English: This figure depicts two linear modes during the formation of structure in our universe. The two modes have the same wavelength. The dashed line is a cold dark matter (CDM) mode and the solid line is a baryon mode. The modes enter the (Hubble) horizon volume during radiation domination. The evolution of the modes up to this point is not very important, as it looks different for different choices of gauge. Upon horizon entry the baryonic mode is coupled to the baryon-radiation fluid, and oscillates as pressure prevents overdensities from collapsing below the Jeans mass. The CDM mode grows logarithmically during radiation domination. At matter-radiation equality the CDM mode begins to grow more quickly. At subsequent decoupling of radiation from the baryons (and consequent enormous drop in the Jeans mass for baryons), baryons start falling into the gravitational potentials established by the CDM, and the baryonic mode begins to closely mimic the CDM mode. This figure thus illustrates the pivotal importance of CDM for structure formation. Horizon entry and decoupling are marked at the top of the figure. The calculation was performed for current best-fit values of the Lambda-CDM model. The mode has wavelength exactly one-thousandth the Hubble radius today, or approximately 4.2 Mpc. The figure includes all linear physics except that decoupling is treated as an instantaneous process occurring at redshift 1100. The basic formalism used to compute all this can be found in T. Padmanabhan, Structure formation in the universe (cited in physical cosmology).