Editing Primordial fluctuations (section)

==Adiabatic/isocurvature fluctuations==

[[Adiabatic]] fluctuations are density variations in all forms of [[matter]] and [[energy]] which have equal fractional over/under densities in the [[number density]].  So for example, an adiabatic [[photon]] overdensity of a factor of two in the number density would also correspond to an [[electron]] overdensity of two.  For isocurvature fluctuations, the number density variations for one component do not necessarily correspond to number density variations in other components.  While it is usually assumed that the initial fluctuations are adiabatic, the possibility of isocurvature fluctuations can be considered given current cosmological data.  Current [[cosmic microwave background]] data favor adiabatic fluctuations and constrain uncorrelated isocurvature [[cold dark matter]] modes to be small.