The S-matrix for a QFT in 4D Minkowski space is an inherently holographic object, i.e. defined
at the (conformal) boundary of spacetime. A section of this boundary is the celestial 2-sphere
and Lorentz group acts on it by conformal transformations. I will briefly review scattering,
when translated from the basis of plane waves (translation eigenstates) to the conformal basis
(dilatation eigenstates). The resulting object is called a celestial amplitude and the change of
basis is implemented for massless particles by a Mellin transform. I will apply this formalism to
amplitudes of Goldstone bosons with an emphasis on their soft theorems. The illustrative example
will be the U(1) (non)-linear sigma model.