Sunday, November 19, 2006

A "Short" History Of The Dark Side - Part 1

I am often asked what an astronomer does and what the main problems facing astronomy are today. The first question is easily answered, an astronomer spends most of their time in front of a PC trying to make sense of confused data that is never enough for the task. Occasionally you get to go observing to out of the way places like Hawaii or Chile, or to conferences in equally exotic locations where you argue over minor points inside a lecture theatre from dawn till dusk, avoiding the always lovely weather and interesting locals. Somedays it seems to be hard work, but on others you find something that no one has ever known before and on others still you get to sit 4 and a half kilometers in the air on top of a huge volcano and watch the sun set over the Pacific in absolute quiet. Its probably the best job in the world on days like those.

The other question is more difficult, it depends on which sub-field you work on, but I would guess that most people would agree that the most pressing area of research at the moment is investigating the so called "Dark Sector". That part of the Universe that is due to exotic particles or strange forces of nature. In this and the following posts in the series I'm going to try to pull together what I understand about Dark Matter and Dark Energy, perhaps even offer a few opinions. This first post does not actually deal with unusual objects or forces but explains a similar set of observations that can be explained using standard physics, I am doing this so that in the later posts I can explain the fundamental differences between the two cases, so onto the main post.

As an astronomer I am used to the fact that we are rarely able to see everything we need to to understand a given object completely. This is simply a by-product of the fact that we don't have infinitely sensitive instruments, so there will always be objects that are difficult or impossible to detect, objects like brown dwarfs and isolated neutron stars or black holes.

In many systems we cannot see these objects directly but we can observe the influence in other ways, in particular through the effects of their gravity, for example in the cores of Globular Clusters the velocity of the stars is so high that it can't be explained by all the mass we can see, if there wasn't some unseen mass whose gravity was holding the GC together the cluster would simply blow apart. This is fairly strong evidence that there must be something else at work here. Happily the amount of missing mass in GCs is consistent with what we would expect of the type of stellar populations that make up a GC, so we would expect some fraction of stars that are not large enough to make it to main sequence (Brown Dwarfs) and some stars to have already expended all of their fuel and to have died by now (white dwarfs, neutron stars and black holes). These objects are simply too faint to be seen in the GC which is why their mass is "missing". When you add up all the mass that should be in these stars its about enough to explain the mass deficit in GCs entirely in terms of normal (baryonic) matter.

In the second post in this series we will look at observations of galaxies and galaxy clusters and examine why the approach used for GCs cannot explain missing mass in these systems.