1,741 research outputs found
Newtonian Limit of Conformal Gravity
We study the weak-field limit of the static spherically symmetric solution of
the locally conformally invariant theory advocated in the recent past by
Mannheim and Kazanas as an alternative to Einstein's General Relativity. In
contrast with the previous works, we consider the physically relevant case
where the scalar field that breaks conformal symmetry and generates fermion
masses is nonzero. In the physical gauge, in which this scalar field is
constant in space-time, the solution reproduces the weak-field limit of the
Schwarzschild--(anti)DeSitter solution modified by an additional term that,
depending on the sign of the Weyl term in the action, is either oscillatory or
exponential as a function of the radial distance. Such behavior reflects the
presence of, correspondingly, either a tachion or a massive ghost in the
spectrum, which is a serious drawback of the theory under discussion.Comment: 9 pages, comments and references added; the version to be published
in Phys. Rev.
Open Questions in Classical Gravity
We discuss some outstanding open questions regarding the validity and
uniqueness of the standard second order Newton-Einstein classical gravitational
theory. On the observational side we discuss the degree to which the realm of
validity of Newton's Law of Gravity can actually be extended to distances much
larger than the solar system distance scales on which the law was originally
established. On the theoretical side we identify some commonly accepted but
actually still open to question assumptions which go into the formulating of
the standard second order Einstein theory in the first place. In particular, we
show that while the familiar second order Poisson gravitational equation (and
accordingly its second order covariant Einstein generalization) may be
sufficient to yield Newton's Law of Gravity they are not in fact necessary. The
standard theory thus still awaits the identification of some principle which
would then make it necessary too. We show that current observational
information does not exclusively mandate the standard theory, and that the
conformal invariant fourth order theory of gravity considered recently by
Mannheim and Kazanas is also able to meet the constraints of data, and in fact
to do so without the need for any so far unobserved non-luminous or dark
matter.Comment: UCONN-93-1, plain TeX format, 22 pages (plus 7 figures - send
requests to [email protected]). To appear in a special issue of
Foundations of Physics honoring Professor Fritz Rohrlich on the occasion of
his retirement, L. P. Horwitz and A. van der Merwe Editors, Plenum Publishing
Company, N.Y., Fall 199
Comprehensive Solution to the Cosmological Constant, Zero-Point Energy, and Quantum Gravity Problems
We present a solution to the cosmological constant, the zero-point energy,
and the quantum gravity problems within a single comprehensive framework. We
show that in quantum theories of gravity in which the zero-point energy density
of the gravitational field is well-defined, the cosmological constant and
zero-point energy problems solve each other by mutual cancellation between the
cosmological constant and the matter and gravitational field zero-point energy
densities. Because of this cancellation, regulation of the matter field
zero-point energy density is not needed, and thus does not cause any trace
anomaly to arise. We exhibit our results in two theories of gravity that are
well-defined quantum-mechanically. Both of these theories are locally conformal
invariant, quantum Einstein gravity in two dimensions and Weyl-tensor-based
quantum conformal gravity in four dimensions (a fourth-order derivative quantum
theory of the type that Bender and Mannheim have recently shown to be
ghost-free and unitary). Central to our approach is the requirement that any
and all departures of the geometry from Minkowski are to be brought about by
quantum mechanics alone. Consequently, there have to be no fundamental
classical fields, and all mass scales have to be generated by dynamical
condensates. In such a situation the trace of the matter field energy-momentum
tensor is zero, a constraint that obliges its cosmological constant and
zero-point contributions to cancel each other identically, no matter how large
they might be. Quantization of the gravitational field is caused by its
coupling to quantized matter fields, with the gravitational field not needing
any independent quantization of its own. With there being no a priori classical
curvature, one does not have to make it compatible with quantization.Comment: Final version, to appear in General Relativity and Gravitation (the
final publication is available at http://www.springerlink.com). 58 pages,
revtex4, some additions to text and some added reference
Virial mass in DGP brane cosmology
We study the virial mass discrepancy in the context of a DPG brane-world
scenario and show that such a framework can offer viable explanations to
account for the mass discrepancy problem. This is done by defining a
geometrical mass that we prove to be proportional to the virial
mass. Estimating using observational data, we show that it
behaves linearly with and has a value of the order of , pointing
to a possible resolution of the virial mass discrepancy. We also obtain the
radial velocity dispersion of galaxy clusters and show that it is compatible
with the radial velocity dispersion profile of such clusters. This velocity
dispersion profile can be used to differentiate various models predicting the
virial mass.Comment: 12 pages, 1 figure, to appear in CQ
Oscillating instanton solutions in curved space
We investigate oscillating instanton solutions of a self-gravitating scalar
field between degenerate vacua. We show that there exist O(4)-symmetric
oscillating solutions in a de Sitter background. The geometry of this solution
is finite and preserves the symmetry. The nontrivial solution
corresponding to tunneling is possible only if the effect of gravity is taken
into account. We present numerical solutions of this instanton, including the
phase diagram of solutions in terms of the parameters of the present work and
the variation of energy densities. Our solutions can be interpreted as
solutions describing an instanton-induced domain wall or braneworld-like object
rather than a kink-induced domain wall or braneworld. The oscillating instanton
solutions have a thick wall and the solutions can be interpreted as a mechanism
providing nucleation of the thick wall for topological inflation. We remark
that invariant solutions also exist in a flat and anti-de Sitter
background, though the physical significance is not clear.Comment: 25 pages, 11 figues. Some typos corrected, references added, and Ch3.
modified according to referee's comment
Chandra Observations of the X-Ray Jet of 3C273
We report results from Chandra observations of the X-ray jet of 3C~273 during
the calibration phase in 2000 January. The zeroeth-order images and spectra
from two 40-ks exposures with the HETG and LETG+ACIS-S show a complex X-ray
structure. The brightest optical knots are detected and resolved in the 0.2-8
keV energy band. The X-ray morphology tracks well the optical. However, while
the X-ray brightness decreases along the jet, the outer parts of the jet tend
to be increasingly bright with increasing wavelength. The spectral energy
distributions of four selected regions can best be explained by inverse Compton
scattering of (beamed) cosmic microwave background photons. The model
parameters are compatible with equipartition and a moderate Doppler factor,
which is consistent with the one-sidedness of the jet. Alternative models
either imply implausible physical conditions and energetics (the synchrotron
self-Compton model) or are sufficiently ad hoc to be unconstrained by the
present data (synchrotron radiation from a spatially or temporally distinct
particle population).Comment: 3 figures; Figure 1 in color. Accepted for publication by ApJ Letter
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