So, that was one conference. It's somewhat entertaining on a number of levels; firstly, being in a room with lots of really clever people is a very good thing; secondly, watching those really clever people disagree violently with each other is amusing; thirdly, getting new ideas for my own research has to help with the impending third year of Ph.D. studies nightmare.
Should you, the dear reader, be interested in the nature of Dark Energy, a brief summary: Monday and Tuesday were devoted to experimental techniques and observational results. It saddened me slightly to see some of the theorists take time off during these sessions, because Physics has to be driven by experiment to work (otherwise it's simply Mathematics... oh, wait, what department am I in again? Still, I learnt a fair bit about the Cosmic Microwave Background baloon experiments (MAXIMA and BOOMERanG), the Type Ia Supernovae observations, Weak Lensing, all apparently pointing towards the ‘Concordance Cosmology’ of (Ωm, ΩΛ) = (0.3, 0.7).
The last plenary session on Tuesday was devoted to the question “Is evidence for Dark Energy compelling?” Based on the previous paragraph, one would have to say ‘yes’, as the observations strongly point towards a non-zero Cosmological Constant. But wait! The CMB results depend on assuming only adiabatic perturbations; we don't have a model for the Type Ia supernovae, and there is the problem of the cosmic distance ladder; and weak lensing observations can easily be contaminated by strong lensing effects. Is it possible that systematic experimental effects can lead to a false concordance (or, more cynically, is it possible that experimentalists will choose the method of analysis that leads to an answer close to the one that they're expecting)? Sadly, the history of science points to a ‘yes’ answer to that question, too. Based on this, I skipped Tuesday afternoon's session to go shopping.
Wednesday to Friday were more theoretical days (well, the days themselves weren't theoretical, but the talks were on theoretical subjects), so I skipped fewer talks. Highlights: Gia Dvali, not so much for his talk's content as for the way he said it – he actually made an 09:00 start tolerable; Sacha Vilenkin, for the bravery in extolling the virtues of the anthropic principle to a mostly hostile audience; and, of course, having my own work presented (all the glory and none of the responsibility). Maybe a side note about the anthropic principle is in order: it comes in a number of flavours, ranging in character from “We're here” through “We're here because we're here” to “Everything in the Universe is your fault”. As presented by Vilenkin, it was a very reasonable argument, essentially saying that, given that we exist, we have a non-uniform prior probability on cosmological parameters, so we shouldn't use a uniform prior when we do Bayesian statistics. This seemed reasonable to me (maybe he shouldn't have said that the anthropic principle ‘predicted’ an ΩΛ of 0.7) but didn't meet with much approval among my peers. It's a shame, because the anthropic principle is a useful tool in the chest of a physicist (notably used by Fred Hoyle in the prediction of the resonance in Carbon-12, at just the right energy for the triple-α collision to work...
The conclusion from the Colloque was really along the lines of “We have no real idea what Dark Energy is like or where it comes from. But that's not a problem, because it leaves us plenty of room for writing articles which everyone else can cite.” Though I did like the attitude of the final session chair: “If I could ask God one question, it would be ‘How many dimensions does the Universe have?’; hopefully He would answer with a number... a real number... if we're really lucky, an integer...”