Astronomers have mapped the universe’s dark matter in 50 independent directions using the European
Southern Observatories’ Very Large Telescope (VLT). Their results, announced in December, show that it
is unlikely that mass alone would stop the current expansion of the universe, and they support the
existence of a non-zero cosmological constant.
Dark matter is, by definition, invisible and
non-radiating. To map it, astronomers observed more than 70 000 normal galaxies and studied how their
shape was deformed. This deformation, which is known as cosmic shear, is caused when the light from
distant galaxies is deflected by large clumps of dark matter along the line of sight. The larger the mass of
dark matter, the greater the distortion of the image and the more galaxies appear to be aligned. The
phenomenon provided a new focus of astronomical interest last year (see June 2000 Features).
The analysis of
such a large sample of galaxies means that limits can be set on the overall mass density of the universe.
These observations are a big step forward for astronomers. It is the first time that calculations of cosmic
shear have been made in enough different directions from the Earth to give an accurate estimate of overall
mass density. The results are in excellent agreement with other methods and suggest that the total matter
in the universe is less than half of that needed to stop Big Bang expansion.
