Tetrad formalism for exact cosmological observables

Understand

The standard description of cosmological observables is incomplete, because it does not take into account the correct angular parametrization of the sky, i.e.

• the one determined by the observer frame.
• The corresponding corrections must be taken into account for reliable results at non-linear orders.
• This can be accomplished by introducing an orthonormal basis, or "tetrad", at the observer point, representing the frame with respect to which observations are performed.