Dark matter

Evidence from big bang nucleosynthesis, the cosmic microwave background and structure formation suggests that about 25% of the mass of the universe consists of non-baryonic dark matter, whereas only 4% consists of visible, baryonic matter. The gravitational effects of dark matter are well understood, as it behaves like cold, non-radiative dust which forms around haloes around galaxies. Dark matter has never been detected in the laboratory: the particle physics nature of dark matter is completely unknown. However, there are a number of candidates, such as a stable supersymmetric particle, a weakly interacting massive particle, an axion, and a massive compact halo object. Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations (MOND) or an effect from brane cosmology.

The physics at the center of galaxies (see active galactic nuclei, supermassive black hole) may give some clues about the nature of dark matter.

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.

Spiral galaxies

The earliest modern theory of the formation of our galaxy (known by astronomers as ELS, after the initials of the authors of that paper, Olin Eggen, Donald Lynden-Bell, and Allan Sandage[3]) describes a single (relatively) rapid monolithic collapse, with the halo forming first, followed by the disk. Another view published in 1978 (known as SZ for its authors, Leonard Searle and Robert Zinn[4]) describes a more gradual process, with smaller units collapsing first, then later merging to form the larger components. An even more recent idea is that significant portions of the stellar halo could be stellar debris from destroyed dwarf galaxies and globular clusters that once orbited the Milky Way. The halo would then be a "newer" component made of "recycled" old parts.

In recent years, a great deal of focus has been put on understanding merger events in the evolution of galaxies. Rapid technological progress in computers have allowed much better simulations of galaxies, and improved observational technologies have provided much more data about distant galaxies undergoing merger events. After the discovery in 1994 that our own Milky Way has a satellite galaxy (the Sagittarius Dwarf Elliptical Galaxy, or SagDEG) which is currently gradually being ripped up and "eaten" by the Milky Way, it is thought these kinds of events may be quite common in the evolution of large galaxies. The Magellanic Clouds are satellite galaxies of the Milky Way that will almost certainly share the same fate as the SagDEG. A merger with a fairly large satellite galaxy could explain why M31 (the Andromeda Galaxy) appears to have a double core.

The SagDEG is orbiting our galaxy at almost a right angle to the disk. It is currently passing through the disk; stars are being stripped off of it with each pass and joining the halo of our galaxy. Eventually, only the core of SagDEG will exist. Although it will have the same mass as a large globular cluster like Omega Centauri and G1, it will appear rather different, as it has far lower surface density due to the presence of substantial amounts of dark matter, while globular clusters appear, mysteriously, to contain very little dark matter.

Further examples of satellite dwarf galaxies that are in the process of merging with the Milky Way are the Canis Major Dwarf Galaxy, discovered in 2003 and thought to be responsible for the Monoceros Ring, and the Virgo Stellar Stream, discovered in 2005.