In a completely generic way, structure formation involves the amplification of density enhancements through the actions of self gravity.

In a static case, once sufficient mass is available to overcome the internal energy in some volume, that mass will collapse and the size of the density enhancement will increase. This condition is known as the Jeans criteria - we will return to it later.

In a normal static situation, the internal energy or pressure of a gas is simply related to its temperature. Hence, cool clouds of gas can gravitational collapse while hot clouds would dissipate.

However, in the early universe there is an additional source of "pressure" and that is radiation pressure. Since the radiation and matter are coupled, then the radiation is trying to smooth out the distribution of matter (e.g. wash out any density enhancements).

At a general level, its unclear if purely baryonic density fluctuations could survive the effects of radiation pressure (also called radiation drag). To mitigate the effects of radiation pressure, it is desireable to have a new form of gravitating matter, one that can gravitationally clump but not interact with radiation. This matter, often called dark matter, could therefore provide the seed density fluctuations around which structure will later grow.

We can now also do a quick estimate, based on observed large scale structure, what we might expect the for the amplitude of any anisotropy in the microwave background.

Structure will amplify in the matter dominated universe as z^3/2. Since the redshift at decoupling was z ~ 1100, the amplification factor is 40,000. Hence, perturbations as small as 1/40,000 could have been amplified to produce the factor of 2 overdensities that we observe in large scale structure today. Hence, the expected temperature anisotropy at the surface of last scattering is 1/40000 = 2.5 x 10^-5 which is consistent with the WMAP observations.

To illustrate the profound effects of radiation pressure on the suppression of the growth of structure in the radiation dominated era (e.g. the first 300,000 years), we can do a Jeans Mass analysis.

The Jeans criterion is that the gravitational potential energy of a could of gas must overcome its internal energy (IE) in order to for collapse to occur. The IE for a fluid is its pressure times its volume. This criteria can be stated in terms of density and pressure as shown below:

In the radiation dominated era the pressure is significant and is 1/3c²ρ . After the radiation era, the pressure will drop by about a factor of 10⁹ (the effective photon to baryon ratio).

At recombination the density of the universe is ~ 10^-21 g/cc.

The Jeans mass prior to recombination is 5 x 10¹⁸ solar masses and we don't observe structures this large.

After recombination the jeans mass lowers to 2 x 10⁵ solar masses, which is the mass of a globular cluster.