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We discuss a new formalism for light propagation which can be used within theregime of validity of geometric optics, but with no limitation on the scales ofinterest: from inside the Galaxy to the ultra-large scales of cosmology. One ofour main results is that within this framework it is possible to calculate allrelevant observables (image magnification, parallax, position drift or propermotion) by simply differentiating the photon trajectory with respect to theinitial data. We then focus on a new observable, which we name the distanceslip: it is defined as the relative difference between the angular diameterdistance and the parallax distance. Its peculiarity lies in the fact that itsvalue is independent of the momentary motions of both the source and theobserver and that for short distances it shows a tomographic property, beingproportional to the amount of matter along the line of sight. After describingfurther its properties and methods of measurement, we specialize our study ofthe distance slip to cosmology. We show that it does not depend on the Hubbleconstant H_0 and that its dependence on the other cosmological parameters isdifferent from other distance indicators. This suggests that the distance slipmay contain new information.

Geometric optics in relativistic cosmology: new formulation and a new observable