Spatial correlations of primordial density fluctuations in the standard cosmological model

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.