Abstract

I will discuss ongoing work on a new formalism for the rigorous computation of cosmological observables, which is important in the era of precision cosmology. The conceptual key is that what we measure are the components of the photon momentum vector and polarization tensor with respect to an orthonormal basis privileged by the observer, i.e. a tetrad. Working exclusively in such a basis all over space-time (the tetrad formalism of GR) leads to explicitly coordinate-independent expressions where the involved quantities are the ones that are actually being measured. Most importantly, it also allows one to parametrize these quantities on the past light-cone by using the fundamental observables that are the redshift and observed angles, thus leading to the relations that are actually constrained by observations. After discussing the required geometrical framework (“observer 3-cylinder”), which is constructed at the fully non-linear background-independent level, I will illustrate its features by focusing on the case of weak lensing observables.