Tuesday, September 20, 2011

Snapshots from the trenches: Stockholm clusters workshop

As a part of contributing to this blog, I am going to try to write quick-ish summaries of any papers I co-author and any conferences or workshops I attend. Though, unlike the commitment to a new post every six weeks, I make no promises about how well I'll keep to this particular aim. These posts will inevitably be the most technically demanding of my posts, but also, potentially the most rewarding. They will be demanding because I won't choose to spend a lot of time writing them, but, more importantly, because the physics I will be discussing will be fresh off the press. As a result, it will not be written in a coherent, packaged, easily understood, form.

But, here's the thing, research is never in that form, even for those who are immersed in the field. It is only when we look back on past research with the benefit of time that we can see it in a form that can be packaged in a complete and coherent picture (the galaxy clusters stuff I wrote last week took physicists more than decades to properly understand). By this time it is no longer 'research', but simply 'past results'.

Often scientific knowledge is only popularised for the consumption of those outside of the field once it has reached this final, 'packagable', state. This is great, but the awesome thing about a blog like this (and the many other similar blogs) is that we can also give you a real-time view inside the research trenches. We can discuss what people are working on, thinking about and discovering, right now. It may be more difficult to follow than a coherent look back, but it should be more exciting to follow, and, as other blogs have shown, it is definitely possible to do. You don't have to follow every line of every calculation, to be pointed to a final number, or final figure in a paper and, when given the context, use that to reach the same conclusions as the authors. Yes, even in the actual, raw, unadulterated, not fit for the eyes of the young and innocent, scientific, papers! That's the goal, at least.

It shouldn't end up being too much more difficult than keeping track of various sports teams and players during a season of professional sport.

I think that giving this view from inside the trench is really important. Popularising scientific results in neat packages isn't a bad thing, it is great to help us all understand what is known and what has been achieved. The problem is when it is only the final package that the public ever sees. Then what is hidden from view is how we know the result and what we did to learn it. When it is only ever the final result that is seen it gives a skewed perception of how science proceeds, which I think can be damaging.

Stockholm clusters workshop

OK, so the above was all just an introduction to give me a chance to discuss a workshop I attended in Stockholm last week. Unfortunately, my internal blogging-engine lost all its steam writing the stuff above, so I'll be brief describing the Stockholm stuff.

The workshop was at the Oskar Klein Centre for Cosmoparticle Physics. The workshop's title was Clusters of Galaxies as Cosmic Laboratories. Perhaps with that information known, my choice of blogpost last Monday isn't so surprising.

It was quite an observational workshop, which means it included a lot of the people actually detecting and measuring the galaxy clusters. But it also had a reasonably large proportion of people like me who are interested in analysing the galaxy clusters to learn things about the universe as a whole. There are quite a few conferences/workshops like this popping up around the world as more and more galaxy clusters are being found and more and more people want to know what they are capable of telling us.

Lots of things went down at the workshop, but I'll only focus on one personal lingering impression I had. That is:

Precision cosmology is about more than just accurate measurements.

It is popular to say that we've entered the age of precision cosmology. The intended meaning is that cosmological things are now being measured to such a level of accuracy that that we are no longer restricted to general statements about the universe, we can now determine specific, quantitative, things about it. Well, OK, isn't like we could never say specific things about the universe, the point is that the number of these specific things is now growing significantly and rapidly.

On the whole I think that this “precision cosmology” sentiment is true. However precision cosmology also requires careful interpretation. We don't measure any cosmological things directly. We measure much more real, earthly, mundane stuff and then reach conclusions about cosmology by interpreting this stuff. If we get the interpretation wrong then no matter how precise the measurement is we will reach false conclusions.

In the context of the Stockholm workshop the issue is the mass of galaxy custers. We can't just go to a cluster with a set of scales and measure the mass of each and every particle. Most often, we can't even indirectly measure the gravitational effect of the cluster's mass on neighbouring objects. So we need to measure quantities like the temperature of X-rays emitted by the cluster, or the effect hot gas inside the cluster has on light as it passes through the cluster. These are the quantities we measure precisely. Then, we use theory and simulations to convert this into the total mass of the cluster. Some of the effects (particularly statistical biases) that were either ignored or treated only approximately in the past can have a large enough effect on the final mass that it affects the cosmological interpretation.

The fact that this is true is not a problem, so much as a sign that the field is coming of age. Adolescence brings with it attention, but at the same all those little things you thought you didn't have to worry about suddenly become important. The cosmological use of galaxy clusters has a bright future and the fact that subtle statistical effects are now important is simply evidence of this.

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