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.
The statistical properties of the primordial fluctuations can be inferred from observations of anisotropies in the cosmic microwave background and from measurements of the distribution of matter, e.g., galaxy redshift surveys.
Since the fluctuations are believed to arise from inflation, such measurements can also set constraints on parameters within inflationary theory.
Primordial fluctuations are typically quantified by a power spectrum which gives the power of the variations as a function of spatial scale.
Within this formalism, one usually considers the fractional energy density of the fluctuations, given by: where
can then be defined via the ensemble average of the Fourier components: There are both scalar and tensor modes of fluctuations.
[clarification needed] Scalar modes have the power spectrum defined as the mean squared density fluctuation for a specific wavenumber
, i.e., the average fluctuation amplitude at a given scale: Many inflationary models predict that the scalar component of the fluctuations obeys a power law[why?]
[1] The scalar spectral index describes how the density fluctuations vary with scale.
As the size of these fluctuations depends upon the inflaton's motion when these quantum fluctuations are becoming super-horizon sized, different inflationary potentials predict different spectral indices.
These depend upon the slow roll parameters, in particular the gradient and curvature of the potential.
On the other hand, models such as monomial potentials predict a red spectral index
[2] The presence of primordial tensor fluctuations is predicted by many inflationary models.
2015 CMB data from the Planck satellite gives a constraint of
For isocurvature fluctuations, the number density variations for one component do not necessarily correspond to number density variations in other components.
Current cosmic microwave background data favor adiabatic fluctuations and constrain uncorrelated isocurvature cold dark matter modes to be small.