1,044 research outputs found
A New Type of Singularity Theorem
A new type of singularity theorem, based on spatial averages of physical
quantities, is presented and discussed. Alternatively, the results inform us of
when a spacetime can be singularity-free. This theorem provides a decisive
observational difference between singular and non-singular, globally
hyperbolic, open cosmological models.Comment: 6 pages, no figures. Contribution to appear in the Proceedings of the
Spanish Relativity Meeting ERE-07, "Relativistic Astrophysics and Cosmology
Equations for general shells
The complete set of (field) equations for shells of arbitrary, even changing,
causal character are derived in arbitrary dimension. New equations that seem to
have never been considered in the literature emerge, even in the traditional
cases of everywhere non-null, or everywhere null, shells. In the latter case
there arise field equations for some degrees of freedom encoded exclusively in
the distributional part of the Weyl tensor. For non-null shells the standard
Israel equations are recovered but not only, the additional relations
containing also relevant information. The results are applicable to a
widespread literature on domain walls, branes and braneworlds, gravitational
layers, impulsive gravitational waves, and the like. Moreover, they are of a
geometric nature, and thus they can be used in any theory based on a Lorentzian
manifold.Comment: 32 pages, no figures. New paragraph and new footnote, plus some added
references. Version to be publishe
The universal 'energy' operator
The "positive square" of any tensor is presented in a universal and unified
manner, valid in Lorentzian manifolds of arbitrary dimension, and independently
of any (anti)-symmetry properties of the tensor. For rank-m tensors, the
positive square has rank 2m. Positive here means future, that is to say,
satisfying the dominant property. The standard energy-momentum and super-energy
tensors are recovered as appropriate parts of the general square. A richer
structure of principal null directions arises.Comment: 13 pages, no figures; minor improvements and corrections. This is a
larger, expanded version, containing proofs and explanations not available in
the short Note to be published in CQ
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