WMAP Year Three

The Wilkinson Microwave Anisotropy Probe (WMAP) was launched to the L2 point a few years ago and measures the cosmic microwave background radiation (CMB). The CMB is radiation that comes from about 300,000 years after the big bang, at a time when the universe had cooled enough for electrons and protons to join together to make the first atoms. Before this point, the free electrons and protons were a plasma which interacted with photons (particles of light). With the formation of neutral atoms, the photons were able to do their own thing and head off through the universe until the WMAP detector got in the way.

The WMAP instruments have been measuring this microwave radiation (which you can pick up on your TV set) for several years and in particular, they are measuring the difference in the amount of this radiation from different directions in the sky. Any differences in the temperature - a region with more microwaves can be considered as hotter - of different parts of the sky tell us a lot about the early universe and let astronomers describe, mathematically, the type of universe we are living in. This is good to do because it can tell us about the very early life of the universe and tell us what will happen to it next

Back in 2003 the first results were published by the WMAP team. These helped herald a new era of "precision cosmology" where the uncertainties in the values were reduced to a couple of percent (subject to certain assumptions). It was expected that as time went on, and more data were collected, a second year's set of results would be published that had smaller uncertainties. However, the publication of the results kept getting delayed. The speculation was that the WMAP team were having some difficulty calibrating the data for some reason. This sounds entirely plausible considering how difficult the observations are. Finally, (thanks to Megan, Andrew Jaffe and Sean Carroll for the heads-up) the team have overcome the difficulties they were having and released the results of years two and three.

The main result from WMAP is called the power spectrum and this basically tells you how much variation there is on different scales. To be confusing, the power spectrum is usually plotted on a graph as multipoles - think of this as the inverse of the angular size - with the low multipoles at the left (big angles on the sky) and high multipoles (small angles on the sky) on the right. The new power spectrum is displayed on this plot showing the data (little black marks with uncertainty bars) and the theoretical model (red line) that fits the data. The agreement is generally better than it was before, as you might expect when you add in more data and your theory was along the right lines. However, the leftmost point on the graph (multipole 2) is still lower than it would be expected to be. Strange.

The parameter which tells you how the size of the fluctuations in temperature vary with angular size is called n (scalar spectral index) and this appears to have a value of 0.95±0.02 which is slightly less than the value of 1 it would have if it were uniform. This basically means that there is more structure in the universe on larger scales and this seems to agree with the sort of numbers you get from models of a universe that has inflation early on. Perhaps some more evidence for inflation it seems.

One result that many astronomers were waiting for was related to the polarization of the radiation coming from different directions. There are several ways you can compare the polarization (technically labelled TT, TE, EE, BB). As the BB or "B-mode" signal is very weak, it is the hardest to detect and WMAP didn't see it. This non-detection was expected but has helped to put upper limits on the strength of the signal anyway. To detect it requires more sensitive instruments such as ESA's Planck satellite, due for launch in 2007/8.

There were plenty of other results that come out of these observations but I have trouble understanding what some of the other parameters physically mean, so I haven't detailed them here. Plus, I'm still on holiday and it is sunny outside.

Finally, my congratulations to the WMAP team for all their hard work in getting everything calibrated and the results released. Great job folks.

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Posted in astro blog by Stuart on Friday 17th Mar 2006 (04:12 UTC) | Permalink
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