Wednesday, March 3, 2010

Light and the Age of the Universe - The Discovery and Analysis of the CMB

Discovery of the Cosmic Microwave Background

The Comic Microwave Background was discovered pretty much by accident by Arno Penzias and Robert Wilson who were working for Bell Laboratories, looking for signals from radio waves reflected from balloons. In the course of their experiments, they had to eliminate all noise sources such as radio broadcasts, and even a "white dielectric substance" left on the inside of the detector horn by a family of pigeons who had taken nest there. Once they had got rid of and accounted for every bit of noise they could, they noticed that there was a constant microwave hiss, from every direction, day and night - they had discovered the Cosmic Microwave Background.

The antenna where they made this discovery is now a national monument in the US:

They still did not know what they had found however, but when a friend of theirs told them about a still unpublished paper by Jim Peebles talking about the possibility of finding a signal like theirs, and what it would mean, they began to realise the significance of their discovery. The papers by Peebles and his colleagues, and the paper published by Penzias and Wilson were published together in Astrophysical Journal Letters. Penzias and Wilson won the 1978 Nobel Prize for their work.

The Cosmic Background Explorer

It was thought from early on after the discovery, that there would be small anisotropies (differences depending on direction) in the CMB, but ground based measurements were not good enough to measure them. It was not until the COsmic Background Explorer (COBE) was launched in 1989 that these anisotropies were first observed.

These fluctuations were very small, just one part in 10,000 of the average temperature. The resolution was still relatively low however, and so there was still much detail to be found. One additional important piece of evidence however came out of this - the match between the theoretically predicted Black Body curve based on the Big Bang model, and the experimental curve. The two matched precisely:

These results earned another Nobel Prize, but this time for the principal investigators on the COBE project; George Smoot and John Mather. The CMB wasn't the only thing that COBE was analysing however, and there were other important experiments and discoveries made. A good outline of the COBE satellite's other results can be found here.

The Wilkinson Microwave Anisotropy Probe (WMAP)

The next satellite to look at the CMB was WMAP. This time dedicated to the analysis of the CMB. After the success of COBE, WMAP was designed to not only view the CMB at higher resolution and sensitivity, but also to look at other features of the CMB such as polarization in order to give a better understanding of the early universe. There are a number of interesting results from WMAP, which will continue to operate until (currently) September 2010, and more details can be found here.

A brief summary of some of the WMAP results
  • The universe is 13.73 billion years old (the most accurate figure we have))
  • The universe is very flat (Euclidean)
  • Around 23% of the universe is dark matter.
  • The anisotropies appear to be random (though there are some hints of deviations from simple randomness which could give further clues into the early nature of the universe)

The Future Exploration of the CMB

This article has provided only a brief outline of the discovery and analysis of the Cosmic Microwave background. There are a number of features that have not been discussed, such as doppler shift, polarization and so on, and there is still much work to be done in understanding the details of the CMB. Although WMAP only has a few months of life left, the European Planck observatory, which started to take measurements in 2009, and is expected to begin to release results in 2012.

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