Relations between physical observables: what is better?
  We investigate some possible relations between physical observables and
estimate the "cosmic variance" which affects these measurements. We focus on
redshift and angular-distance and we discuss the difference in considering the
redshift as function of the angular-distance rather than the usually considered
inverse relation. Already at linear level in metric perturbations, we find a
significant difference. Indeed, even if both relations are led by source radial
velocity for close enough sources, this effect is suppressed by 2 orders of
magnitude in the redshift/angular-distance relation. This fact can
significantly reduce the theoretical uncertainty for close sources already
investigated in the literature for the angular-distance/redshift relation and
open a new scenario for clarifying the tension in the measurement of $H_0$ from
local sources rather than from the CMB.
