XXV Workshop on Geometric Methods in Physics 2-8.07.2006

Marc Lachieze-Rey

Frames and observers in (cosmological) spacetime

The covariance (or diffeomorphism invariance) of general relativity implies that a physical result should not depend on a choice of frame, or of a space + time splitting. On the other hand, frames are convenient to perform explicit calculations, to interpret observational results from the point of view of an observer, or to apply a quantization procedure.
After general comments about frames and covariance, I will consider the various properties of possible frames in relativity and cosmology. I will introduce three particular kinds of frames with specific properties, and comment their pertinence in relativity and cosmology:
(i) the ‚ radar frames‚ provide a given observer with a precise and operational ability of synchronization in a large neighborhood.
(ii) Beside nice mathematical properties, the ‚osculating frame‚ allows exact and practical calculations in cosmology.
(iii) The GPS frames present an unique collection of properties : they are covariant, independent of any observer, and allow direct measurement properties. They are very well suited for astrometry, monitoring, cosmography and tests of gravitation.