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AAO Press Releases

Thursday, 15 April 1993

The meaning of the ripples

The stars and gas revealed by telescopes account for only three percent of the material in the universe; the remainder is a mysterious component referred to as "dark matter". Finding out the nature of the dark matter is one of the greatest challenges confronting physicists today.

Considerable progress has been possible by combining observations made from Australia and the Canary Islands with the results of two surveys of the sky undertaken by satellites.

It now seems that there are two intermingled types of dark matter: "hot" material in the form of neutrinos, and "cold" matter of a more subtle nature.

"The Meaning of the Ripples" is the title of a public lecture to be given by Professor Michael Rowan-Robinson of the Anglo-Australian Telescope Board and Queen Mary Westfield College London. The lecture is co-hosted by Questacon - The National Science and Technology Centre in Canberra.

Using information gleaned from both the IRAS and COBE satellites, Professor Rowan-Robinson will explore how a universe that began explosively, in the Big Bang, was nevertheless able to collapse to produce the billions of galaxies and stars and, ultimately, ourselves.

The first clue to the process came from a survey made by the Infrared Astronomical Satellite IRAS in 1983. Away from the plane of the Milky Way, IRAS revealed a huge population of galaxies. Ground-based observations of thousands of these galaxies using telescopes in the Canary Islands and Australia led to a three-dimensional map of the galaxy distribution of unprecedented depth in space.

With this map it was possible to explain the paradox of our Galaxy's rapid motion through space in terms of the gravitational attraction of a dozen clusters of galaxies within 300 million light years of us. This motion was detected in 1977 when it was recognised that the microwave background, the relic of the fireball phase of the Big Bang, was slightly brighter in one half of the sky than the other.

This explanation of our Galaxy's motion also gave a measurement of the mean density of matter in the universe. Surprisingly this turned out to be thirty times higher than the mean density of normal "Baryonic" matter (the stuff the Milky Way and the earth are made of), showing that the universe is filled with some as yet unknown form of "non-baryonic" dark matter.

The IRAS three-dimensional map of the galaxy distribution also showed that the universe was "lumpier" on very large scales than expected according to current theories of how galaxies are formed.

The announcement in April 1992 that the COBE Cosmic Background Explorer satellite had detected minute "ripples" in the microwave background radiation took the world's cosmologists by surprise. Although these ripples correspond to structures much larger than we are able to map in the galaxy distribution, they give a snapshot of the Universe only 300,000 after the Big Bang, and show how galaxies and clusters grew by the action of gravity on primordial ripples.

By tying together the COBE and IRAS data it became possible to determine the nature of the dark matter which, because of its dominance in the universe, controls gravity. Hence the two forms of dark matter have been revealed.

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Roger Bell
01 Jan 1998