One of the most exciting areas of research in cosmology today concerns the nature and distribution of the so-called "dark matter", which comprises the vast majority of the total mass in the Universe. Although this matter is undetectable with conventional astronomical methods (hence, the name "dark"), we can detect its presence from its gravitational influence on light travelling through its potential.

By measuring distortions introduced into the images of far-away galaxies, we can deduce information about the intervening matter distribution through which the galaxies' light has passed. By making such measurements of the "shear distribution" over large enough pieces of sky, we can build up a picture of the large scale distribution of the dark matter in the Universe.

One important quantity we can measure in this way is the "shear power spectrum" which contains all the two-point statistical information of the shear field. Since this quantity is simply a weighted projection of the underlying 3-d mass power spectrum, it represents a powerful probe of the clustering properties of the dark matter on cosmological scales. Measuring the shear power spectrum can also lead to strong constraints on cosmological parameters, in particular the matter density of the Universe and the normalisation of the mass power spectrum.

We have performed a full Maximium Likelihood analysis to measure the shear power spectrum from the COMBO-17 survey. These measurements are shown in the above figure along with the parameter constraints we find from combining our measurements with the 2dF Galaxy Redshift Survey and the most recent CMB experiments.

Contact person:  Michael Brown, ROE

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