7 research outputs found
Rehabilitating space-times with NUTs
We revisit the Taub-NUT solution of the Einstein equations without time
periodicity condition, showing that the Misner string is still fully
transparent for geodesics. In this case, analytic continuation can be carried
out through both horizons leading to a Hausdorff spacetime without a central
singularity, and thus geodesically complete. Furthermore, we show that, in
spite of the presence of a region containing closed time-like curves, there are
no closed causal {\em geodesics}. Thus, some longstanding obstructions to
accept the Taub-NUT solution as physically relevant are removed.Comment: 9 pages revtex 4, Journal versio
NUT wormholes
We show that supercritically charged black holes with NUT provide a new
setting for traversable wormholes. This does not require exotic matter, a price
being the Misner string singularities. Without assuming time periodicity to
make Misner strings unobservable, we show that, contrary to expectations,
geodesics do not stop there. Moreover, since there is no central singularity
the space-time turns out to be geodesically complete. Another unpleasant
feature of spacetimes with NUTs is the presence of regions where the azimuthal
angle becomes timelike, signalling the appearance of closed timelike
curves (CTCs). We show that among them there are no closed timelike or null
geodesics, so the freely falling observers should not encounter causality
violations. Considering worldlines of charged particles, we find that, although
these can become closed in the vicinity of the wormhole throat for large enough
charge-to-mass ratio, the non-causal orbits are still disconnected from the
distant zones. All these findings support our feeling that wormholes with NUTs
deserve to be taken seriously. Integrating the geodesic equations completely,
we demonstrate the existence of timelike and null geodesics connecting two
asymptotic regions of the wormhole, such that the tidal forces in the throat
are reasonably small. We discuss bounds on the NUT charge which follow from the
Schwinger pair creation and ionization thresholds and speculate that such NUT
wormholes could be present in some galactic centers.Comment: Journal version. New references added, discussion part slightly
modifie
Time delay in the Einstein-Straus solution
The time delay of strong lensing is computed in the framework of the
Einstein-Straus solution. The theory is compared to the observational bound on
the time delay of the lens SDSS J1004+4112.Comment: 20 pages, 4 tables, 1 figur