Mud Wrestling and Microwaves

Reading through an interesting blog post about the new results from Planck by the ever-reliable Jonathan Amos (the BBC’s very own “spaceman”), I was reminded of a comment I heard made by Martin Rees (now Lord Rees) many years ago.

The remark concerned the difference between cosmology and astrophysics. Cosmology, said Lord Rees, especially the part of it that concerns the very early Universe, involves abstract mathematical concepts, difficult yet logical reasoning and the ability to see deep things in complicated spatial patterns. In that respect it’s rather like chess. Astrophysics, on the other hand, which is not at all elegant and has so many messy complications that it is sometimes difficult even to work out what is going on or what the rules are, is more like mud wrestling.

The following image, which I borrowed from Jonathan Amos’ piece, explains why I was reminded of this and why some cosmologists are having to abandon chess for mud wrestling, at least for the time being. The picture shows the nine individual frequency maps (spanning the range from 30 GHz to 857 GHz) obtained by Planck.

What we cosmologists really want to see is a pristine map of the cosmic microwave background, the black-body radiation that pervades the entire Universe. It’s black body form means that it would have the same brightness temperature across all frequencies, and would also be statistically homogeneous (i.e. looking roughly the same all across the sky).

What you actually see is a mess. There are strong contributions from the disk of our own Galaxy, some of it extending quite a way above and below the plane of the Milky Way. You can also see complicated residuals produced by the way Planck scans the sky. On top of that there is radiation from individual sources within our Galaxy, other Galaxies and even clusters of Galaxies (which I mentioned a couple of days ago). These “contaminants” constitute valuable raw material for astronomers of various sorts, but for cosmologists they are an unwanted nuisance. Unfortunately, there is no other way to reach the jewels of the CMB than by hacking through this daunting jungle of foregrounds and instrumental artefacts.

Looking at the picture might induce one of two reactions. One would be to assume that there’s no way that all the crud can be removed with sufficient accuracy and precision to do cosmology with what’s left. Another is  to appreciate how well cosmologists have done with previous datasets, especially WMAP, have confidence that they’ll solve the numerous problems associated with the Planck data, but understand why  will take another two years of high-powered data analysis by a very large number of very bright people to extract cosmological results from Planck.

There might be gold at the end of the pipeline, but until then it’s going to be mud, glorious mud…


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