39 research outputs found

### Do Spinors Frame-Drag?

We investigate the effect of the intrinsic spin of a fundamental spinor field
on the surrounding spacetime geometry. We show that despite the lack of a
rotating stress-energy source (and despite claims to the contrary) the
intrinsic spin of a spin-half fermion gives rise to a frame-dragging effect
analogous to that of orbital angular momentum, even in Einstein-Hilbert gravity
where torsion is constrained to be zero. This resolves a paradox regarding the
counter-force needed to restore Newton's third law in the well known spin-orbit
interaction. In addition, the frame-dragging effect gives rise to a {\it
long-range} gravitationally mediated spin-spin dipole interaction coupling the
{\it internal} spins of two sources. We argue that despite the weakness of the
interaction, the spin-spin interaction will dominate over the ordinary inverse
square Newtonian interaction in any process of sufficiently high-energy for
quantum field theoretical effects to be non-negligible.Comment: V2: published version, mostly minor clarifications from V

### On the extension of the concept of Thin Shells to The Einstein-Cartan Theory

This paper develops a theory of thin shells within the context of the
Einstein-Cartan theory by extending the known formalism of general relativity.
In order to perform such an extension, we require the general non symmetric
stress-energy tensor to be conserved leading, as Cartan pointed out himself, to
a strong constraint relating curvature and torsion of spacetime. When we
restrict ourselves to the class of space-times satisfying this constraint, we
are able to properly describe thin shells and derive the general expression of
surface stress-energy tensor both in its four-dimensional and in its
three-dimensional intrinsic form. We finally derive a general family of static
solutions of the Einstein-Cartan theory exhibiting a natural family of null
hypersurfaces and use it to apply our formalism to the construction of a null
shell of matter.Comment: Latex, 21 pages, 1 combined Latex/Postscript figure; Accepted for
publication in Classical and Quantum Gravit

### Negative Energy Densities in Extended Sources Generating Closed Timelike Curves in General Relativity with and without Torsion

Near a spinning point particle in (2+1)-dimensional gravity (or near an
infinitely thin, straight, spinning string in 3+1 dimensions) there is a region
of space-time with closed timelike curves. Exact solutions for extended sources
with apparently physically acceptable energy-momentum tensors, have produced
the same exterior space-time structure. Here it is pointed out that in the case
with torsion, closed timelike curves appear only for spin densities so high
that the spin energy density is higher than the net effective energy density.
In models without torsion, the presence of closed time-like curves is related
to a heat flow of unphysical magnitude. This corroborates earlier arguments
against the possibility of closed timelike curves in space-time geometries
generated by physical sources.Comment: (to be published in Phys. Rev. D), 5 pages, REVTEX 3.0, NORDITA 93/62
A (Sept. 10/Revised Nov. 1, 1993

### Gravitational Phase Operator and Cosmic Strings

A quantum equivalence principle is formulated by means of a gravitational
phase operator which is an element of the Poincare group. This is applied to
the spinning cosmic string which suggests that it may (but not necessarily)
contain gravitational torsion. A new exact solution of the Einstein-
Cartan-Sciama-Kibble equations for the gravitational field with torsion is
obtained everywhere for a cosmic string with uniform energy density, spin
density and flux. A novel effect due to the quantized gravitational field of
the cosmic string on the wave function of a particle outside the string is used
to argue that spacetime points are not meaningful in quantum gravity.Comment: 22 pages, to be published Phys. Rev. D. Some minor changes have been
made and a reference has been added to the paper of D.V. Gal'tsov and P.S.
Letelier, Phys. Rev. D 47 (1993) 4273, which first contained the metric (2.2)
external to the cosmic string. The present paper extends this solution to a
regular solution inside the string as wel

### Volterra Distortions, Spinning Strings, and Cosmic Defects

Cosmic strings, as topological spacetime defects, show striking resemblance
to defects in solid continua: distortions, which can be classified into
disclinations and dislocations, are line-like defects characterized by a delta
function-valued curvature and torsion distribution giving rise to rotational
and translational holonomy. We exploit this analogy and investigate how
distortions can be adapted in a systematic manner from solid state systems to
Einstein-Cartan gravity. As distortions are efficiently described within the
framework of a SO(3) {\rlap{\supset}\times}} T(3) gauge theory of solid
continua with line defects, we are led in a straightforward way to a Poincar\'e
gauge approach to gravity which is a natural framework for introducing the
notion of distorted spacetimes. Constructing all ten possible distorted
spacetimes, we recover, inter alia, the well-known exterior spacetime of a
spin-polarized cosmic string as a special case of such a geometry. In a second
step, we search for matter distributions which, in Einstein-Cartan gravity, act
as sources of distorted spacetimes. The resulting solutions, appropriately
matched to the distorted vacua, are cylindrically symmetric and are interpreted
as spin-polarized cosmic strings and cosmic dislocations.Comment: 24 pages, LaTeX, 9 eps figures; remarks on energy conditions added,
discussion extended, version to be published in Class. Quantum Gra

### Gravitational field around a screwed superconducting cosmic string in scalar-tensor theories

We obtain the solution that corresponds to a screwed superconducting cosmic
string (SSCS) in the framework of a general scalar-tensor theory including
torsion. We investigate the metric of the SSCS in Brans-Dicke theory with
torsion and analyze the case without torsion. We show that in the case with
torsion the space-time background presents other properties different from that
in which torsion is absent. When the spin vanish, this torsion is a
$\phi$-gradient and then it propagates outside of the string. We investigate
the effect of torsion on the gravitational force and on the geodesics of a
test-particle moving around the SSCS. The accretion of matter by wakes
formation when a SSCS moves with speed $v$ is investigated. We compare our
results with those obtained for cosmic strings in the framework of
scalar-tensor theory.Comment: 22 pages, LaTeX, presented at the "XXII - Encontro Nacional de Fisica
de Particulas e Campos", Sao Lourenco, MG, Brazi

### Cosmological model with macroscopic spin fluid

We consider a Friedmann-Robertson-Walker cosmological model with some exotic
perfect fluid with spin known as the Weyssenhoff fluid. The possibility that
the dark energy may be described in part by the Weyssenhoff fluid is discussed.
The observational constraint coming from supernovae type Ia observations is
established. This result indicates that, whereas the cosmological constant is
still needed to explain current observations, the model with spin fluid is
admissible. For high redshifts $z > 1$ the differences between the model with
spin fluid and the cold dark matter model with a cosmological constant become
detectable observationally for the flat case with $\Omega_{\text{m},0}=0.3$.
From the maximum likelihood method we obtain the value of
$\Omega_{\text{s},0} = 0.004 \pm 0.016$. This gives us the limit
$\Omega_{\text{s},0} > -0.012$ at the $1\sigma$ level. While the model with
``brane effects'' is preferred by the supernovae Ia data, the model with spin
fluid is statistically admissible. For comparison, the limit on the spin fluid
coming from cosmic microwave background anisotropies is also obtained. The
uncertainties in the location of a first peak give the interval $-1.4 \times
10^{-10} < \Omega_{\text{s},0} < -10^{-10}$. From big bang nucleosynthesis we
obtain the strongest limit $\Omega_{\text{s},0} \gtrsim -10^{-20}$. The
interconnection between the model considered and brane models is also pointed
out.Comment: RevTeX4, 15 pages, 10 figures; some minor change

### Gravitational field around a time-like current-carrying screwed cosmic string in scalar-tensor theories

In this paper we obtain the space-time generated by a time-like
current-carrying superconducting screwed cosmic string(TCSCS). This
gravitational field is obtained in a modified scalar-tensor theory in the sense
that torsion is taken into account. We show that this solution is comptible
with a torsion field generated by the scalar field $\phi$. The analysis of
gravitational effects of a TCSCS shows up that the torsion effects that appear
in the physical frame of Jordan-Fierz can be described in a geometric form
given by contorsion term plus a symmetric part which contains the scalar
gradient. As an important application of this solution, we consider the linear
perturbation method developed by Zel'dovich, investigate the accretion of cold
dark matter due to the formation of wakes when a TCSCS moves with speed $v$ and
discuss the role played by torsion. Our results are compared with those
obtained for cosmic strings in the framework of scalar-tensor theories without
taking torsion into account.Comment: 21 pages, no figures, Revised Version, presented at the "XXIV-
Encontro Nacional de Fisica de Particulas e Campos ", Caxambu, MG, Brazil, to
appear in Phys. Rev.