1,429 research outputs found
Riemann-Cartan Space-times of G\"odel Type
A class of Riemann-Cartan G\"odel-type space-times are examined in the light
of the equivalence problem techniques. The conditions for local space-time
homogeneity are derived, generalizing previous works on Riemannian G\"odel-type
space-times. The equivalence of Riemann-Cartan G\"odel-type space-times of this
class is studied. It is shown that they admit a five-dimensional group of
affine-isometries and are characterized by three essential parameters : identical triads () correspond to locally
equivalent manifolds. The algebraic types of the irreducible parts of the
curvature and torsion tensors are also presented.Comment: 24 pages, LaTeX fil
ISWI remodeling complexes in Xenopus egg extracts: Identification as major chromosomal components that are regulated by INCENP-aurora B
We previously characterized major components of mitotic chromosomes assembled in Xenopus laevis egg extracts and collectively referred to them as Xenopuschromosomeâassociated polypeptides (XCAPs). They included five subunits of the condensin complex essential for chromosome condensation. In an effort to identify novel proteins involved in this process, we have isolated XCAP-F and found it to be theXenopus ortholog of ISWI, a chromatin remodeling ATPase. ISWI exists in two major complexes in Xenopus egg extracts. The first complex contains ACF1 and two low-molecular-weight subunits, most likely corresponding to Xenopus CHRAC. The second complex is a novel one that contains theXenopus ortholog of the human Williams syndrome transcription factor (WSTF). In the absence of the ISWI complexes, the deposition of histones onto DNA is apparently normal, but the spacing of nucleosomes is greatly disturbed. Despite the poor spacing of nucleosomes, ISWI depletion has little effect on DNA replication, chromosome condensation or sister chromatid cohesion in the cell-free extracts. The association of ISWI with chromatin is cell cycle regulated and is under the control of the INCENP-aurora B kinase complex that phosphorylates histone H3 during mitosis. Apparently contradictory to the generally accepted model, we find that neither chromosome condensation nor chromosomal targeting of condensin is compromised when H3 phosphorylation is drastically reduced by depletion of INCENP-aurora B
Equivalence of three-dimensional spacetimes
A solution to the equivalence problem in three-dimensional gravity is given
and a practically useful method to obtain a coordinate invariant description of
local geometry is presented. The method is a nontrivial adaptation of Karlhede
invariant classification of spacetimes of general relativity. The local
geometry is completely determined by the curvature tensor and a finite number
of its covariant derivatives in a frame where the components of the metric are
constants. The results are presented in the framework of real two-component
spinors in three-dimensional spacetimes, where the algebraic classifications of
the Ricci and Cotton-York spinors are given and their isotropy groups and
canonical forms are determined. As an application we discuss Goedel-type
spacetimes in three-dimensional General Relativity. The conditions for local
space and time homogeneity are derived and the equivalence of three-dimensional
Goedel-type spacetimes is studied and the results are compared with previous
works on four-dimensional Goedel-type spacetimes.Comment: 13 pages - content changes and corrected typo
A cosmological model in Weyl-Cartan spacetime
We present a cosmological model for early stages of the universe on the basis
of a Weyl-Cartan spacetime. In this model, torsion and
nonmetricity are proportional to the vacuum polarization.
Extending earlier work of one of us (RT), we discuss the behavior of the cosmic
scale factor and the Weyl 1-form in detail. We show how our model fits into the
more general framework of metric-affine gravity (MAG).Comment: 19 pages, 5 figures, typos corrected, uses IOP style fil
On limits of spacetimes -- a coordinate-free approach
A coordinate-free approach to limits of spacetimes is developed. The limits
of the Schwarzschild metric as the mass parameter tends to 0 or are
studied, extending previous results. Besides the known Petrov type D and 0
limits, three vacuum plane-wave solutions of Petrov type N are found to be
limits of the Schwarzschild spacetime.Comment: 19 p
The Principle of Symmetric Criticality in General Relativity
We consider a version of Palais' Principle of Symmetric Criticality (PSC)
that is applicable to the Lie symmetry reduction of Lagrangian field theories.
PSC asserts that, given a group action, for any group-invariant Lagrangian the
equations obtained by restriction of Euler-Lagrange equations to
group-invariant fields are equivalent to the Euler-Lagrange equations of a
canonically defined, symmetry-reduced Lagrangian. We investigate the validity
of PSC for local gravitational theories built from a metric. It is shown that
there are two independent conditions which must be satisfied for PSC to be
valid. One of these conditions, obtained previously in the context of
transverse symmetry group actions, provides a generalization of the well-known
unimodularity condition that arises in spatially homogeneous cosmological
models. The other condition seems to be new. The conditions that determine the
validity of PSC are equivalent to pointwise conditions on the group action
alone. These results are illustrated with a variety of examples from general
relativity. It is straightforward to generalize all of our results to any
relativistic field theory.Comment: 46 pages, Plain TeX, references added in revised versio
On the limits of Brans-Dicke spacetimes: a coordinate-free approach
We investigate the limit of Brans-Dicke spacetimes as the scalar field
coupling constant omega tends to infinity applying a coordinate-free technique.
We obtain the limits of some known exact solutions. It is shown that these
limits may not correspond to similar solutions in the general relativity
theory.Comment: LaTeX, 16 pp, report DF/UFPB/02-9
The type N Karlhede bound is sharp
We present a family of four-dimensional Lorentzian manifolds whose invariant
classification requires the seventh covariant derivative of the curvature
tensor. The spacetimes in questions are null radiation, type N solutions on an
anti-de Sitter background. The large order of the bound is due to the fact that
these spacetimes are properly , i.e., curvature homogeneous of order 2
but non-homogeneous. This means that tetrad components of are constant, and that essential coordinates first appear as
components of . Covariant derivatives of orders 4,5,6 yield one
additional invariant each, and is needed for invariant
classification. Thus, our class proves that the bound of 7 on the order of the
covariant derivative, first established by Karlhede, is sharp. Our finding
corrects an outstanding assertion that invariant classification of
four-dimensional Lorentzian manifolds requires at most .Comment: 7 pages, typos corrected, added citation and acknowledgemen
Vacuum solutions which cannot be written in diagonal form
A vacuum solution of the Einstein gravitational field equation is given that
follows from a general ansatz but fails to follow from it if a certain
symmetric matrix is assumed to be in diagonal form from the beginning.Comment: 18 pages, latex, no figures. An Acknowledgement, 4 references, and
the section "Note added" are adde
Symmetric vacuum scalar--tensor cosmology
The existence of point symmetries in the cosmological field equations of
generalized vacuum scalar--tensor theories is considered within the context of
the spatially homogeneous cosmologies. It is found that such symmetries only
occur in the Brans--Dicke theory when the dilaton field self--interacts.
Moreover, the interaction potential of the dilaton must take the form of a
cosmological constant. For the spatially flat, isotropic model, it is shown how
this point symmetry may be employed to generate a discrete scale factor duality
in the Brans--Dicke action.Comment: 10 pages, latex, To appear in Class. Quantum Gra
- âŠ