Planck finds a mystery

The Wilkinson Microwave Anisotropy Probe (WMAP) has been perhaps the most spectacularly successful cosmology experiment of all time. WMAP painstakingly mapped the cosmic microwave background and its results have allowed scientists to determine key parameters of the Universe – it’s age, its geometry and so on – with a precision that would have been unthinkable just ten years ago. In 2004, WMAP also found hints of something unusual happening closer to home: it found a weak signal corresponding to haze of microwave radiation, roughly spherical in shape, centred around the centre of our Milky Way galaxy.

If such a haze exists then astronomers have an interesting challenge in trying to explain its origin. But does it exist? Some researchers argued that the haze might be nothing more than an artefact of data analysis, since they doubted whether WMAP had the sensitivity to distinguish such a weak signal from the general microwave background and the strong emission from galactic dust.

Well, the European Planck mission has a sensitivity that exceeds WMAP and it can observe the Universe over a greater range of frequencies. We can expect great things for cosmology when the Planck team releases its full results. One intermediate result of the Planck Collaboration, released recently on the arxiv server, is that the microwave haze found by WMAP does indeed exist. What is particularly interesting, however, is that the haze isn’t spherical; it’s stretched out like a cigar. Furthermore, the sharp edge to the haze suggests that whatever causes it is a sporadic rather than a continuous phenomenon (since a continuous process would lead to a diffuse haze).

Astronomers don’t yet have a good explanation for the haze. One initial suggestion for the WMAP observation was that dark matter annihilation created electrons and positrons which, when they spiralled in the Milky Way galaxy’s magnetic field, generated the observed microwave haze. But the Planck discovery of a squashed rather than spherical haze tends to discount that idea.  Planck has found a mystery. I’m hoping that when the Planck Collaboration publish its full results from the mission there’ll be several more discoveries to ponder.