Friday, 12 July 2013

Structure Formation- Revisiting Dark Matter

Looking up at the night sky and into our interstellar neighbourhood, we see matter clumped into galaxies, clusters and super-clusters. Quite distinct from the early universe, where the relatively low levels of anisotropies in the cosmic microwave background (CMB) serves as a cue to the smooth and sleek distribution of baryonic matter at recombination. The evolution of large scale structure is thought to have arisen from the gravitational instability and collapse of regions initially denser than usual; hence such regions expand slower than the average expansion of the universe. The minute perturbations layed down quantum mechanically produce relative density fluctuations and depends on the balance between two effects. Firstly, the self-gravitation of matter in the over-dense region which has a tendency to cause collapse and secondly, the maintenance of hydrostatic equilibrium that serves to prevent collapse. A key cosmological parameter is the Jeans mass which plays the role of a border or limit between these two effects, if a region exceeds the Jeans mass, it will collapse. Similarly, the horizon distance (at any moment in the chronology of the universe, the maximum interval a signal could transverse in the time that had passed up to that moment) plays a critical role in stability against collapse; an overdense region exceeding the horizon distance can't support itself. Something interesting happens at recombination, at about 3000K and 300,000 years after the big bang; the, Jeans mass falls sharply to about the mass of globular clusters and before recombination, the interaction of photons with free electrons contributed to the overall pressure. After the recombination epoch, when the electrons cease to interact with photons, the only protection against collapse comes from the internal pressure of gas. However, gravitational collapse with cold (non-relativistic) dark matter seems to have caused density fluctuations before recombination; the dominant influence on baryons is the gravitational attraction of regions which have acquired over-densities of cold dark matter. This means that baryons were drawn into those collapsing clouds of dark matter and kick-started galaxy formation; a hierarchical process (bottom-up) whereby cold dark matter drew condensations against the overall expansion of the universe into redshifts between the orders of 100 and 40. Cool gas was drawn into dark matter halos into well-defined disks to produce the first spiral galaxies, so this provides an intuitive reason why ellipticals have no young stars because only where gas can collect and coalesce can stellar formation proceed. After recombination, the decoupling of photons from baryons allowed them to travel unhindered, causing the universe to become transparent and ushing in a period of darkness (the dark ages). Such dark ages ceased 400 million years (reionisation) after with the inception of the maiden generations of galaxies and other objects (quasars and Pop. III stars) that emit UV radiation, forming the initial ionised portion of cosmic gas which exponentially increases until the complete ionisiation of hydrogen. This highlights a point where ionised gas became just as important as cold dark matter in structure formation.

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