Hierarchical Structure Formation

Under models for hierarchical structure formation, little things collapse first and big things collapse later. This is the natural outcome of Cold Dark Matter, and has made it the leading cosmological model of structure formation.

So what really does collapse first?

In order to collapse, remember, a parcel of gas must be Jeans unstable. So let's calculate the Jeans mass immediately after recombination, when structure can first collapse.

Temperature: ~ 3000 K
Baryonic Density: we can get this from the present day baryonic density.
if we insert rough numbers, we get

So putting these numbers in, we find that just after recombination, the Jeans mass is

MJ ~ 2x106 Msun

which is just about the mass of a dwarf galaxy. Anything smaller than that could not collapse. Anything larger than that could, but more massive objects are characteristically less dense, and will take longer to collapse.

As larger structure develops, it collapses gravitationally and the formal Jeans equation does not really apply. Rather than massive clouds collapsing as one (monolithic collapse), we instead have ensembles of smaller objects (ie protogalaxies or galaxies) falling together to form larger structures.

Building structure hierarchically

This is a "merger tree", describing the evolution of structure in a hierarchical universe (from Lacey and Cole MNRAS 262 627 1993)

So when did this object form?

Can we watch this kind of structure form? Numerical simulations of cosmology! Here is the collapse of a portion of the universe which becomes a galaxy cluster. It is shown in comoving coordinates, where the expansion of the Universe is scaled out of the picture.

Courtesy Paul Bode, UPenn

This hierarchical collapse continues on smaller scales to form galaxy clusters.