A "Short" History Of The Dark Side - Part 2

In the previous post we saw how in certain circumstances it is possible to explain supposedly missing mass in terms of standard physics. I'm now moving onto a case where it appears that this isn't possible, the strange "Dark Matter" that seems to permeate space.


Its been known since the 1930s that if you add up the mass seen in clusters of galaxies (like that shown above) there isn't enough mass to account for the motion of the galaxies in the cluster. This is exactly analogous to the situation in GCs, where instead of individual stars appearing to be moving to fast its now entire galaxies moving too quickly. Over the years some of this missing mass has been found, in the form of very tenuous hot gas that resides between the galaxies in the cluster. The image below shows the Centaurus cluster as seen in X-Rays, the X-Rays are produced by the hot gas in between galaxies in the cluster. The gas itself is also of great use in determining the mass in the cluster, this is because we can measure the temperature and density of the gas and from this infer the gravity that must be present to stop the gas expanding out of the cluster and into intercluster space. Although this gas is very diffuse when its mass is added up it still adds up to more than the mass contained in the galaxies in the cluster but still it only makes up a small fraction of the total mass we know must be in the cluster from the motions of the galaxies.


Over time other manifestations of this missing mass has been seen, it was observed that spiral galaxies rotated too quickly to be explained by just the visible mass, then it was noticed that the stars in elliptical galaxies where also speeding around too quickly, finally that the GCs and dwarf galaxies around normal galaxies were themselves moving too quickly to be explained by the luminous mass of the parent galaxy. Other effects were noticed that do not rely on the kinematics of objects, it was observed that the bending of light due to the gravity of galaxy clusters and individual galaxies was too severe to be explained by the visible mass.

As it became clear that this invisible mass was a real phenomenon and not due to some problems with our models people attempted to explain this missing mass. Initially people attempted to explain this missing mass in terms of stellar remnants (like in the GCs) and/or gas and dust that doesn't emit light strongly. With our increasing ability to observe at different wavelengths of light where we would expect gas and dust to be emitting radiation it became clear that this gas and dust could only explain a small fraction of the missing mass. Similarly stellar population modeling showed that it was almost impossible to explain the missing mass as being due to stellar remnants, it would require far too many stars to have already died by now, this would only be possible if initially most stars that formed in a galaxy were very massive. This is not observed in nearby galaxies and from what we know of star formation is not expected to be the case in most situations.

So what is the solution to the puzzle? Well the one that is most popular is inclusion of some matter which is not made of the same material as normal atoms, this Dark Matter is non-baryonic and only interacts with luminous matter through the force of gravity, if it did interact in any other way we would be able to see it. This solution seems like a fudge, except that it can be used to fit all of the problems I have listed above and more, something which the other contending model has difficulty with. This other approach (called Modified Newtonian Dynamics or MOND) is to assume that gravity behaves differently on different scales, this approach can reproduce many of the observed effects but not all, and is generally distrusted because gravity in general and general relativity in particular is seen to work so well in every observation we have to date. At the present much work is being done to investigate and try and detect a Dark Matter particle, if one is found it will be the crowning achievement of modern astronomy, if it is not observed then we have major problems.

However just as the astronomical community was reaching consensus on the existence of DM another set of observations appeared that has led to another dark substance. We will look at this in my next post in the series.

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.

More Great News from Sunderland

I love this story, a couple of old ladies in their 70s are wanted by Police in Sunderland in relation to a stolen wallet. Now they may have had nothing to do with it, but still I love the way the news has been going in Sunderland recently, idiots with fireworks and possibly criminal grannies. Brilliant.

Attempted Darwin Award

I love this story from the bbc. Its perfect Darwin award material, if only the damage had been done about 5cm away. In short some genius decided to launch a firework rocket from his backside, apparently its been done on Jackass, well without the extra help the Jackass team had it all went a bit wrong. The aptly name Black Cat Thunderbolt rocket (it all happend in Sunderland home to the Black Cats, or the filthy Maccam B**tards if your from nearer the Tyne) then proceeded to cause a scorched rectum, thats got to sting.

Then There Were 4

It looks as if India is about to join the very exclusive club of nations that can independently support manned space operations. According to a space.com story found here the Indian Space Research Agency IRSO (?) has decided to get ambitious and propose a manned space program costing $3 Billion a year which is more than 3 times its annual budget at present. Good luck to them. I hope things go well and that a bit more cooperation between the space going countries can be achieved.

If only governments over here could be just a little more abitious, I note that looking at the wikipedia article on GDP that the richest "country" in the world is the EU with 18 times as much GDP as India. Of course there are many complications here that means you can't take a simplistic GDP view of things but you would still think that the europeans together could make a go at an independent manned space program. Its not like a space program is a total waste of money, all of the money is spent on Earth and invested in real economies. Anyway I suppose we should be pleased for the ambitious ground based programs we have here, CERN, ITER, ESO etc.

Government Interference

Reading through Live Science I came across another one of those articles that makes you want to scream. It can be found here. I suggest anybody interested in governments meddling with research should check it out. The short version is that the Centres for Disease Control (CDC) in the US has been attempting to suppress research that does not fit with President Bush's abstinence only solution for unwanted pregnancies and STD transmission. We can only hope that now he is essentially left quacking after the mid term elections common sense may prevail and science can be left to enlighten rather than forced to fit particular prejudices.

The Earth

As an astronomer I tend to spend a lot of time looking upwards and outwards at some of the most incredible and powerful events in the Universe. So much so that I often forget just how amazing the Earth can be, the BBC News website tends to have a different interesting science picture everyday, sort of a Science Pic of the Day if you will, todays picture is a perfect reminder of just how incredible things can be on Earth.

It shows an area of Namibia called the Brandberg Massif, the whole thing is a single lump of granite which punched its way through the Earths crust 120 million years ago during the break up of the Gondwana supercontinent. The dark ring around the massif is caused by the rocks that were pushed out of the way as it broke the surface.

That something that size (it covers 650 square km) can push its way through the Earth crust just shows the immense pressures and energies still bound up in our active seemingly benign little ball of rock.

Another Great Astronomy Pic

I've just come across this great picon the badastronomy blog. Its really cool as it combines radio (red), optical and X-ray (blue) images into one. In the image you can see the radio jets thrown out by a supermassive black hole in the central galaxy of a large cluster, these jets are busily blasting a trillion solar masses of gas out of their way to form a cavity around the galaxy. This cavity can clearly be seen in the X-Ray image as the dark regions that line up with the jets. It just shows what extra information you can get by combining the information from many different wavelengths.

For a much fuller and more literate explanation of the whole thing check out the badastronomer blog here.