I’m in the Sunday morning session of the Sloan Digital Sky Survey conference (let it never be said that astronomers don’t work hard, even on weekends), and the first talk of the day is by Simon White of the Max Planck Institute in Garching, Germany.
Simon is usually worth listening to, and was introduced by today’s chairman as someone who’d talked at literally every conference he can remember, so in the absence of structured writing I will try and keep you updated here.
He’s started by reminding us that looking at the largest scale structures was the original primary goal of the survey. In the early 90s, as the Sloan was being designed, it was still a relatively new proposition that the structures we see all formed by gravity acting on the tiny fluctuations that we observe in the early Universe, which were themselves detected in our earliest glimpse of the Universe, the cosmic microwave background, by the COBE satellite in 1992.
9.07 : The survey has done this and more – for example, gravitational lensing was considered only to be a curiosity.
9.11 : The world changed rapidly between this point and the survey producing data. For example, astronomers studying distant supernovae realised the Universe’s expansion is speeding up, rather than slowing down. Perhaps more importantly, we were able to measure the geometry of the Universe, and discovered that it is effectively ‘flat’; this is a measure of the energy density in the Universe.
9.13 : The simulation included in the proposal turns out to be not too bad, despite the fact that they had to randomly scatter the galaxies in the absence of other information. By 1996, the resolution of the simulation was good enough to allow us to model the formation of the galaxies themselves – although as with most of these simulations we’re only talking about following the evolution of the dominant dark matter.
9.22 : Simon’s working his way through a list of (fairly technical) observations from the Sloan which compare the distribution of galaxies, and groups of galaxies with the predictions of our standard model of cosmology. Although agreement is currently good, the results are surprisingly sensitive and (at least according to Simon) offer the possibility of distinguishing between the presence of dark matter versus theories which change gravity.
9.24 : Scratch the last bit; a new paper has made him think again about truly testing dark matter.
9.25 : What about looking for the shapes of dark matter halos by fitting profiles to the observed systems? And now we’re on to looking at beautiful simulations. One simulation of the Milky Way’s dark matter has what I think is hundreds of millions of particles whose position and movement are being modelled. This being a science conference, he skipped the movie. Boo!
9.28 : The conclusions from the simulation are many and varied, but include the prediction that the dark matter in our local neighbourhood will essentially be smooth. It will actually be in streams, but there are hundreds of thousands of them and so we won’t be able to distinguish between what the models predict and a truly smooth distribution.