848 research outputs found
Absolute spacetime: the twentieth century ether
All gauge theories need ``something fixed'' even as ``something changes.''
Underlying the implementation of these ideas all major physical theories make
indispensable use of an elaborately designed spacetime model as the ``something
fixed,'' i.e., absolute. This model must provide at least the following
sequence of structures: point set, topological space, smooth manifold,
geometric manifold, base for various bundles. The ``fine structure'' of
spacetime inherent in this sequence is of course empirically unobservable
directly, certainly when quantum mechanics is taken into account. This issue is
at the basis of the difficulties in quantizing general relativity and has been
approached in many different ways. Here we review an approach taking into
account the non-Boolean properties of quantum logic when forming a spacetime
model. Finally, we recall how the fundamental gauge of diffeomorphisms (the
issue of general covariance vs coordinate conditions) raised deep conceptual
problems for Einstein in his early development of general relativity. This is
clearly illustrated in the notorious ``hole'' argument. This scenario, which
does not seem to be widely known to practicing relativists, is nevertheless
still interesting in terms of its impact for fundamental gauge issues.Comment: Contribution to Proceedings of Mexico Meeting on Gauge Theories of
Gravity in honor of Friedrich Heh
Modified gravity and the origin of inertia
Modified gravity theory is known to violate Birkhoff's theorem. We explore a
key consequence of this violation, the effect of distant matter in the Universe
on the motion of test particles. We find that when a particle is accelerated, a
force is experienced that is proportional to the particle's mass and
acceleration and acts in the direction opposite to that of the acceleration. We
identify this force with inertia. At very low accelerations, our inertial law
deviates slightly from that of Newton, yielding a testable prediction that may
be verified with relatively simple experiments. Our conclusions apply to all
gravity theories that reduce to a Yukawa-like force in the weak field
approximation.Comment: 4 pages, 3 figures; published version with updated reference
Quantum Cosmology for the General Bianchi Type II, VI(Class A) and VII(Class A) vacuum geometries
The canonical quantization of the most general minisuperspace actions --i.e.
with all six scale factor as well as the lapse function and the shift vector
present-- describing the vacuum type II, VI and VII geometries, is considered.
The reduction to the corresponding physical degrees of freedom is achieved
through the usage of the linear constraints as well as the quantum version of
the entire set of classical integrals of motion.Comment: 23 pages, LaTeX2e, No figure
A note on wavemap-tensor cosmologies
We examine theories of gravity which include finitely many coupled scalar
fields with arbitrary couplings to the curvature (wavemaps). We show that the
most general scalar-tensor -model action is conformally equivalent to
general relativity with a minimally coupled wavemap with a particular target
metric. Inflation on the source manifold is then shown to occur in a novel way
due to the combined effect of arbitrary curvature couplings and wavemap
self-interactions. A new interpretation of the conformal equivalence theorem
proved for such `wavemap-tensor' theories through brane-bulk dynamics is also
discussed.Comment: 8 pages, LaTeX, to appear in the Proceedings of the 2nd Hellenic
Cosmology Workshop, National Observatory of Athens, April 21-22, 2001,
(Kluwer 2001
Nonminimal Couplings in the Early Universe: Multifield Models of Inflation and the Latest Observations
Models of cosmic inflation suggest that our universe underwent an early phase
of accelerated expansion, driven by the dynamics of one or more scalar fields.
Inflationary models make specific, quantitative predictions for several
observable quantities, including particular patterns of temperature anistropies
in the cosmic microwave background radiation. Realistic models of high-energy
physics include many scalar fields at high energies. Moreover, we may expect
these fields to have nonminimal couplings to the spacetime curvature. Such
couplings are quite generic, arising as renormalization counterterms when
quantizing scalar fields in curved spacetime. In this chapter I review recent
research on a general class of multifield inflationary models with nonminimal
couplings. Models in this class exhibit a strong attractor behavior: across a
wide range of couplings and initial conditions, the fields evolve along a
single-field trajectory for most of inflation. Across large regions of phase
space and parameter space, therefore, models in this general class yield robust
predictions for observable quantities that fall squarely within the "sweet
spot" of recent observations.Comment: 17pp, 2 figs. References added to match the published version.
Published in {\it At the Frontier of Spacetime: Scalar-Tensor Theory, Bell's
Inequality, Mach's Principle, Exotic Smoothness}, ed. T. Asselmeyer-Maluga
(Springer, 2016), pp. 41-57, in honor of Carl Brans's 80th birthda
Black Holes with a Massive Dilaton
The modifications of dilaton black holes which result when the dilaton
acquires a mass are investigated. We derive some general constraints on the
number of horizons of the black hole and argue that if the product of the black
hole charge and the dilaton mass satisfies then the black
hole has only one horizon. We also argue that for there may exist
solutions with three horizons and we discuss the causal structure of such
solutions. We also investigate the possible structures of extremal solutions
and the related problem of two-dimensional dilaton gravity with a massive
dilaton.Comment: 36 pages with 5 figures (as uuencoded compressed tar file) (revised
version has one major change in bound on mass for extremal solution and minor
typos fixed), harvma
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