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Cosmological structure formation is the gravitational amplification of initial linear cosmological density perturbations into the cosmic web and bound objects. Overdense regions attract extra matter while surrounding regions become relatively empty. In the cold dark matter picture, small initial particle velocities preserve fluctuations that stronger free streaming would erase. The resulting spectrum of seed fluctuations influences the populations of galaxies and clusters; gas cooling and feedback add further physics to their visible contents.^[ref]

George R. Blumenthal helped formulate the cold dark matter account of the formation of galaxies and larger cosmic structures. The 1984 paper with Sandra Faber, Joel Primack and Martin Rees considered matter whose early velocity dispersion was sufficiently small that free streaming would not erase fluctuations on galactic scales.

This connects a proposed microscopic property of dark matter to observable cosmic structure: surviving density fluctuations grow through gravity, and their initial spectrum constrains the later distribution of galaxies. The paper evaluated a model with calculable consequences, rather than merely adding unseen mass to individual systems.^[ref]

Joel Primack helped formulate cold dark matter as a quantitative framework for galaxy formation. The 1984 study with George R. Blumenthal, Sandra Faber and Martin Rees linked a primordial fluctuation spectrum to the formation of galaxies and galaxy clusters when collisionless particles have sufficiently small early velocity dispersion.^[ref]

Here “cold” concerns the particles' early random velocities, not whether the later galaxies are thermally cold.

cold dark matter is nonrelativistic and effectively collisionless on the scales of interest. Its small velocity dispersion limits free streaming, allowing small-scale gravitational wells to survive. baryons coupled to photons undergo acoustic motion and damping before recombination; dark matter does not share that photon pressure, and provides potential wells into which baryons can later fall.

For a subhorizon pressureless perturbation, once a Newtonian treatment is applicable,During radiation domination the matter contribution to the background gravity is small; neglecting it gives $\delta \simeq C_1+C_2\log t$. This slow subhorizon growth is the Mészáros effect. During matter domination,in an Einstein-de Sitter universe. Once $\delta$ approaches unity, linear theory fails and collapse, virialization and merging become important.

Modes entering the horizon during radiation domination therefore grow less before equality than longer modes entering later. This scale dependence is encoded in a transfer function, with a schematic linear matter power spectrum $P(k,a)=A{k}^{n_s}{T}^2(k)D^2(a)$. cold dark matter supports hierarchical assembly; a sufficiently large warm- or hot-dark-matter free-streaming length suppresses small-scale structure. Superhorizon density contrasts change under a gauge transformation in cosmological perturbation theory, so the subhorizon growth laws should not be applied indiscriminately outside the horizon. The original cold-dark-matter formation model explicitly connects weak free streaming to the spectrum that seeds galaxies and clusters.^[ref]

Sandra Faber contributed to the cold dark matter explanation of galaxy formation. With George R. Blumenthal, Joel Primack and Martin Rees, she developed the 1984 account in which small early particle velocity dispersion preserves galaxy-scale density fluctuations against free streaming.^[ref]

The central test connects particle behaviour to astronomical structure: the surviving fluctuation spectrum determines which scales can seed galaxies and galaxy clusters.