Thursday, November 22, 2012

Blank On The Map: The Structure of a Scientific Revolution

A few days ago, Peter Coles posted an interesting comment about skepticism on his blog. In it, he showed the first graphs with data showing evidence for the acceleration of the expansion of the Universe from observations of distant supernovae. His point was that though the plot isn't much to look at, the statistical evidence for cosmic acceleration from supernovae data was quickly established and very soon became very widely believed.

Coles goes on to make some points about global warming skepticism, and most of the long discussion in the comments that follow is concerned with this. Which is unfortunate, as none of that discussion provides an exception to the general rule that arguments about global warming are a complete waste of time, because people have already decided what they want to believe before they start, and are subsequently impervious to any evidence, logic or persuasion. So although you may want to read the discussion there, let's not talk about the climate here.

Instead, I want to stick with the cosmology and pick up on a question that is more interesting. Let's take a quick look at the picture of the supernovae data from the High-Z Supernova Search Team and the Supernova Cosmology Project:
Evidence for dark energy from the Hubble diagram of supernovae. The top panel shows the distance modulus of the supernovae as a function of redshift, with three different theoretical model curves. The bottom panel shows the residual distance modulus relative to that in the model with the dotted curve (a Milne universe).
As Coles points out, most people who are not cosmologists (and some who are) look at these data and find them deeply unconvincing. The error bars are large. The points are widely scattered, and at least both the top two curves seem to give reasonably good fits. On top of that, there appear to be two distinct groups of supernovae: one group rather close to us, and the other much further away at higher redshifts (and therefore observed as they were a long time ago). Any inferences we draw from relative calibrations of these two groups rest on the assumption that deep down they are the same kind of objects, and that the Universe they existed in remained the same kind of Universe.

So why exactly did this cause such a revolution in the field? How did so many physicists become convinced, almost overnight, of the existence of this mysterious dark energy, that constitutes most of the energy of the Universe, but that we didn't understand then and don't understand now?

Read the rest at Blank On The Map