In the Dark

What a day.

For a start we had another manic UCAS admissions event. Applications to study physics here have rocketed, by more than 50% compared to last year, so it’s all hands on deck on days like this. Next weekend we have our first Saturday event of the year, and that promises to be even more popular. Still, it’s good to be busy. Without the students, we’d all be on Her Majesty’s Dole. At least some of our advertising is hitting the target.

After that it was back to the business of handing out 1st Semester examination results to my tutees – the Exam Board met yesterday but I skived off because I wasn’t involved in any exams last semester. Then a couple of undergraduate project meetings and a few matters related to postgraduate admissions that needed sorting out.

Finally, being a member of our esteemed Course Committee, I spent a little bit of time trying to assemble some new syllabuses. All our Physics (and Astrophysics) courses are changing next year, so this is a good chance to update the content and generally freshen up some of the material we teach.

In the course of thinking about this, I dug about among some of my old course notes from here there and everywhere, some of which I’ve kept on an old laptop. I chanced upon this cute little graphic, which I don’t think I’ve ever used in a lecture, but I thought I’d put it up here because it’s pretty. Sort of.

What it shows is a simulation of the large-scale structure of the Universe as might be mapped out using a galaxy redshift survey. The observer is in the centre of the picture (which a two-dimensional section through the Universe); the position of each galaxy is plotted by assuming that the apparent recession velocity (which is what a redshift survey measures) is related to the distance from the observer by Hubble’s Law:

where   is the recession velocity,   is the redshift, is Hubble’s constant  and is the radial distance of the galaxy. However, this only applies exactly in a completely homogeneous Universe. In reality the various inhomogeneities (galaxies, clusters and superclusters) introduce distortions into the Hubble Law by generating peculiar velocities

These distort the pattern seen in redshift space compared to real space. In real space the pattern is statistically isotropic, but in redshift space things look different along the line of sight from the observer compared to the directions at right angles as described quite nicely by this slide from a nice web page on redshift-space distortions.

There are two effects. One is that galaxies in tightly bound clusters have high-speed disordered motions. This means that each cluster is smeared out along the line of sight in redshift space, producing artefacts sometimes called “Fingers of God” – elongated structures that always point ominously at the observer. The other effect caused by large-scale coherent motions as matter flows into structures that are just forming, which squashes large-scale features in the redshift direction more-or-less opposite to the first.

These distortions don’t simply screw up our attempts to map the Universe. In fact they help us figure out how much matter might pulling the galaxies about. The number in the upper left of the first (animated) figure is the density parameter, . The higher this number is, the more matter there is to generate peculiar motions so the more pronounced the alteration; in a low density universe, real and redshift space look rather similar.

Notice that in the high-density universe the wall-like structures look thicker (owing to the large peculiar velocities within them) but that they are also larger than in the low-density universe. In a paper a while ago, together with Adrian Melott and others, we investigated  the dynamical origin of this phenomenon, which we called the Bull’s-Eye Effect because it forms prominent rings around the central point. It turns out to be Quite Interesting, because the merging of structures in redshift-space to create larger ones is entirely analogous the growth of structure by hierarchical merging in real space, and can be described by the same techniques. In effect, looking in redshift space gives you a sneak preview of how the stucture will subsequently evolve in real space…