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...”
And now, off to Bordeaux for Libre Software Meeting. I should
stop writing this diary entry, and start writing my talk on “SBCL: The best thing since
sliced bread?”