Simply astronomical the Square Kilometre Array
This topic is sponsored by the International Centre for Radio Astronomy Research.
Australia is playing a leading part in plans to build the world’s largest radio telescope.
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Australia is in the running to host a giant new radio telescope, the astronomical equivalent to the Large Hadron Collider which has been called the biggest science experiment in history.
The Square Kilometre Array (SKA) telescope will be too complex and costly (A$2.9 billion) to be built by any one country. Instead an international consortium of 19 countries has been formed to plan and build it. In October 2006, the consortium announced that two countries had been short listed to host the SKA – Australia and South Africa.
What is the SKA?
In the early 1990s astronomers posed the question: What sort of telescope will we need to investigate the astronomical questions of the new millennium? The answer: A radio telescope with 50 times the sensitivity of any existing telescope and with a total collecting area of one square kilometre, hence the name (Box 1: What is a radio telescope?).
The SKA will not be a single instrument but will consist of several thousand antennas all linked together to form one giant array. The design currently favoured for the array consists of many small dish antennas approximately 10 metres in diameter, and a large number of a new type of flat panel known as an aperture array or 'tile'. The dishes will receive high frequency radio waves and the tiles low frequencies, giving the SKA an exceptionally wide radio 'window' to observe the universe.
Low-frequency receiving tiles will be surrounded by high-frequency receiving dishes
(Image: SKA Project Office/Xilostudios)
Viewed from above the SKA will have a spiral shape. Half of the dishes and all the tiles will be located in a central 5 kilometre by 5 kilometre region. The other 'dish' antennas will be located in groups of a hundred or so at progressively larger distances from the core. The entire array will be spread over thousands of kilometres.
The location of the SKA will be of vital importance. The remote site proposed by Australia meets the important requirements: a large tract of flat land at a relatively low cost and, with its very low population density, a site with extremely low levels of radio interference (eg, from mobile phones and radio stations). The upper atmosphere (ionosphere) above the site also needs to be stable to let signals through from space with minimum interference.
Twentieth century astronomers discovered an expanding universe with billions of galaxies, each filled with billions of stars, along with exotic objects such as black holes, quasars and neutron stars. A major challenge for the next century is to understand how it all got there – the evolution of the universe. To this end, astronomers have identified the following key science goals for the SKA.
The birth and evolution of galaxies
When astronomers study distant galaxies, they are looking back in time. Because of the time taken for light from distant galaxies to reach us across such enormous distances, they are observed as they were billions of years ago. The extraordinary sensitivity of the SKA means that we will be able to look much further back in time, back to a time shortly after the Big Bang when the first stars and galaxies were formed.
Evolution of the universe: the SKA will look back in time to when the first stars and galaxies formed
(Image: NASA/WMAP Science Team)
Many of the secrets of galaxy formation will be revealed by studying the radio emissions of hydrogen. In the early universe hydrogen emitted radiation at a wavelength of 21 centimetres. As light from the very first stars ionised the hydrogen around them, this 21 centimetre radiation stopped. Using spectroscopy to observe the hydrogen radiation at different times, the SKA will show how the first galaxies formed and evolved over time (Box 2: Spectroscopy – a vital tool).
In the late 1990s a group led by Australian astronomer Brian Schmidt made an astonishing discovery. Astronomers had always assumed that the gravitational attraction between galaxies would cause the expansion of the universe to gradually slow down. Schmidt's group showed the opposite – the expansion of the universe is actually accelerating.
Astronomers now believe that all the normal matter we see in stars and galaxies makes up only a small part of the total mass and energy of the universe. Most of it consists of a mysterious mix of dark matter and dark energy that is driving the expansion of the universe. A fundamental challenge will be to shed light on the properties of this unseen matter-energy and to determine whether our laws of physics are outdated. The SKA will help by conducting measurements of a billion galaxies in the universe to clarify the nature of both dark energy and dark matter.
The magnetic universe
An important discovery last century was that stars and galaxies have magnetic fields and that these fields play a vital role in controlling how stars and galaxies form and evolve. Despite their importance, we do not understand how the fields are created or how they maintain their strength over cosmic timescales.
By measuring subtle changes in the radio emissions from millions of distant galaxies, the SKA will be able to map the magnetic fields within the Milky Way and beyond in the vast reaches of intergalactic space. The SKA will reveal what these magnetic fields look like, their origin, and their role in the evolving universe.
Are we alone in the universe?
The SKA may answer a fundamental question as old as mankind: 'Are we alone?' In recent years the number of planets discovered orbiting nearby stars has grown dramatically, though it is unknown whether any harbour life.
Since the 1960s astronomers have carried out numerous searches for extraterrestrial intelligence, hoping to detect signals deliberately beamed in our direction by an advanced civilisation. The SKA will be so powerful it will be able to eavesdrop on other Earth-like planets looking for signals as weak as airport radars. The sensitivity of the SKA will also allow scientists to search for planetary systems in the process of forming and for evidence of complex organic molecules, the building blocks of life.
Testing Einstein's gravity
Einstein's theory of general relativity and gravity has been tested many times by astronomers and come through with flying colours on every occasion (Box 3: Is Einstein still right?).
A major goal of the SKA will be to carry out more rigorous tests of the theory. The ideal system will be a pulsar orbiting in the ultra-intense gravitational field around a black hole. The behaviour of the pulsar will decide whether Einstein's theory needs to be replaced by a new and more powerful theory of gravity.
Although the SKA has these well-defined science goals, it is possible that its most important discoveries will be unexpected – completely new phenomena and new laws of physics.
Through CSIRO and a number of universities, Australian astronomers have been among the driving forces behind the SKA since it was first proposed. If the Australian bid to host the SKA is successful, the bulk of the thousands of antennas will be located at the Murchison Radio-astronomy Observatory in Western Australia. Others will be up to 3000 kilometres away in New South Wales or even 5000 kilometres away in New Zealand. The recent establishment of a major research centre based in Western Australia, the International Centre for Radio Astronomy Research (a joint venture of the University of Western Australia and Curtin University of Technology), can only strengthen Australia's bid for the SKA.
Australia and a number of other countries are participating in the development of several so-called 'pathfinder' arrays that will help to develop and test technologies for the SKA.
One pathfinder is the Murchison Widefield Array currently under construction near the proposed Australian SKA site by a group of researchers from the USA, India and Australia. Over 500 flat-panel tiles will be installed at this facility and observations will begin at low frequencies.
A second and more ambitious stepping stone to the SKA is the Australian SKA Pathfinder (ASKAP) to be built by CSIRO in collaboration with other national and international groups. Located near the proposed SKA site, ASKAP will consist of 36 dish antennas receiving higher-frequency radio signals and working as a single telescope.
Although primarily a test-bed for SKA technologies, ASKAP will be a powerful new telescope in its own right with the ability to cover the entire southern sky every day. ASKAP is expected to discover millions of galaxies – a tantalising preview of even bigger things to come with the Square Kilometre Array in 2020.
Regardless of the final site of the SKA, international collaboration has brought together science and technology to give us an unprecedented understanding of the universe we live in.
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Posted April 2009, edited September 2012.