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1. Description of the Infrastructure and National interest – The Square Kilometre Array (SKA, see https://www.skatelescope.org) is an enormous radio telescope array planned to eventually encompass more than one square kilometre of collecting area. The telescope will operate at metre and centimetre wavelengths (initially from frequencies 50MHz to 15GHz, likely extending to 24GHz), covering one of the two great electromagnetic windows accessible from the Earth’s surface without attenuation (the other being the Visible-NIR window). The project is designed to have a lifetime of at least 50 years, i.e. stretching into the 2070’s. Since for radio telescopes critical capabilities (in particular survey speed) are determined by digital electronics and available computational resources, Moore’s law will ensure that the performance of the telescope will continuously increase over its lifetime. The Square Kilometre Array will be one of the great astronomy, computational and engineering endeavours of the 21st century. This proposal concerns Swedish contributions to the construction and operation of phase 1 of SKA (i.e. SKA1). SKA1 will be built on two sites, one in Australia concentrating on low frequencies (SKA1-low) using arrays of dipoles (See Fig 1-left) and one in South Africa (SKA1-mid), concentrating on higher frequencies and using parabolic dishes (see Fig 1-right). These two arrays will have collecting areas of respectively, 419,000 m2 (i.e. 0.42 km2) distributed over 500 stations, and 32,600 m2 distributed over 197 dishes. The maximum baseline lengths for the two instruments will be 80 km and 150 km respectively. SKA1-low will have a sensitivity and sky survey speed which are respectively 8 and 135 times better than that of the present world leading instrument at metre wavelenths, LOFAR. For SKA-mid the corresponding ratios compared to the largest existing centimeter array, the Jansky Very Large Array (JVLA) in the US, are factors of 5 in sensitivity and 60 in survey speed. The total construction cost of SKA1 has a cap at 650M€ (2013 index). Assuming the processes of forming the final SKA treaty organization and raising the required funding go according to plan, SKA1 is expected to start construction in 2018, to execute first science in 2020 and to become fully operational by 2023. While SKA1 construction is underway the design for SKA phase 2 will be finalized. At metre and centimetre wavelengths respectively SKA2 will have >2 and 10 times more collecting area than SKA1. At centimetre wavelengths SKA2 will have a sensitivity 50 times that of the JVLA and have a survey speed 10,000 times faster. The new SKA2-mid element will also have baselines of up to 3000 km encompassing dishes located in several countries spread over Southern Africa. SKA2 is not expected to be operational until the 2030’s at the earliest. Involvement of partner countries in SKA1 does not imply a commitment to be involved in building SKA2. In this proposal we limit ourselves to SKA1 science.

Fig. 1. Computer generated images of (left) SKA-low dipoles in the Western Australia desert and (right) SKA-mid parabolic dishes in the Karoo desert in South Africa.

The frequencies at which SKA will operate have several unique features compared to other wavelength bands. The radio regime has many important spectral lines including that of atomic hydrogen and mechanisms of radio continuum emission that are uniquely sensitive probes of magnetic fields. Additionally radio waves have an advantage over other frequencies in that they are almost immune to

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obscuration. A further technical advantage of long radio wavelengths is the large field of view of the receiving elements (i.e. 1 – 10 degree diameter) making the instrument ideal for surveying huge volumes of the universe quickly. Interferometry is also easily achieved at radio wavelengths allowing very high spatial resolution, diffraction limited imaging. The combination of the large field of view and high spatial resolution (eventually milli-arcseconds) will give the SKA an unprecedented output data rate. For SKA1 the raw data rate from the antennas will be 150 Tb/s. These raw data will however not be kept but instead continuously be processed into images. This requires 100 PetaFlops of processing at the telescope sites. Total archive storage requirements for these final images will be up to 300 Petabyte per year when SKA1 is fully operational. The SKA will be a huge ICT project driving the development of ‘Big Data’ technologies. The SKA will conduct imaging in both continuum and spectral line emission (especially the 21cm wavelength line of atomic hydrogen) allowing the instrument to map the three-dimensional structure of the universe from the era of the first galaxies to the present day. Up to 10 million galaxies will be detectable in atomic hydrogen and billions of galaxies in radio continuum emission. The SKA’s survey capability will have a huge impact on cosmology and our understanding of galaxy structure, formation and evolution with redshift. The high sensitivity of SKA will allow us to observe a large fraction of galactic pulsars, enabling sensitive tests of fundamental physical theories of gravity and nuclear matter. The SKA will also probe the evolution of magnetism through cosmic history, the formation of proto-planetary disks (forming solar systems such as our own) and detect and study new types of transient sources. As a practical matter the precise SKA design is optimized toward the above highest priority science goals but it should be noted that the SKA is being built as a general observatory and not as a narrowly focused experiment. The SKA is expected to have a major impact on any area of astrophysics that can benefit from radio observations. The national Interest - The SKA represents the future of international radio astronomy and Sweden has a long and ongoing tradition of delivering groundbreaking scientific results using radio astronomical observations. Given this we believe it essential for a large cross section of the Swedish astronomical community to have access to SKA. The Swedish national committee for Astronomy (organized via the Royal Academy of Sciences), in response to a request from VR in 2010, was asked to rank the Swedish interests in four large future astronomy projects, namely the SKA, E-ELT (European Extremely Large Telescope), CTA (Cherenkov Telescope Array) and EST (European Solar Telescope); in this process the SKA was ranked as equal in science interest to the optical E-ELT. At the European level the full SKA science case has been extensively reviewed by expert panels of astronomers within the ASTRONET consortium (formed on the initiative of European science research councils) and has been highly rated. SKA also features prominently in both the ASTRONET ‘Science Vision’ and ‘roadmap’ reports (the latest edition from 2013, see references). Furthermore, the SKA is identified as a priority for European infrastructure investments within the EU sponsored ESFRI roadmap (see ESFRI link in references). Finally the SKA is listed in the recently published 2014 Swedish guide to infrastructures as an important potential future infrastructure for astronomy. 2. Infrastructure vision and science goals – Progress in astronomical research is largely driven by increasing the sensitivity of telescopes, allowing detection of either intrinsically fainter or more distant objects. Additionally, increasing frequency and time resolution opens up new parts of discovery space. SKA therefore combines increased sensitivity with high frequency resolution and high time resolution to enable breakthrough discoveries across all of astronomy. The design of the telescope is however guided by six, carefully selected, broad themes, viz; 1- Probing the Cosmic Dawn and the Epoch of Reionization (EoR) – The SKA will probe the second phase transition of the universe and trace the sources of ionizing radiation from the first stars and black holes and observe earlier epochs than are accessible with any other type of telescope.

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2- Galaxy Evolution, Cosmology, and Dark Energy – The SKA will study the evolution of galaxies and the process of conversion of gas into stars through cosmic time. By mapping the distributions of galaxies in 3D space the SKA will also constrain structure formation in the universe and the equation of state of dark energy. 3- Strong-field Tests of Gravity using Pulsars and Black Holes – The SKA will test the properties of matter under extreme densities, seek to detect long wavelength gravitational waves by pulsar timing and test General-Relativity using pulsars orbiting black holes. 4- The Origin and Evolution of Cosmic Magnetism – The SKA will seek to discover if magnetic fields in the universe are primordial or not and how they evolve over cosmic time. 5- The Cradle of Life – The SKA will image solar systems in formation, detect pre-biotic molecules in the interstellar medium and look out for possible signals from extra-terrestrial intelligence. 6- Exploration of the Unknown – Given the large advance in sensitivity and survey speed over earlier radio telescopes it is likely that many new phenomena will be detected by SKA. The original assessment of the science impact of SKA was presented in the book ‘Science with the Square Kilometre Array’ (Carilli and Rawlings 2004). This science case has recently been updated and expanded, in a process which started with an open science meeting held in Sicily in June 2014 with over 250 attendees. Based on the presentations given at that meeting and other submissions a new science book is being produced with 9 summary chapters and 121 contributed chapters. The final version of this book will be available in July 2015 (see key references for more information) and demonstrates the extremely wide range of astronomy that will be advanced with the SKA. In the process of optimizing the final configuration of SKA1 over the last six months a set of critical science projects were defined drawn from the above key science areas (see Braun et al 2014, ‘Science Priority Outcomes’ in the list of key references for details). Of these critical projects those within the areas of Epoch of Reionization and tests of physics using pulsars were given the highest priority. An exhaustive engineering process then looked at a very large number of possible SKA1 architectures to optimize the science that could be achieved within the cost cap of 650 M€; the final recommended optimum design for SKA1 was then approved by the SKA board in its meeting of 5th March 2015 (a fuller description of the technical aspects of this design is given in Appendix B). Details on the key science that can be achieved with this final SKA1 design are given in the following section. 3. Scientific Motivation – Below we give a longer scientific description of the main thematic science areas of the SKA, highlighting some of the main areas of Swedish interest and involvement. 3.1 Probing the Cosmic Dawn and the Epoch of Reionization – Approximately 13 billion years ago, the first stars and galaxies formed in the then cold and neutral Universe. The ionizing ultraviolet light from these stars partly escaped from the galaxies and entered the regions in-between the galaxies, the so-called Intergalactic Medium. As it did so, it both heated and ionized this medium, causing the last global phase transition in the Universe during what is commonly referred to as the Epoch of Reionization (EoR). The EoR is at the forefront of modern cosmological research and the SKA will transform our understanding of it by mapping the distribution of atomic hydrogen (`HI') throughout the entire period (shown schematically in Fig. 2). The SKA will thus show how and when the HI gradually disappeared as the Intergalactic Medium became ionized, providing detailed maps showing which parts of the Universe reionized when and thus measuring the total production of ionizing photons in these regions. These groundbreaking measurements will not only teach us about the distribution and star formation properties of the first generations of galaxies, but will also provide invaluable cosmological information as this period was the last one during which most of the normal, baryonic, matter distribution in our Universe was observable. In addition to mapping the disappearance of the atomic hydrogen, the SKA will also be the first instrument to detect how the even earlier first generations of stars changed the quantum state of the still cold and neutral medium and how the first sources of x-rays, including the progenitors of the supermassive black holes, raised its temperature to

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several hundreds of Kelvin. This period before the EoR is usually called the Cosmic Dawn and the SKA will be the only existing telescope that can directly observe this crucial period in the history of the Universe. All this can be achieved through observations of the red-shifted 21cm signal. However, the SKA will also provide an un-obscured view of the gas content inside these early galaxies and active galactic nuclei via observations of highly red-shifted emission from low-lying molecular transitions (e.g., from CO). Sweden is heavily involved in EoR studies with the SKA precursor instrument LOFAR (including a station at Onsala) and in planning the Cosmic Dawn/EoR observations for the SKA (see Mellema et al 2013). There is also considerable interest in Stockholm and Uppsala in the nature of the first stars and galaxies that are the source of ionizing photons during this era (Östlin, Hayes, Zackrisson) and how ionizing photons escape from galaxies.

Fig.2. Epoch of Reionization observations with the SKA. Top figure illustrates how the first stars and galaxies form as a function of redshift (yellow dots) producing surrounding regions of warm atomic hydrogen (red) and then ionized regions (black). The bottom panels taken from Mellema et al (2013) show a simulation of the radio emission from atomic hydrogen at two different redshifts, one during early and one during late reionization.

3.2 Galaxy Evolution, Cosmology, and Dark Energy – After reionization, substantial amounts of atomic hydrogen are only found inside galaxies. The SKA will have sufficient sensitivity to the 21 cm line of atomic hydrogen to detect individual galaxies similar to our own out to redshifts z ~ 1. Many more galaxies will also be detected in radio continuum tracing their star-formation rates. Another tracer of star-formation is radio supernovae (an interest in Stockholm and Gothenburg) which can be detected in totally optically obscured regions and also provide unique information on the environments in which they explode. One of the key questions for 21st century astronomy is the assembly of galaxies; the SKA will probe how galaxies convert their gas into stars over a significant fraction of cosmic time, all the way from high redshift to detailed studies of local galaxies (see Fig 3). Additionally red-shifted molecular gas can be observed by the high frequency receivers of SKA to trace molecular evolution of galaxies. Galaxy evolution through cosmic time is a major theme of astronomers in Gothenburg (Aalto, Knudsen and others) and from Stockholm (Hayes, Freeland and others) and Uppsala (Zackrisson). In addition to studying galaxies themselves SKA HI observations will also trace the cosmological 3D distribution of galaxies to detect the effects of baryonic acoustic oscillations (BAOs), remnants of early density fluctuations in the Universe, acting as a ‘cosmic ruler’ imposing a fixed linear length on the distribution of galaxies. The SKA1 will average over a large enough sample of galaxies to

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measure the angular size of BAO signals to constrain the scale size of the universe versus redshift and in turn the equation of state of dark energy. The number of galaxies studied by SKA1 will be comparable to those spectroscopically studied by the EUCLID satellite, although the biases of the two instruments will be different. However, it has been recently realised that for cosmological measurements it is not necessary to detect individual galaxies but that the large scale distribution of galaxies can be mapped through the technique of 'intensity mapping' of their 21cm signal. The SKA1_mid will through this technique be able to map the large scale distribution of galaxies out to redshift 3. Other SKA1 constraints on cosmology come from weak lensing effects (distortion of the shapes of background sources from foreground mass concentrations) and the integrated Sachs-Wolf effect (ISW) which causes correlations in Cosmic Microwave Background brightness related to the foreground mass as measured by radio sources. By combining radio observations of weak lensing and ISW with Type Ia supernova measurements (Raccanelli et al 2012) exotic forms of dark energy with redshift variable equation of state pressure/density = w(a) can be distinguished from cosmological constant models with wa=0. There is a long tradition in Sweden of using Type Ia supernovae to constrain cosmology (that will continue via Swedish involvement in the EUCLID satellite) and this on-going research can be very fruitfully combined with the SKA results.

Fig 3. The result of VLA observations of atomic hydrogen in a sample of 34 galaxies ranging from giant spirals to dwarfs (THINGS sample, Walter et al. 2008, ApJ 136, 2648). These observations allow the derivation of galactic total mass distributions (baryonic plus dark matter) out to large radii and the distribution/kinematic of galactic atomic gas. The latter gas phase, accreted from the cosmic web in the intergalactic medium, likely provides the ultimate fuel source for star formation in spirals. The SKA will allow such detailed imaging studies for tens of thousands of galaxies over a range of redshifts.

3.3 Strong-field Tests of Gravity using Pulsars and Black Holes – Radio emitting rotating neutron stars (Pulsars) have extreme conditions of density and magnetic field making them important physical laboratories. Furthermore, pulsars are also uniquely suitable for high accuracy kinematical studies by pulse timing or astrometry. The potential of pulsars has already been demonstrated by the award of two separate Nobel Prizes in Physics. In the SKA era pulsars will continue to probe fundamental physics in numerous ways (see Kramer 2014). The SKA will enormously expand the known population of both known normal and millisecond pulsars (totaling 9,000 for SKA1, about 25% of all pulsars in the Milky Way). By measuring the time of arrival of their pulses, the SKA can constrain the orbits of pulsars in binaries via the Doppler effect. For a subset of these, this will probe General Relativity in the strong field limit around black holes in both pulsar- stellar mass black hole binaries and pulsars orbiting the central supermassive black hole in the centre of our galaxy. Topics probed in gravitational physics using these observations will include; ‘the no-hair theorem’, frame dragging and cosmic censorship conjecture; such observations will also provide accurate determination of spin and mass of black holes. Accurate timing of binary pulsar orbits also constrains the pulsar material stiffness and hence the equation of state of matter at nuclear densities, testing high-energy physics models. Complementary precision timing of single pulsars will allow us to construct the so called ‘Pulsar Timing Array’ (PTA), in effect a vast galactic-sized machine with the SKA at its centre for the detection of nano-Hz gravitational waves (see Fig 4). This set up will allow the detection of stochastic gravitational wave backgrounds from the merging supermassive black hole population in either the early or nearby Universe or gravitational waves from exotic phenomena such as cosmic strings. PTA

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observations will provide excellent spatial locations for individual periodic or burst sources and hence enable follow-up observations at multiple frequencies. Furthermore, by exploiting the long baselines between pulsars, direct geometrical distances to gravitational wave sources closer than 100 Mpc can be determined via ‘timing parallax’ measurements of wavefront curvature (see Deng and Finn 2012). PTA observations will also set limits on theories of gravity including the properties of the graviton. In Sweden Vlemmings is PI of a pulsar astrometry project on a SKA precursor instrument (eMERLIN). This project will determine positions, proper motions and parallax distances of a sample of pulsars. These fundamental quantities are needed before using pulsars as physical probes; they are also needed to allow a better understanding of the galactic distributions and space velocities of different pulsar classes constraining stellar population models. The techniques developed in the above project will be directly transferable to SKA1, which will have similar maximum baseline length. More generally in Sweden there is scientific interest in the tests of gravity and gravitational wave detection enabled by SKA pulsar observations. Such interest is especially strong at NORDITA and Stockholm University, where there is also interest in potential multi-messenger observations of the sources of gravitational waves including neutrinos from IceCube, Cosmic rays from the Auger Array and high energy Gamma rays from HESS and CTA.

Fig. 4. Left: Principle of SKA detection of gravitational waves using via timing of samples of pulsars (Pulsar Timing Array, PTA). The passage of very long-wavelength gravitational waves through our region of the galaxy affects space-time and causes slight variations in the time of arrival of the lighthouse-like radio beams from rotating pulsars. Right; Limits on gravitational waves from existing PTAs compared to other methods and expected levels of stochastic GW from various sources (van Haasteren et al 2011). For SKA1 limits on PTAs should be at lest an order of magnitude better. 3.4 The Origin and Evolution of Cosmic Magnetism – Magnetic fields play an important role throughout astrophysics, including in particle acceleration, cosmic ray propagation and galaxy star formation. Unlike gravity, which has been present since the earliest times in the Universe, magnetic fields have likely been generated or enhanced from very small primordial seed fields in galaxies and clusters of galaxies. By measuring the Faraday rotation toward large numbers of background sources, the SKA will track the evolution of magnetic fields in galaxies, galaxy clusters and the intergalactic medium over a large fraction of cosmic time. An important area of research in Sweden is the area of cosmic dynamos both in the Sun and also in external galaxies (Brandenburg 2014). Such dynamo models predict helical magnetic fields, which can be distinguished by their characteristic properties in measurements of changes of the plane of polarization of synchrotron emission due to Faraday rotation (see Brandenburg and Stepanov 2014; Horellou and Fletcher 2014). 3.5 The Cradle of Life - The existence of life elsewhere in the Universe has been a topic of speculation for millennia. In the latter half of the 20th Century, these speculations began to be informed by observational data, including organic molecules in interstellar space, and the discovery of

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both proto- planetary disks and planets orbiting nearby stars. With its sensitivity and resolution, the SKA will be able to observe the cm-wavelength thermal radiation from dust and cm-sized particles inside regions the ‘snow line’ for proto-planetary disks closer than 100 pc, thereby probing a key stage in the planetary formation process. Theoretical work on the process of particle formation and growth is a carried out at Lund (Johansen and collaborators). Initial results of radio observations using the Jansky VLA of emission from cm-sized particles in proto-planetary disks (Perez et al 2012) are already providing new observational insight into particle growth. The SKA1 with its much larger collecting area and longer baselines will probe disks both at higher spatial resolution and at lower frequency (probing larger particles). On much larger scales in molecular clouds, the SKA will search for complex pre-biotic molecules (Bergman 2013). Finally, detection of transmissions from another civilization would provide immediate and direct evidence of life elsewhere in the Universe. SKA will provide sufficient sensitivity to enable, for the first time, searches for unintentional emissions or ‘leakage’ from civilizations around the nearest 104 stars.

Fig. 5. Millisecond radio bursts; A new class of fast radio transient (Thornton et al 2013). Inset shows pulse shape versus frequency and main plot shows pulse arrival time versus frequency. The latter shows a large dispersion consistent with a cosmological origin. Up to 104 such bursts per day are inferred on the sky: their origin is as yet unknown.

3.6 Exploration of the Unknown – In addition to the major science themes listed above, and recognizing the long history of discovery at radio wavelengths (pulsars, cosmic microwave background, quasars, masers, the first extra-solar planets around pulsars, etc.), the international science community has also recommended that the design and development of the SKA has an “Exploration of the Unknown” philosophy. Wherever possible, the design of the telescope is being developed in a manner to allow maximum flexibility and evolution of its capabilities to probe new parameter space. One particular area where many discoveries may occur is in the detection of new transient phenomena. This potential is highlighted by the recent confirmation of a large population of millisecond duration Fast Radio Bursts of unknown origin (See Fig 5 and Thornton et al 2013). The large dispersion measure (DM) of these sources (delay in arrival time versus frequency due to intervening ionized intergalactic medium) implies that they are at cosmological distances. The origin of these bursts is fascinating in themselves (Petrov et al. 2015); additionally there is the exciting prospect of using these bursts as cosmological probes. If the DM scales in the simple way with redshift as expected, it should be possible to determine the galaxy correlation function in 3D and detect the expected angular scale of Baryonic Acoustic Oscillations versus redshift, so constraining cosmological models and the nature of dark energy (see Science goal 2 above). Within Sweden there is theoretical interest in fast transients in Lund (Davies, Church ) and in Onsala/Gothenburg (Yang, Torkelsson). Torkelsson (University of Gothenburg) is also participating in the VAST survey that will use ASKAP, an SKA pathfinder, for monitoring transient and variable radio sources. There is also strong observational interest in transients via the involvement of Stockholm University as a partner in the US based optical intermediate Palomar Transient Facility (iPTF) and its successor the Zwicky Transient Facility.

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4. The infrastructure users – Although the final details have yet to be established, access to the SKA will depend on membership and contribution to the construction with only a limited amount of international "open time" being available. Membership of SKA will allow applying for observing time but no other user fees beyond the national membership fee will be charged. At this moment the prospective users of SKA have been organized in so-called Science Working Groups each with a specific scientific focus. These groups supply the SKA with input for the design and also receive information from the SKA Head Office over progress of the project. Within Sweden there is a large and rapidly growing scientific interest in SKA. Eleven Swedish scientists are active members of one or more Science Working Groups. For the new SKA Science book, 8 Swedish scientists have contributed to 15 chapters. The "Swedish SKA science meeting" in 2014 attracted 45 participants from across a broad range of research areas: reionization, cosmology, galaxies, transient phenomena, magnetic fields and proto-planetary disks. From the Swedish astronomical community, 23 scientists (listed in Appendix B) have indicated that they support this proposal. Of these, 9 are from Chalmers University of Technology, 6 from Stockholm University, 3 from Lund University, 2 from Uppsala University, and 1 from Gothenburg University, KTH and NORDITA. SKA thus has substantial support from all astronomy groups in Sweden. In terms of the specific areas of Swedish science interest, it should be noted that a Swedish scientist (see Mellema et al 2013) is one of the leaders of the headline science area of Epoch or Reionization studies. There is also a wide interest within Sweden (especially in Gothenburg and Stockholm) in the core SKA area of galaxy evolution. In the area of pulsars a key science project on one of the SKA precursor instruments (eMERLIN) is led by a PI from Gothenburg (see Vlemmings 2012). SKA Magnetism studies are also of great interest to Swedish scientists (at NORDITA, Stockholm and Gothenburg). Finally, there is interest in SKA observations of proto-planetary disks (Lund) and Transients (Lund, Stockholm and Gothenburg). Further evidence of the level of interest are several recent meetings, including a conference "Galactic Magnetism in the era of LOFAR and SKA" (see link in key references) from 23 - 27 September 2013 organized by NORDITA and the "Swedish SKA science meeting" (see link in key references) 9 January 2014 organized in Stockholm. The prominent role of Sweden and Swedish scientists within the SKA project led to Stockholm being selected as the site for the next large international SKA science meeting "SKA Key Science", 24 - 27 August 2015. 5. Access to scientific results – As the SKA and all of its organisational structures are still being designed, there is no official policy on data rights yet. However, the proposed arrangement is that the observational data will be owned by the SKA, not by the observers who proposed the observations, and that the data will be made public after a proprietory period. The latter may be as short as one year for small projects or several years for projects which require data collection during several years. Details about how data will be made available have not yet been worked out although there exists extensive expertise within the (radio)astronomical community on making observational data easily accessible for example through the Virtual Observatory middleware. 6. Ethical considerations – There are no ethical considerations to consider for this infrastructure.