Square kilometre array
Understanding the evolution of the Universe, galaxies and stars requires looking back in time as far as possible. But the radiation from distant objects is incredibly weak and its detection needs huge collecting areas. Increasing sensitivity provided by the collecting area will reveal new classes of cosmic objects, distant and nearby, which are too faint or too short-lived to have been detected so far. One of these huge telescopes is the Square Kilometre Array (SKA)  in the radio wavelength part of the electromagnetic spectrum, planned for a dual-site construction between 2018 and 2025 in Southern Africa and Australia. Radio waves carry signals from gas clouds emitted even before the formation of the first stars. The SKA will also constrain fundamental physics on gravitation and magnetism. It will conduct astro-biological observations, potentially including the detection of life elsewhere in the Universe via their radio signals.
Radio waves provide a number of advantages: unlike optical waves, they are not absorbed by interstellar dust and they mostly do not suffer from distortions in the atmosphere, except for the shortest wavelengths of a few mm and below. The radio window for ground-based observations spans frequencies from about 10 MHz (30 m wavelength), below which the Earth's ionosphere blocks cosmic radio waves, to frequencies between 10 GHz (3 cm) and 1 THz (0.3 mm), depending on height above sea-level and water content of the troposphere.
Radio waves emerge from objects widely different from the well-known sources of light. Observations at radio wavelengths led to the modern view of the Universe: discovery of the cosmic microwave background (CMB, see below), the first notion of non-thermal emission from charged particles in magnetic fields, discovery of quasars, pulsars, masers and extrasolar planets. Some of the most spectacular objects in the Universe are radio sources whose radiation is emitted from hot gas and charged particles around black holes (quasars) and in the magnetospheres around neutron stars (pulsars), remainders of supernova explosions. Cold gas in galaxies, invisible in the optical range, can be radio-bright when emitting in specific radio spectral lines. Radio waves tell us that the Universe does not only consist of stars, gas and dark matter, but is also permeated by superfast "cosmic ray" particles and magnetic fields which emit synchrotron emission over a wide (continuous) frequency range in the radio, while they escape detection in most other spectral ranges.
Radio astronomy is another window to the Universe where known objects look different and new objects shine. The radio window allows us to look deep into space and hence deep into the past, and we can observe how the gas, fast particles and magnetic fields have developed over time. Scientists worldwide are extremely excited about the possibilities offered by the SKA.
Key Science Projects
The large investment in the SKA requires convincing justification. Apart from the expected technological spin-offs, five main science questions ("Key Science Projects") drive the SKA (see the SKA homepage  and Further Reading below for details):
- Probing the dark ages
The SKA will use the emission of neutral hydrogen to observe the most distant objects in the Universe. The strongest line emission of hydrogen is in the radio range at a frequency of 1.4 GHz (21 cm wavelength) which corresponds to the energy difference of the hyperfine transition when the spin of the electron flips with respect to that of the proton.
According to present-day cosmological models, the Universe became transparent about 380,000 years after the big bang (at a redshift of about 1100). The radiation released at that time is now prominent in the radio range as the Cosmic Microwave Background (CMB) (Durrer 2008), measured in great detail by NASA’s WMAP satellite  and since 2009 by ESA’s PLANCK satellite . Matter (mostly hydrogen) remained neutral and smoothly distributed over the next billion years, called the dark ages, until the first stars and black holes formed, followed by the formation of galaxies. The energy output from the first energetic stars and the jets launched near young black holes (quasars) started to heat the neutral gas, forming bubbles of ionized gas as structure emerged. This is called the Epoch or Reionization (see Fan et al. 2006 for a review). The signatures from this exciting transition phase should still be observable with help of the radio line of hydrogen, though extremely redshifted by a factor of about 10 when arriving at our telescopes today (Fig. 1). The lowest SKA frequency will allow us to detect hydrogen at redshifts of up to 20, well into the dark ages, to search for the transition from a neutral to an ionized Universe, and hence provide a critical test of our present-day cosmological model.
- Galaxy evolution, cosmology, and dark energy
The expansion of the Universe is currently accelerating, a poorly understood phenomenon, for which a multitude of possible explanations have been proposed: Einstein's cosmological constant , a time-dependent energy called quintessence, topological defects, the effects of "other" Universes and many more. Since the correct answer is not known, physicists and astronomers named the phenomenon dark energy  (see also Frieman et al. 2008 for a review).
One important method of distinguishing between these various explanations is to compare the distribution of galaxies at different epochs in the evolution of the Universe to the distribution of matter at the time when the Cosmic Microwave Background (CMB, see above) was formed, about 380,000 years after the Big Bang. Small distortions ("ripples") in the distribution of matter, called baryon acoustic oscillations, should persist from the era of CMB formation until today. Tracking if and how these ripples change in size and spacing over cosmic time can then tell us if one of the existing models for dark energy is correct or if a new idea is needed.
The SKA will use the hydrogen emission from galaxies to measure the properties of dark energy. The strongest line emission of hydrogen is in the radio range at a frequency of 1.4 GHz (21 cm wavelength), but redshifted to lower frequencies/longer wavelengths for distant galaxies. A deep all-sky SKA survey will detect hydrogen emission from galaxies out to redshifts of about 1.5, at a distance of about 9 billion light years, or at a time when the Universe was about 4.7 billion years old. The galaxy observations will be “sliced” in different redshift (time) intervals and hence reveal a comprehensive picture of the Universe's history.
The same data set will give us unique new information about the evolution of galaxies. How the hydrogen gas was concentrated to form galaxies, how fast it was transformed into stars, and how much gas did galaxies acquire during their lifetime from intergalactic space and by merging with other galaxies? Present-day telescopes have difficulty in detecting intergalactic hydrogen clouds with no star formation activity and distant dwarf galaxies, but these sorts of radio sources will be easily detectable by the SKA. The hydrogen survey will simultaneously give us the synchrotron radiation intensity of all galaxies which is a measure of their star-formation rate and magnetic field strength.
- Tests of General Relativity and detection of gravitational waves with pulsars and black holes
The radio-astronomical discovery of pulsars and the indirect detection of gravitational waves from a pulsar-star binary system were rewarded with two Nobel prizes for physics. Pulsars are precise clocks and can be used for further experiments in fundamental physics and astrophysics. Einstein’s Theory of General Relativity has precisely predicted the outcome of every test experiment so far. However, no tests in the strong gravitational field around black holes have yet been made. The SKA will search for a radio pulsar orbiting around a black hole (Fig. 2), the remnants from the supernova explosions of two massive stars in a binary system, measure time delays in extremely curved space with much higher precision than with laboratory experiments and hence probe the limits of General Relativity (Lorimer & Kramer 2004).
Regular high-precision observations with the SKA of a network of pulsars with periods of milliseconds opens the way to detect gravitational waves with wavelengths of many light years, as expected for example from two massive black holes orbiting each other with a period of a few years resulting from galaxy mergers in the early Universe. When such a gravitational wave passes by the Earth, the nearby space-time changes slightly at a frequency of a few nHz (about 1 oscillation per 30 years). The wave can be detected as apparent systematic delays and advances of the pulsar clocks in particular directions relative to the wave propagation on the sky.
We expect that more than 20,000 new pulsars will be detected with the SKA, compared to about 2000 known today. Almost all pulsars in the Milky Way (Fig. 3) and several 100 bright pulsars in nearby galaxies will become observable.
- Origin and evolution of cosmic magnetism
Electromagnetism is one of the fundamental forces, but little is known about its role in the Universe. Large-scale electric fields induce electric currents and are unstable, whereas magnetic fields can exist over long times because, mysteriously, single magnetic charges (monopoles) are missing in the Universe. Data suggest that all interstellar and probably intergalactic space is permeated by magnetic fields, but these are extremely hard to observe. Radio waves provide two tools: synchrotron radiation emitted by cosmic-ray electrons spiraling around magnetic field lines with almost the speed of light, and Faraday rotation of the polarization plane when a polarized (synchrotron) radio wave passes through a medium with magnetic fields and thermal electrons. Both methods have been applied to reveal the large-scale magnetic fields in our Milky Way, nearby spiral galaxies (Fig. 4), and in galaxy clusters, which are probably amplified and maintained by dynamo action , but little is known about magnetic fields in the intergalactic medium (Wielebinski & Beck 2005). Furthermore, the origin and evolution of magnetic fields is still unknown. The first "seed" fields may originate in the very young Universe or may have been ejected from the first quasars, stars, or supernovae.
The SKA will measure the Faraday rotation towards several tens of million polarized background sources (mostly quasars), allowing us to derive the magnetic field structures and strengths of the intervening objects, such as, the Milky Way, distant spiral galaxies, clusters of galaxies, and in intergalactic space.
- The cradle of life
The presence of life on other planets is a fundamental issue for astronomy and biology. The SKA will contribute to this question in several ways. Firstly, it will be able to detect the thermal radio emission from centimeter-sized "pebbles" in protoplanetary systems (Fig. 5) which are thought to be the first step in assembling Earth-like planets. The SKA will allow us to detect a protoplanet separated from the central star by spacings of order the Sun-Earth separation out to distances of about 3000 light years.
Biomolecules are observable in the radio range, for example, "cold sugar" glycolaldehyde (CH2OHCHO) which has several lines between 13 and 22 GHz. Prebiotic chemistry - the formation of the molecular building blocks necessary for the creation of life - occurs in interstellar clouds long before that cloud collapses to form a new solar system with planets.
Finally, the SETI (Search for Extra Terrestrial Intelligence)  project (see Tarter 2001 for a review) will use the SKA to find hints of technological activities. Ionospheric radar experiments similar to those on Earth will be detectable out to several thousand light years, and Arecibo-type radar beams, like those that we use to map our neighbor planets in the solar system, out to as far as a few ten thousand light years. SETI will also search for such artificial signals superimposed onto natural signals from other objects.
Core science drivers
From the five Key Science Projects (see above) two major science goals have been identified that drive the technical specifications for the first phase (SKA1):
- Origins: Understanding the history and role of neutral hydrogen in the Universe from the dark ages to the present-day
- Fundamental Physics: Detecting and timing binary pulsars and spin-stable millisecond pulsars in order to test theories of gravity.
Exploration of the Unknown
While the experiments described above are exciting science, the history of science tells us that many of the greatest discoveries happen unexpectedly and reveal objects which are completely different from those which had been envisaged during the planning phase of a new-generation telescope. For example, the serendipitous discovery of pulsars was made with a low-frequency telescope at Cambridge/UK that had been designed to measure the effects of the ionized interplanetary medium on radio waves. The unique sensitivity of the SKA will certainly reveal new classes of cosmic objects which are totally beyond our present imagination. We are looking forward to such surprises.
Similar to present-day radio interferometers, like the Very Large Array (USA), the Westerbork Synthesis Radio Telescope (Netherlands), the Australia Telescope Compact Array and the Allen Telescope Array (USA), the SKA will consist of many antennas which are spread over a large area. The resolving power is proportional to the frequency and to the largest baseline between the outermost antennas and hence is much higher than for single dish telescopes. The signals are combined in a central computer (correlator). While the radio images from present-day interferometric telescopes are generally produced offline at the observer's institute, the enormous data rates of the SKA will demand online image production with automatic software pipelines.
With a collecting area of about one square kilometer, the SKA will be about ten times more sensitive than the largest single dish telescope (305 m diameter) at Arecibo (Puerto Rico) , and fifty times more sensitive than the currently most powerful interferometer, the Jansky Very Large Array (JVLA, at Socorro/USA) . The SKA will continuously cover most of the frequency range accessible from ground, from 50 MHz to 3 GHz (corresponding to wavelengths of 10 cm to 6 m) in the first phase, later to be extended to at least 14 GHz (2 cm). The third major improvement is the enormously wide field of view, ranging from about 40 square degrees at 50 MHz to about 18 square degree at 1.4 GHz. The speed to survey a large part of the sky, particularly at the lower frequencies, will hence be ten thousand to a million times faster than what is possible today.
To meet these ambitious specifications and keep the cost to a level the international community can support, planning and construction of the SKA requires many technological innovations such as light and low-cost antennas, detector arrays with a wide field of view, low-noise amplifiers, high-capacity data transfer, high-speed parallel-processing computers and high-capacity data storage units. The realization needs multifold innovative solutions which will soon find their way into general communication technology.
The frequency range spanning more than two decades cannot be realized with one single antenna design, so this will be achieved with a combination of different types of antennas. Under investigation are the following designs for the low and mid-frequency ranges:
1. SKA-low: An aperture array of simple dipole antennas with wide spacings (a "sparse aperture array") for the low-frequency range (about 50-350 MHz) (Fig. 6). This is a software telescope with no moving parts, steered solely by electronic phase delays. It has a large field of view and can observe towards several directions simultaneously.
2. SKA-mid: An array of several thousand parabolic dishes of 15 meters diameter each for the medium frequency range (about 350 MHz - 14 GHz), each equipped with wide-bandwidth single-pixel "feeds" (Fig. 7). The surface accuracy of these dishes will allow a later receiver upgrade to higher frequencies. The central region will contain about 50% of the total collecting area and comprise (1) separate core stations of 5 km diameter each for the dish antennas and the two types of aperture arrays (Fig. 6), (2) the mid-region out to about 180 km radius from the core with dish and aperture array antennas aggregated into "stations" distributed on a spiral arm pattern, and (3) "remote" stations with about 20 dish antennas each out to distances of at least 3000 km and located on continuations of the spiral arm pattern. The overall extent of the array determines the angular resolution, which will be about 0.03 seconds of arc at 350 MHz and 0.001 seconds of arc at 10 GHz.
3. SKA-survey: A compact array of parabolic dishes of 12–15 meters diameter each for the medium-frequency range, each equipped with a multi-beam, phased array feed with a huge field of view and several receiving systems covering about 350 MHz – 4 GHz. This array will allow fast surveys of the sky.
As an "Advanced Instrumentation Programme" for the full SKA, two additional technologies for substantially enhancing the field of view in the 1-2 GHz range are under rapid development: aperture arrays for medium frequencies with dense spacings (Fig. 8) and phased-array feeds for the parabolic dishes of SKA-survey (see below). The technologies for a wide field of view are currently less mature than the dishes and the low-frequency dipole array but have the promise of significant scientific benefit in further increasing the survey speed once they prove feasible and cost effective. The detailed design for low and mid frequencies will be ready until 2016.
Technical developments around the world are being coordinated by the SKA Science and Engineering Committee and its executive arm, the SKA Project Office. The technical work itself is funded from national and regional sources, and is being carried out via a series of verification programs. The global coordination was supported by funds from the European Commission under a program called PrepSKA, the Preparatory Phase for SKA , whose primary goals were to provide a costed system design and an implementation plan for the telescope by 2012.
A number of telescopes provide examples of low frequency arrays, such as the European LOFAR (Low Frequency Array) telescope, with its core in the Netherlands , the MWA (Murchison Widefield Array) in Australia , PAPER (Precision Array to Probe the Epoch of Reionization), also in Australia , and the LWA (Long Wavelength Array) in the USA . All these long wavelength telescopes are software telescopes steered by electronic phase delays ("phased aperture array").
The first LOFAR stations saw "first light" in 2007 in the frequency band 10-80 MHz and in 2009 in the frequency band 110-240 MHz (Fig. 9). Full operation of LOFAR with 38 Dutch and 8 stations in other European countries started in 2013.
Examples of dishes with a single-pixel feed are operating already in the USA (Allen Telescope Array, ATA)  and are under development in South Africa (MeerKAT) . The first 12 m prototype dish of the MeerKAT array was completed in 2009.
Dense aperture arrays comprise up to millions of receiving elements in planar arrays on the ground which can be phased together to point in any direction on the sky. Due to the large reception pattern of the basic elements, the field of view can be up to 250 square degrees. Dense aperture arrays have been the subject of a European Commission-funded design study named SKA Design Study (SKADS) which has resulted in a prototype array of 140 square meters area (EMBRACE) .
The technology of dense phased-array feeds (PAF) can also be adapted to the focal plane of parabolic dishes. Such a "radio camera" is composed of many elements (pixels) which are controlled and combined electronically. This allows the dishes to observe over a far wider field of view than when using a classical single-pixel feed. Prototypes of such wide-field cameras are presently constructed in Australia (ASKAP) , the Netherlands (APERTIF)  and in Canada (AFAD) . The first of the 36 dish antennas (12 meter size) of ASKAP in Western Australia have already been equipped with PAF prototypes.
To summarize the various international activities: ASKAP, MWA and MeerKAT are SKA Precursor telescopes and are located on the two candidate sites, Australia and South Africa, respectively. SKA Pathfinder telescopes develop technology or science projects related to the SKA, such as LOFAR, EMBRACE, APERTIF, ATA, LWA, the Arecibo dish and the EVLA dish array. SKA Design Studies include the SKA Design Study (SKADS, Europe), the SKA Program (Canada) and the Technology Development Project (TDP, USA).
To obtain radio images, the data from all stations have to be transmitted to a central computer and processed online. Compared to LOFAR with a data rate of about 300 Gigabits per second and a central processing power of 27 Tflops, the SKA will produce much more data and need much more processing power - by a factor of at least one hundred. Following "Moore’s law" of increasing computing power, a processor with sufficient power should be available by the end of this decade. The energy consumption for the computers and cooling will be tens of MegaWatts.
Timeline and site
Construction of the SKA is planned to start in 2018. In the first phase (until 2020) about 10% of the SKA will be erected (SKA Phase 1, SKA1), with completion of construction (SKA Phase 2, SKA2) at the low and mid frequency bands by about 2025, followed by construction at the high band. The total costs of the SKA are 650 million € for SKA1 plus about 1,500 million € for SKA2 (estimate from 2007), to be shared among the countries of the worldwide collaboration.
In 2011, the SKA Organisation was founded, with presently ten members (Australia, Canada, China, Germany, Italy, the Netherlands, New Zealand, South Africa, Sweden and the United Kingdom) and one associated member (India). On 25 May 2012, the Members of the SKA Organisation agreed on a dual site solution for the SKA with two candidate sites fulfilling the scientific and logistical requirements: Southern Africa, extending from South Africa, with a core in the Karoo desert, eastward to Madagascar and Mauritius and northward into the continent, and Australia, with the core in Western Australia.
Building of SKA1
190 15-meter dishes of SKA1 will be built in South Africa, combined with the 64 MeerKAT dishes in South Africa and equipped with three single-pixel receivers for the frequency range 350-3050 MHz ("SKA1-mid"). 60 15-meter dishes will be added to the 36 dishes of the ASKAP array in Australia and equipped with phased-array feeds for the frequency range 650-1670 MHz ("SKA1-survey"). The low-frequency sparse aperture array of about 250,000 dipole antennas for the frequency range 50-350 MHz will be built in Australia ("SKA1-low").
Further reading on the SKA
The Square Kilometre Array, download from: http://www.skatelescope.org/wp-content/uploads/2011/03/SKA-Brochure_June2011_web_small.pdf
C. Carilli and S. Rawlings: Science with the Square Kilometre Array, New Astronomy Reviews, vol. 48, Elsevier, Amsterdam (2004)
P.E. Dewdney, P.J. Hall, R.T. Schilizzi and T.J.L.W. Lazio: The Square Kilometre Array, Proceedings of the IEEE, 97, 1482-1496 (2009)
P. Hall: The SKA: an Engineering Perspective, Experimental Astronomy, vol. 17, Springer, Berlin (2005)
J. Lazio, M. Kramer and B. Gaensler: Tuning in to the Universe, Sky & Telescope 7/2008, p.20
B.F. Burke, F. Graham-Smith: An Introduction to Radio Astronomy, 3rd ed., Cambridge University Press (2009)
R. Durrer: The Cosmic Microwave Background, Cambridge University Press (2008)
X. Fan, C.L. Carilli and B. Keating: Observational Constraints on Cosmic Reionization, Annual Reviews in Astronomy & Astrophysics, 44, 415-462 (2006)
J.A. Frieman, M.S. Turner and D. Huterer: Dark Energy and the Accelerating Universe, Annual Reviews in Astronomy & Astrophysics, 46, 385-432 (2008)
D.R. Lorimer and M. Kramer: Handbook of Pulsar Astronomy, Cambridge University Press (2004)
J. Tarter: The Search for Extraterrestrial Intelligence (SETI), Annual Reviews in Astronomy & Astrophysics, 39, 511-548 (2001)
R. Wielebinski and R. Beck (eds.): Cosmic Magnetic Fields, Springer, Berlin (2005)
T.L. Wilson, K. Rohlfs and S. Hüttemeister: Tools of Radio Astronomy, 5th ed., Springer, Berlin (2009)