Simply astronomical – the Square Kilometre Array

Key text

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).

Dark energy

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.

Australia's contribution

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.

Related site: MWA: From the outback to the cosmos A video describing the Murchison Widefield Array. (MIT Haystack Observatory, USA)

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.

Related site: ASKAP overview Provides information on ASKAP science and technology. (CSIRO, Australia)

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.

Related Nova topics:

Astronomy in the deep freeze

Box 1: What is a radio telescope?

The electromagnetic spectrum
(Image: © NASA)

Radio waves have a longer wavelength than visible light so the telescopes need to be very large to produce sharp images of astronomical objects. The larger the collecting area of a telescope, the more sensitive it is and therefore able to detect faint objects at great distances. After reaching the dish (or reflector), radio waves are reflected onto a receiver. The receiver detects and amplifies the signals which can then be processed and stored in a computer ready for analysis.

Two or more dishes (antennas) can be linked together to form a telescope known as an interferometer. The signals from each antenna can be electronically combined to imitate a single dish equal in size to the distance between them – the greater the separation, the sharper the image produced. Images of the very highest resolution are produced by connecting telescopes across continental scales around the globe, a technique known as Very Long Baseline Interferometry.

Radio telescopes have several advantages over optical telescopes. The radio waves they detect can penetrate interstellar gas and dust that block visible light – so the universe is much more 'see-through' at radio wavelengths. Radio telescopes can also observe some objects such as pulsars that do not radiate visible light. A radio telescope can also operate during the daytime.

Optical telescopes must be sited well away from the lights of busy cities. Similarly, radio telescopes are highly susceptible to man-made interference from television, mobile phones, car ignition systems and industrial machinery. To reduce this interference, radio telescopes are often sited in remote and sparsely populated locations.

Australia has been a pioneer of radio telescopes since the earliest days. The first radio galaxies were discovered from a cliff-top at Dover Heights in Sydney using an antenna no more complicated than a television aerial. At present our largest telescope is the Australia Telescope in Narrabri, northern New South Wales, constructed as a bicentennial project in 1988.

Related sites

• What is radio astronomy? (Square Kilometre Array, Australia)

• How radio telescopes work (National Radio Astronomy Observatory, USA)

• Basics of radio astronomy (Jet Propulsion Laboratory, USA)

• Radio interferometry (CSIRO, Australia)

Box 2: Spectroscopy – a vital tool

Astronomers study three types of spectra. The first is the so-called black body spectrum where the emission from an object such as a star varies smoothly across a continuous range of wavelengths. The region or wavelength emitting the most energy provides information on the temperature of the object (eg, the core of a star).

A second type is known as an absorption spectrum where dark lines appear against a continuous background spectrum. Absorption spectra are produced when light from a distant object passes through a cloud of cool gas in interstellar space. Atoms and ions in the cloud absorb the light at characteristic wavelengths and produce the dark lines. The wavelengths of these lines provide information on the chemical composition of the cloud.

In contrast, an emission spectrum consists of bright lines appearing on a dark background, corresponding to a number of characteristic wavelengths. These spectra can be produced, for example, in clouds of interstellar gas heated by nearby stars and can also be used to determine the chemistry of the cloud.

The three types of spectra are not only used to study visible light, but all parts of the electromagnetic spectrum. An important emission line in radio astronomy is from atomic hydrogen at a wavelength of 21 centimetres. Future studies of 21 centimetre emission are expected to shed light on the early evolution of the universe.

Related sites

• Spectroscopy: unlocking the secret in starlight (CSIRO, Australia)

• Spectra and what scientists can learn from them (NASA, USA)

• Wavebands – photons from space (CSIRO, Australia)

Box 3: Is Einstein still right?

Einstein spent the next ten years developing special relativity into a new theory of gravitation, known as general relativity. This states that, like motion, gravity can also affect time and space. Gravity pulling in one direction is completely equivalent to acceleration in the opposite direction. So an elevator accelerating upwards feels the same as the gravity pulling you to the floor.

A key prediction of general relativity is that light travelling close to a massive object will bend slightly due to gravity. In 1919 this prediction was tested when astronomers observed light from stars passing near the sun during a solar eclipse. Amazingly, they found the light bent as predicted. Overnight Einstein became an international celebrity.

General relativity's ultimate test will be in the extreme gravitational fields around black holes.

Related sites

• Einstein, Friedman and relativity (CSIRO, Australia)

• Relativity and the Cosmos (Einstein's big idea, NOVA, USA)

• Testing Einstein (National Radio Astronomy Observatory, USA)

• Strong field tests of gravity using pulsars and black holes (Square Kilometre Array)

• Putting relativity to the test (National Centre for Supercomputing Applications, USA)

Activities

• Earth and beyond modules (Astronomy WA, Western Australia)
  • Multiwavelength astronomy – students measure the temperature of different wavelengths of radiation. They then learn about different types of electromagnetic radiation and their applications to astronomy through internet research and video links.

• National Radio Astronomy Observatory (USA)
  • Try it at home! Make a radio image! – students learn how radio images are made, then produce their own pixel images from printable worksheets.
  • A tour of Orion, the Hunter – an interactive, online activity that shows students images of Orion in different wavelengths including radio images of atomic and ionised hydrogen. Includes a quiz on the content.
  • Measuring the age of the universe – using hydrogen spectra and galaxy images, students measure the age of the universe.

• Jet Propulsion Laboratory (National Aeronautics and Space Administration, USA)
  • Basics of radioastronomy – covers the science of radioastronomy in detailed chapters with a comprehension activity at the end of each chapter.

• Teachers' lab (Annenberg Media, USA)
  • Stellar spectra – provides a basic overview of spectra. Students then identify elements in a star's spectrum.

• Australia Telescope outreach and education (CSIRO, Australia)
  • Spectroscopy: Unlocking the secret in starlight – includes online information and questions relating to spectroscopy in astronomy.
  • Cosmology – the Big Bang and origin of the universe – includes online information and questions relating to cosmology.

• Smithsonian Astrophysical Observatory (USA)
  • How fast do galaxies move? – an interactive activity (either online or class- based) in which students investigate how fast galaxies are moving using a virtual spectroscope.

• Public Broadcasting Service (USA)
  • Activity: Warped space-time model – this activity demonstrates space-time continuum warping from gravity using a constructed model.

• Lab notes – for teachers (Australian Broadcasting Corporation)
  • Telescope dreaming – includes a series of comprehension and research activities based around an article on the Square Kilometre Array.

Activity 1. Australia or southern Africa? Choosing a site for the Square Kilometre Array

You are members of the International SKA Science and Engineering Committee responsible for deciding which site, Australia or southern Africa, is more suitable for building the Square Kilometre Array (SKA).

To help make your decision your group is to conduct a SWOT analysis for each site. A SWOT analysis is a useful evaluation tool to help analyse the positives and negatives of an issue (in this case the site). It helps to evaluate the issue objectively by considering all sides. The issue is considered for internal factors (eg, radio interference) and external factors (eg, socio-economic issues).

1. Use the resources available in further reading and useful sites to find out about the suitability of both sites for the SKA. Particularly useful information can be found in:

   SKA, Site Characterisation Working Group (see Committees and working groups menu)
   South Africa's competitive advantage
   Array for Australia? Cosmos, 27 September 2006
   Candidate sites for world's largest telescope face first big hurdle, Science, 18 August 2006
   Australia and the Square Kilometre Array
   Australian Science, April 2008, pages 20–22, Telescope wars
   Australian Science, August 2006, pages 16–19, Largest telescope plan becomes even larger

2. Make two copies of the SWOT template below. Your committee is to use the templates to list the positives and negatives for each site.

   Some aspects you may wish to consider in your analysis:

   radio interference/ionospheric conditions
   location
   area
   experience/technology
   cost
   infrastructure
   orientation
   elevation
   socio-economic issues
   support

3. Using the analysis of each site, decide within your committee the best site for the SKA.

4. Record the chosen site using dot points to justify your committee's final decision.

SWOT TEMPLATE

	INTERNAL	EXTERNAL
POSITIVE	STRENGTHS	OPPORTUNITIES
NEGATIVE	WEAKNESSES	THREATS

Further reading

Useful sites

• Square Kilometre Array, international radio telescope for the 21st century, student home page
  Provides clear information on the SKA concept, science and design.
  http://www.skatelescope.org/pages/page_student.htm

• The Square Kilometre Array, the international radio telescope for the 21st century
  A brochure which provides a thorough and clear overview of the SKA and its key science projects in simple language.
  http://www.skatelescope.org/PDF/brochure/SKABrochure_2008.pdf

Square Kilometre Array (Australia)

• Australia and the Square Kilometre Array
  Describes Australia's role in the development of the Square Kilometre Array.
  http://www.ska.gov.au/Documents/InternationalBrochure4pp.pdf

• What is the SKA?
  Briefly outlines the Square Kilometre Array and its purpose.
  http://www.ska.gov.au/media/Documents/Factsheet2withbleed.pdf

• What is the Australian SKA Pathfinder (ASKAP)?
  Briefly describes the purpose and location of the Australian SKA Pathfinder.
  http://www.ska.gov.au/media/Documents/Factsheet3%20ASKAP.pdf

Murchison Widefield Array (Australia)

Describes one of the SKA pathfinders in Australia, the Murchison Widefield Array. This site includes a video describing the array technology.
http://www.mwatelescope.org/

Australia Telescope National Facility, CSIRO

• Australian SKA Newsletters
  Provides up-to-date information on developments related to the SKA from an Australian perspective.
  http://www.atnf.csiro.au/news/auska-newsletter/

• ASKAP
  Presents a range of information on the Australian SKA Pathfinder including the science and technology.
  http://www.atnf.csiro.au/projects/askap/

University of Western Australia

• The Murchison Radio-Astronomy Observatory (MRO)
  Briefly describes the proposed site for the Square Kilometre Array.
  http://www.astro.uwa.edu.au/ska/mro

• Square Kilometre Array
  Briefly describes the proposed Square Kilometre Array and the candidate site in Australia.
  http://www.astro.uwa.edu.au/ska/the_square_kilometre_array

Australian Broadcasting Corporation

• The Square Kilometre Array and detecting complex organic molecules in space (Science Show, 21 February 2009)
  Discusses the detection of organic molecules in space and how the Square Kilometre Array will help.
  http://www.abc.net.au/rn/scienceshow/stories/2009/2497251.htm

• The view beyond the Milky Way (News in Science, 25 May 2000)
  Reports on a survey based in Australia that mapped galaxies using radio emissions from neutral hydrogen.
  http://www.abc.net.au/science/articles/2008/11/25/2428882.htm?site=science

Telescopes from the ground up (Amazing Space, USA)

Provides a clear overview of different types of telescopes and their history.
http://amazing-space.stsci.edu/resources/explorations/groundup/

Glossary

dark energy. A form of energy believed to make up 73 per cent of the universe. Dark energy has been proposed to account for the accelerating expansion of the universe. It is unknown whether it is constant throughout the universe or whether it varies in space and time. The properties of dark energy are investigated by observing its effects on the universe.

dark matter. Matter that is not visible but makes up around 22 per cent of the universe (normal matter only makes up 5 per cent). The observable matter in clusters of galaxies is not enough to create a gravitational field that would hold them together. Hence it is believed unseen dark matter accounts for the remaining mass.

ion. A positively or negatively charged atom or group of atoms.

neutron star. A middle-sized star that has used up its nuclear fuel so no longer has an energy source. This causes the star to undergo gravitational collapse. Neutron stars have an extremely high density; a teaspoon of their matter would weigh several million tonnes. Pulsars are believed to be neutron stars.

pulsar. A star that emits radiation at regular intervals. Believed to be neutron stars, pulsars emit radio signals as they rotate at very high speeds.

quasar. An abbreviation for quasi-stellar due to the resemblance of quasars to stars. Quasars are extremely distant, bright objects from the early universe. They are thought to be the cores of distant galaxies.

sensitivity. The ability of an object to detect weak signals eg, the sensitivity of a radio telescope is its ability to detect weak radio signals.

spectroscopy. The technique of detecting and analysing the spectrum of an object to get information on its chemical and physical nature (eg, temperature, motion). Using a spectroscope the radiation or light from an object is dispersed into its different colours or wavelengths (like a rainbow). The position of emission and absorption lines in the spectrum provides information on what chemicals are present. For example, emission at a wavelength of 21 centimetres corresponds to hydrogen. Large telescopes have spectroscopes to measure the properties of astronomical objects.

spectrum. Plural spectra. The distribution of electromagnetic radiation when it is dispersed (eg, the dispersal of visible light into a rainbow). Astronomers gain different information about astronomical objects by examining their spectra from different parts of the electromagnetic spectrum (eg, visible light, radio waves, X-rays).

wavelength. The distance between two adjacent wave crests. Visible light and X-rays are both electromagnetic waves and differ from each other only in the length of the wave. The wavelength of visible light ranges from 400 to 700 nanometres while the wavelength of X-rays ranges from about 0.01 to 10 nanometres. The relatively long wavelength of visible light sets the limit of how small an image it can produce. For more information see Electromagnetic radiation (Back to basics, Australian Academy of Science).