The square kilometer array: a revolution in radio astronomy

Telescopes in the 20^th century revealed an expanding universe with billions of star-filled galaxies, black holes, planets, and gas clouds. Now, in the 21^st century, we have the opportunity to address unanswered questions about the origin and evolution of the universe. Radio emission from neutral hydrogen (HI) provides the best observational window to view the era after the Big Bang, when gas first turned into stars and galaxies. For two decades, astronomers have recognized the need for an exceptionally sensitive radio telescope that can probe the gaseous component of the early universe and detect HI. The Square Kilometer Array (SKA) will detect HI in the local universe and also reveal the faintest (highly redshifted) emissions from the furthest reaches of the universe.¹

To achieve such unique sensitivity, SKA's size must far exceed that of any existing telescope. Its huge collecting area (around one square kilometer, hence the name) will provide unprecedented sensitivity and, with the much larger fields of view offered by novel receptor technologies, enable a far higher survey speed than anything currently in use. Its enormous footprint of receptors spread across an entire continent will also give exceptionally high angular resolution. SKA will provide astronomers with a clear picture of how galaxies have evolved over a large fraction of the history of the universe. It will also help elucidate the role of magnetism in the cosmos, the nature of gravity, and possibly even detect life beyond Earth.

From its conception, the SKA project has been an international collaboration and it now encompasses some 70 institutes in 20 countries. Participating organizations are designing and building prototype systems, many of which will be integrated into the final instrument. Two regions, Australia/New Zealand and southern Africa, have been short-listed in the competition to host the telescope. The final site selection will be made in 2012. The center of the array will be situated in a desert region in either Western Australia or the Northern Cape province in South Africa—see Figure 1(A)—with antennas extending across the continent: see Figure 1(B).²

Figure 1. (A) Fifty percent of the collecting area will be concentrated in the central region. (B) Antennas, logarithmically spaced in spiral arms, will extend more than 3000km from the central region. (Image credit Swinburne Astronomy Productions.)

SKA will initially cover the frequency range from 70MHz to 10GHz (4m to 3cm wavelengths), with a future extension to 25GHz or more. In the higher part of the frequency band, the antennas will comprise up to 3000 dishes (see Figure 2). In the lower part, the antennas will be fields of dense aperture-array tiles and sparse aperture-array dipoles (see Figure 3).³ Signals received by the antennas will be transferred to a central signal-processing system and high-performance computer by optical fiber links carrying up to 420Gb/s per dish and 16Tb/s per aperture array. A special-purpose central-processing system will process as much as one petabyte of astronomical data every 20s, requiring exascale computing and exabyte data storage.

Figure 2. Each dish will be approximately 15m in diameter and carry low-noise, innovative feed and receiving systems. (Image credit Swinburne Astronomy Productions.)

Figure 3. The dense (A) and sparse (B) aperture arrays have no moving parts and can observe several large areas of sky simultaneously. (Image credit Swinburne Astronomy Productions.)

A set of key science projects, selected to fundamentally progress modern astronomy, physics, and astrobiology, have been identified as priorities for SKA.⁴ The final frontier in cosmology is to fill in the gap between 300,000 and a billion years after the Big Bang, a period known as the Dark Ages. We expect SKA to observe the collapse of primordial gas to form the first stars. Although new IR telescopes should also see the first stars, the birth of these stars is only detectable through radio emission of highly redshifted HI.

The accelerated expansion of the universe has been attributed to the mysterious action of ‘dark energy.’ SKA's vast sensitivity will enable detection of extremely redshifted HI emission, providing an unprecedented 3D map that will be able to track young galaxies and help identify the nature of this dark energy. Detecting alterations in the precise timing of pulsars (pulsating compact radio sources), some of which orbit black holes, could provide evidence for the existence of gravitational waves and enable astronomers to investigate the nature of gravity and explore the limits of Einstein's theory of general relativity.

Despite our understanding of electromagnetism, we still cannot answer basic questions about the origin and evolution of cosmic magnetic fields. A polarized radio wave propagating through a magnetized region undergoes a systematic distortion known as Faraday rotation. By detecting this effect, SKA will be able to create 3D maps of cosmic magnets. This new approach will reveal what cosmic magnets look like and how they stabilize galaxies, influence the formation of stars and planets, and regulate solar and stellar activity.

The gas from which stars form is known to contain surprisingly complex organic molecules. SKA's sensitivity will enable it to probe still more complex molecules and perhaps even detect the building blocks of life. SKA will also be able to detect exceptionally faint signals, no stronger than those generated by terrestrial TV and radar, from nearby stars.

The SKA will be a telescope of superlatives. It will be the largest, the most sensitive, and have the fastest survey speed. It will be able to uncover more about our universe than any previous radio telescope. The target construction cost is 1,500 million Euros. Construction of the first elements is scheduled for 2015, the first experiments will take place in 2017, and the array will be completed in 2022.

We acknowledge the contributions from the many institutions around the world that support the SKA project.