Monthly Archives: May 2012

Collective Marvelling

Shaun Hotchkiss has rounded up a few young upstart science bloggers, including myself, and proposed that we form a research blogging network. Check us out over at Collective Marvelling!


Colours in the Mac OS X terminal

My snobbish conviction that Mac OS X is somehow an inferior operating system has been growing of late. I long to install Ubuntu 12.04 on my Power Mac at work (centrally managed by Oxford Physics IT services), just so that I don’t have to recompile every bloody component of my toolchain every time I start a new project.

Anyway, for evidence that Mac OS X isn’t a “proper” operating system, note the lack of colours in the default terminal profile. Microsoft Windows (vehemently not a proper operating system) also lacks colours on the command line by default. You do the maths.

To get colours in the terminal on a Mac, add the following to your ~/.bashrc:

export CLICOLOR = 1

Acceleration paper published

Hooray! The acceleration paper was published in Phys. Rev. D a couple of days ago.

I was quite pleased with how this one turned out – it’s a nice clarification, I think. Tim had the idea of using the blueshift in collapsing regions to mimic acceleration in the Hubble diagram, which is pretty cool in itself. It was also good to find a concrete example of the link between acceleration of the average and acceleration in the Hubble diagram that Syksy Rasanen has discussed in a couple of papers (see our discussion for references).

Of course, we’re not claiming that “dark energy is backreaction” or anything nearly as strong as that, but I think it does extend the backreaction debate a little. The papers by Ishibashi, Green, and Wald, which seem to show that inhomogeneities on small scales don’t affect the background evolution much, suggest that backreaction effects can’t have any bearing on dark energy. I suppose our paper responds to theirs by saying “yes, perhaps they can’t dynamically, but what about non-linear optical effects?”

Philip Bull, Timothy Clifton (2012). Local and non-local measures of acceleration in cosmology. Phys. Rev. D: 10.1103/PhysRevD.85.103512

Recent numpy/matplotlib finds: Log plotting and map projections

With a change of research focus comes a change of tools. I’m fiddling with clusters of galaxies at the moment (GR stuff has been put slightly to one side for a little while), so I’ve been re-educating myself in the art of handling catalogues. I discovered a handful of neat numpy/matplotlib routines in the process, which I thought I’d share:

  • Map projections: You can project plots differently by using the projection keyword argument for subplot, e.g. subplot(111, projection="mollweide")
  • Logarithmic sampling: If you need to sample some function on a logarithmic interval (e.g. a cluster pressure profile), use numpy’s logspace function instead of the usual linspace.
  • Split log axes: Sometimes you want log axes for a plot that includes both positive and negative values. Rather than messing around with taking the absolute value of the negative numbers and then changing the line style, you can use SymmetricalLogScale.

Lambda is an unnatural tuning

Here’s an interesting comment from John Barrow, who’s currently speaking at the Arrow of Time mini-series here in Oxford. With many theories, if you need to add an extra parameter, you typically make the theory more complicated (and in a sense, less symmetrical). With an FLRW model, however, you find that your model is less symmetrical when you take away a certain parameter – the cosmological constant. FLRW with Lambda asymptotes to de Sitter space far into the future, a spacetime which is maximally symmetric. Removing Lambda removes this asymptotic behaviour, and so you have effectively made the theory less symmetric. Weird, huh?