  Starting from the massless form factor expansion for the two-point dynamical
correlation functions obtained recently, I extract the long-distance and
large-time asymptotics of these correlators.
  The analysis yields the critical exponents and associated amplitudes
characterising the asymptotics.
  The results are obtained on the basis of exact and first principle based
considerations: they \textit{do not rely, at any stage}, on some hypothetical
correspondence with a field theory or the use of any other phenomenological
approach. Being based on form factor expansion, the method allows one to
clearly identify which contributions to the asymptotics issues from which class
of excited states. All this permits to settle the long-standing question of the
contribution of bound states to the asymptotics of two-point functions. For
instance, when considering the long-distance $m$ behaviour of equal-time
correlators, the analysis shows that while, \textit{in fine}, the bound states
only produce contributions that are exponentially small in $m$, they also play
a key role in cancelling out certain power-law contributions which, should they
be present, would break explicitly the universality structure of the
long-distance behaviour.
