  We revisit the {\it origin of structures problem} of standard
Friedmann-Robertson-Walker cosmology to point out an unjustified approximation
in the prevalent analysis. We follow common procedures in statistical mechanics
to revise the issue without the disputed approximation. Our conclusions
contradict the current wisdom and reveal and unexpected scenario for the origin
of primordial cosmological structures. We show that standard physics operating
in the cosmic plasma during the radiation dominated expansion of the universe
produce at the time of decoupling scale invariant density anisotropies over
cosmologically large comoving volumes. Scale invariance is shown to be a direct
consequence of the causality constrains imposed by the short FRW comoving
horizon at decoupling, which strongly suppress the power spectrum of density
fluctuations with cosmologically large comoving wavelength. The global
amplitude of these cosmological density anisotropies is fixed by the power
spectrum in comoving modes whose wavelength is shorter than the causal horizon
at the time and can be comparable to the amplitude of the primordial
cosmological inhomogeneities imprinted in the cosmic microwave background
radiation.
