, a
considerably more challenging problem than the flatness problem. We also
discuss the related \Lambda and quasi-\Lambda problem in these theories. We
conclude with an appraisal of the difficulties these theories may face.Comment: 21 pages, 6 figure
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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