The posts below constitute a brief introduction to the Big Bang model: the three planks of evidence, the problems of singularity, horizon and flatness, and the theory of inflation. Before we go on to discuss the Standard Model of cosmology (yes, there is such a thing), two further concepts are necessary: the old puzzle of Dark Matter and the new puzzle of Dark Energy.
Dark matter is thought to make up about 70% of the matter of the universe. Although we can’t see it, we presume it exists because of its gravitational effect on visible matter. Put differently, we don’t insist that all matter be ‘visible’ i.e. interact with the electromagnetic force. Instead, we include the possibility that some matter may be seen only by its gravitational effect on other matter.
DM was first postulated by Fritz Zwicky in the 1930s to account for a discrepancy between the calculated velocity of spiral galaxies and that observed. Nowadays, it has been proposed to account for the motion of many astrophysical phenomena from the smaller scales to the largest e.g. local stellar dynamics, galaxy rotation, galaxy cluster dynamics, X-ray halos, gravitational lensing and cluster streaming.
Calculations for galaxy rotation based on ordinary matter (curve A) and experimental points (curve B)
A second pointer of evidence for Dark Matter comes from cosmology, in particular from the cosmic microwave background (see post below). In order to to relate the miniscule variations in temperature seen in the CMB to galaxy formation in the early universe, all current models invoke the existence of DM. Even more importantly, the existence of DM is necessary to provide enough gravity to explain the flatness of the universe, as measured from the CMB (in conjunction with the postulate of dark energy – see next week).
It should be pointed out that not everyone agrees with the postulate of Dark Matter. Skeptics point to the possibility that our laws of gravity (both Newtonian and Einsteinian) may be failing at the largest scales – a theory known as modified Newtonian dynamics or MOND. However, most cosmologists now consider this possibility unlikely, due to the astrophysical and cosmological evidence above.
Best of all, the first hint of direct evidence DM was reported in a study of galaxy collision in 2007. If Dark Matter really exists, one might expect to observe ‘galaxy splitting’ in the case of galaxy collision. This is because the DM of each galaxy should interact little with the other, while the ordinary matter of each will interact strongly (just as a couple crossing a crowded room soon become separated if one is more social than the other!). Researchers at the University of Arizona are pretty sure this is exactly what they observed (see here). A similar result was reported by NASA in September 2008.
The famous bullet cluster collision (2007)
What could Dark Matter be made up of? Clearly, DM particles must be weakly interacting (otherwise we would see them) and possibly massive – i.e. weakly interacting massive particles or WIMPs. It is currently thought that the most likely candidates might be supersymmetric particles. (As we saw before, the theory of supersymmetry (SUSY) arises out of attempts to unify three of the fundamental forces – the theory postulates that every normal particle has a heavier supersymmetric partner). It turns out the most likely candidiate for DM is the neutralino, the lightest SUSY particle which cannot decay further.
Many groups around the world have been constructing experiments to look for particles that might be candidates for Dark Matter – you can find a post on a lecture on this subject by Tim Sumner of the Zeplin III experiment here and there is a very good overview of the Zeplin experiment itself here . However, this is straying into the area of particle physics; for cosmologists, establishing the existence of DM unequivocally is the real challenge.