331 research outputs found
Observational Constraints on the Averaged Universe
Averaging in general relativity is a complicated operation, due to the
general covariance of the theory and the non-linearity of Einstein's equations.
The latter of these ensures that smoothing spacetime over cosmological scales
does not yield the same result as solving Einstein's equations with a smooth
matter distribution, and that the smooth models we fit to observations need not
be simply related to the actual geometry of spacetime. One specific consequence
of this is a decoupling of the geometrical spatial curvature term in the metric
from the dynamical spatial curvature in the Friedmann equation. Here we
investigate the consequences of this decoupling by fitting to a combination of
HST, CMB, SNIa and BAO data sets. We find that only the geometrical spatial
curvature is tightly constrained, and that our ability to constrain dark energy
dynamics will be severely impaired until we gain a thorough understanding of
the averaging problem in cosmology.Comment: 6 pages, 4 figure
Privacy through uncertainty in location-based services
Location-Based Services (LBS) are becoming more prevalent. While there are many benefits, there are also real privacy risks. People are unwilling to give up the benefits - but can we reduce privacy risks without giving up on LBS entirely?
This paper explores the possibility of introducing uncertainty into location information when using an LBS, so as to reduce privacy risk while maintaining good quality of service. This paper also explores the current uses of uncertainty information in a selection of mobile applications
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