Primordial fluctuations are density variations in the early universe which are considered the seeds of all
structure in the universe. Currently, the most widely accepted explanation for their origin is in the context of
cosmic inflation. According to the inflationary paradigm, the exponential growth of the
scale factor during inflation caused
quantum fluctuations of the inflaton field to be stretched to macroscopic scales, and, upon leaving the horizon, to "freeze in". At the later stages of radiation- and matter-domination, these fluctuations re-entered the horizon, and thus set the
initial conditions for
structure formation.