Mapping the Universe in 3-D: Unlocking its Cosmological Secrets

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 from us.

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 galaxy’s 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 Hubble’s 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 Hubble’s discovery, redshifts and distances had been recorded for as few as 30,000 galaxies! In the 1990’s 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 universe’s 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.

Warrick Couch, Roberto De Propris,
Marton Hidas and Suzanne Kenyon

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