Australasian Science Magazine

Artist’s impression representing the first phase of the Square Kilometre Array at night, with its two instruments SKA1 LOW (in Australia, on the left) and SKA1 MID (in South Africa, on the right). SKA1 LOW will comprise 130,000 dipole antennas while SKA1 MID will comprise 200 dishes, including 64 MeerKAT dishes. Credit: SKA Organisation

By Willem van Straten

The Square Kilometre Array is an unprecedented international collaboration to build the world's largest radio telescope and address some of the most fundamental questions of modern science.

When it comes to radio waves, most of us first think about the various devices that we use for communication, such as the radios in our cars and the phones in our pockets. However, there are many natural sources of radio emission, and as soon as we stop listening to ourselves we discover a universe full of signals from some of the most extraordinary places.

Our Sun is the brightest object in the radio sky. Its solar flares release enormous amounts of energy and produce crackling radio noise that can be detected easily with a small antenna.

Neutron stars are the collapsed remnants of once-bright stars. They are often visible only by their radio emissions, which sweep past the Earth like a lighthouse beam with each rotation of the star.

Supermassive black holes at the hearts of distant galaxies accelerate jets of material to almost the speed of light. This produces relativistic electrons that emit radio waves as they spiral around the embedded magnetic field.

Hydrogen, the most abundant element in the universe, can spontaneously emit a photon with a well-defined radio wavelength near 21 cm.

Finally, at the current limit of observable reality, the hot fireball that was our universe shortly after the Big Bang is visible today as a faint radio afterglow known as the cosmic microwave background.

To receive signals from the most distant reaches of our galaxy and across the universe requires a sensitive telescope. The sensitivity of a radio antenna is determined, among other things, by its collecting area.

Collecting area is the most outstanding feature of the Square Kilometre Array (SKA) project, an ambitious international collaboration to build the largest and most sensitive radio telescope. The SKA will also stand out in a number of other indicators that typically determine the performance of a radio telescope, including:

• field of view, which enables it to survey more sky in less time;
• angular resolution, which provides the ability to distinguish smaller features in an image;
• frequency resolution, which improves a wide variety of measurements, including better separation of molecular lines and more precise estimation of cosmological redshifts; and
• temporal resolution, which facilitates the measurement of phenomena that fluctuate on very short time scales.

Because it will be composed of a large number of small antennas spread over great distances, the SKA will have exceptional sensitivity, a large field of view, and the ability to produce high-resolution images of the radio sky. Furthermore, high time and frequency resolution will be furnished by the sophisticated instrumentation that will process the signals from each antenna.

To image the radio sky, the SKA will employ a technique known as interferometry, which exploits the fine patterns that emerge when waves interfere with each other to make high-resolution images of the radio sky. The signals from hundreds of antennas distributed across several hectares will be combined in pairs, creating tens of thousands of interference patterns, each like the diffraction pattern seen in the double-slit experiment of classical wave optics. These are decoded to produce an image of the radio sky with an angular resolution that is thousands of times better than would have been possible with only a single dish.

The SKA will produce exquisitely detailed maps of the neutral hydrogen in our universe, which will help astronomers understand how the first galaxies formed (see Stuart Wyithe, p.20). These maps will also show us how galaxies evolved within the large-scale cosmological structure of the universe and provide valuable new insights into the poorly understood physics of dark matter, dark energy and inflation (see Martin Meyer and Chris Blake, p.23).

The SKA will simultaneously measure the polarisation of the radio emission from relativistic electrons in the most distant galaxies. Melanie Johnston-Hollitt (p.26) describes how this information will be decoded to study the magnetic fields through which the radio waves travelled on their way to Earth, from the magnetism within our galaxy to the large-scale fields that span cosmological distances.

The SKA’s imaging processor is optimised for the study of large-scale structures, like galaxies and cosmic magnetic fields, that evolve very slowly over time. However, to study compact objects that are too small to be imaged, such as neutron stars and stellar mass black holes, a telescope must also be sensitive to radio signals that fluctuate on time scales shorter than a microsecond. Therefore, in addition to its imaging capability, the SKA will be equipped with time domain processors for the study of transient events (Tara Murphy, p.29) and pulsars (George Hobbs, p.32). In time domain mode, the signals from multiple antennas in the array will be added together to produce a single synthesised beam that focuses the combined sensitivity of the antennas on a very small patch of sky.

This is perfect for high precision studies of individual compact objects, such as known pulsars. However, the greatly reduced beam size presents a significant challenge when searching for new pulsars and radio transient events.

To survey such large areas of sky in a reasonable amount of time, the SKA will tile the primary beam with thousands of synthesised beams. The data generated by this multi-beam technique would fill over 14,000 Terabyte disks per day.

It is currently not feasible to store such large volumes of data, even for a short amount of time. Therefore, the survey engine that will search for pulsars and transient events must process the data as quickly as it is recorded, making this task one of the primary computational challenges facing the SKA.

Both imaging and time domain detection techniques are used in the search for extraterrestrial life, which ranges from surveys for the planetary environments and the organic molecules that support life to searches for the radio signals broadcast – intentionally or not – by other civilisations. Carol Oliver and Ian Morrison (p.34) describe how the sensitivity of the SKA will allow us to search a greater volume of our neck of the galaxy for evidence of intelligent neighbours.

To process the enormous volumes of data produced by the SKA, Peter Quinn (p.37) argues that astronomers must research and develop new computational skills and computing architectures. The required innovation in big data science will be driven by big questions in astrophysics and cosmology.

The scale and nature of the SKA project is unprecedented in radio astronomy. Therefore it is being designed and developed in stages, beginning with the precursor telescopes that are currently operating and/or under construction, progressing into the initial phase of SKA1 construction in 2018 and entering the final phase of SKA2 construction in 2023.

The current plan is to build two arrays: a low-frequency dipole array hosted in Western Australia (Fig. 1), and a mid-frequency dish array in South Africa (Fig. 2). The teams who will design, build, operate and use the SKA will come from all over the world.

This special edition of Australasian Science celebrates the international SKA project by providing an overview of the broad range of scientific questions that astronomers hope to answer using the SKA. These questions probe the depths of human curiosity, from the origin of our universe and the nature of space and time to the evolution of galaxies and the emergence of intelligent life forms to contemplate such things.

Willem van Straten is a Senior Lecturer at Swinburne University of Technology, and Guest Editor of this edition of Australasian Science.