Formation and evolution of large-scale structure

Understanding the formation and evolution of the largest and earliest structures (ie, quasars, galaxies, clusters and superclusters) is one of the largest efforts in cosmology. Cosmologists study a model of hierarchical structure formation in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling. The most straightforward way to study structure in the universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the universe and measure the matter power spectrum. This is the approach of the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.

An important tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into filaments, superclusters and voids. Most simulations contain only non-baryonic cold dark matter, which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy.

Other, complementary techniques will allow cosmologists to measure the distribution of matter in the distant universe and to probe reionization. These include:

* The Lyman alpha forest, which allows cosmologists to measure the distribution of neutral atomic hydrogen gas in the early universe, by measuring the absorption of light from distant quasars by the gas.
* The 21 centimeter absorption line of neutral atomic hydrogen also provides a sensitive test of cosmology
* Weak lensing, the distortion of a distant image by gravitational lensing due to dark matter.

These will help cosmologists settle the question of when the first quasars formed

Big Bang Nucleosynthesis

Big Bang Nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when the universe was about three minutes old and its temperature fell enough that nuclear fusion ceased. Because the time in which big bang nucleosynthesis occurred was so short, only the very lightest elements were produced, unlike in stellar nucleosynthesis. Starting from hydrogen ions (protons), it principally produced deuterium, helium-4 and lithium. Other elements were produced in only trace abundances. While the basic theory of nucleosynthesis has been understood for decades (it was developed in 1948 by George Gamow, Ralph Asher Alpher and Robert Herman) it is an extremely sensitive probe of physics at the time of the big bang, as the theory of big bang nucleosynthesis connects the abundances of primordial light elements with the features of the early universe. Specifically, it can be used to test the equivalence principle, to probe dark matter and test neutrino physics. Some cosmologists have proposed that big bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.