When we look
up at the sky on a clear night, it has the appearance of a dark
sphere illuminated by the many thousands of celestial objects
some faint, some bright that are visible to the naked eye.
If we do the same thing with a powerful telescope, we see orders
of magnitude more objects, many of which are other galaxies like
our own. In both cases, we see just a projection of objects onto
the plane of the sky, without any sense of how far away they are
Thanks to Edwin
Hubble discovering more than 70 years ago that galaxies are receeding
from us at a speed proportional to their distance, we have a very
simple way of measuring how far away they are. This can be done
through obtaining a galaxys spectrum, measuring how far it
has been shifted towards redder wavelengths as a result of its recessional
motion (its redshift), and using the redshift to infer its distance
from Hubbles Law.
While this is
not a difficult measurement, it initially was a slow process since
galaxies could only be observed one at a time and it took some time
to record their relatively faint signals. Even some 50 years after
Hubbles discovery, redshifts and distances had been recorded
for as few as 30,000 galaxies! In the 1990s this situation
was completely revolutionised through the use of robotic positioners
and optical fibres to collect the light from and observe many hundreds
of objects at the one time. The most advanced instrument for doing
this was built here in Australia for the 3.9m Anglo-Australian Telescope.
Called 2dF because of its two-degree field of view,
it is capable of measuring spectra for 400 objects simultaneously.
Since 1995, a group of us here in the School of Physics have been
involved in the 2dF Galaxy Redshift Survey (2dFGRS)
- the largest ever such survey of its kind designed to measure the
redshifts, and hence distances, for 250,000 galaxies and thus create
the highest fidelity three-dimensional map of how galaxies are distributed
in the local universe. At the end of 2001, this mammoth survey was
completed, yielding the exquisite galaxy map shown in the figure
below. It shows that most (90% of) galaxies are located on the surfaces
of big bubbles, with the rest residing in dense, knotty,
clusters systems which have been the focus of the research
efforts here at UNSW.
For the first
time, the galaxy distribution and its structure over hundreds of
millions of light years can be studied in great quantitative detail.
This is very important, since the statistical average of how galaxies
(and hence matter) are clustered on these huge scales bears the
imprint of the matter and energy content of the universe. Analysis
of this clustering has revealed that only ~35% of the universes
energy content can be attributed to matter, with the rest being
in some form of dark energy. Moreover, the matter of
the universe is mostly dark matter, with only 6% being
the normal baryonic matter that planets, stars and galaxies
are made of. The same results were found by analyzing the data in
another way: by looking at how galaxies move under the influence
of gravity. Not only was this agreement reassuring, but it also
confirmed that it is gravity alone that is responsible for forming
the delicate and intricate structures seen in the map below.
Couch, Roberto De Propris,
Marton Hidas and Suzanne Kenyon