ACADEMIA

The National Science Foundation recently awarded a four-year, $12 million Technology Development Program (TDP) grant to a consortium of more than a dozen U.S. institutions, led by Cornell University and including the University of Illinois at Urbana-Champaign, to support development of a massive radio telescope project. The Square Kilometer Array plans to begin operations in 2020 and will be capable of observations that are two orders of magnitude more sensitive than those from existing meter- and centimeter-wavelength facilities. NCSA will lead the project's calibration and processing team. Access' Trish Barker recently spoke with the principal investigator of that TDP grant, Cornell's Jim Cordes, about the challenges of the project and the breakthroughs it could enable. Q: What is the Square Kilometer Array project? How is it different from other radio astronomy telescopes? A: The notion of it has been around since about 1991 when people talked about something called the "hydrogen telescope," meaning it would be used to study the evolution of hydrogen in the universe. And when you do the simplest calculation of what size radio telescope you need to detect a galaxy like ours at a red-shift of 1, which is a distance that's interesting from the point of view of how big our universe is, you need a square kilometer of collecting area using receiver systems currently available. Over the past decade, many technological solutions have been looked at to identify cost-effective ways to build the necessary collecting area along with low-noise receiver systems. The current baseline design includes a large number of small-diameter antennas along with phased arrays. From the start the SKA has been an international project, because that's a big enough collecting area and hence project that no one country can afford it. People around the world thought, 'If we have that much collecting area, we could not only do hydrogen in galaxies and do cosmology, but we could do all kinds of other things,' like using these fairly exotic objects called pulsars to test theories of gravity, including Einstein's theory of general relativity. Once you start talking about a big collecting area, there are a whole bunch of other things you can do. There's an incredibly rich science case: testing gravity, studying cosmology and dark energy, searching for signals from other civilizations, studying magnetism in the universe and where it came from, and so forth. Q: A square kilometer collecting area seems enormous. How does this compare to current radio telescopes? A: Its aggregate collecting area will be about 10 times the biggest aperture we have now, which is the Arecibo telescope in Puerto Rico. However, unlike Arecibo, the SKA will consist of a large number of antenna systems spread over a wide area, probably thousands of kilometers. It's an array, and the biggest array telescope currently operating is the VLA (Very Large Array) in New Mexico. The SKA will be perhaps 50 times the VLA. I guess we could call the SKA the Very, Very, Very Large Array. We've identified two acceptable sites, one in South Africa and the other in western Australia. They're really desolate places so there's not much radio interference, which is a really important factor. The other thing is that the southern sky is, to a large extent, richer because you can see the center of our galaxy overhead for many hours of the day, and that's a fairly significant target for some of the science we want to do. Q: What will researchers be able to observe with SKA that they can't examine today? What new phenomena does this open up for observation? A: Preeminently galaxies—we can detect the spectral line from hydrogen from a Milky Way-type galaxy at fairly high red shift. The reason that's important is because of the discovery 10 years ago that the universe is not only expanding but accelerating. So what are the possibilities for explaining that? Dark energy, meaning the cosmological constant that Einstein invented, is one possibility. But we haven't really looked at how the expansion works in enough detail, with enough precision. There are several projects whose aim is to quantify the properties of dark energy, or whatever it is, by looking at how the distribution of galaxies evolves with cosmological time. And one of the ways you can do that is to make a very large galaxy survey. The ultimate ... is that you would do a survey of about a billion galaxies providing a billion test particles that you could use to probe the expansion of the universe, and see how things evolve over cosmological time. Q: You mentioned using pulsars to test gravity. Can you explain how that would work? A: Pulsars are rotating neutron stars, some of them spinning as fast as 600, 700 times per second. They have more mass than the sun by about 40 percent. We see them as pulses because they are like rotating lighthouses. So how can we use those? If a pulsar is orbiting another object—especially if it's another neutron star or a black hole, and if the orbit is compact—we've got a perfect situation. Basically, we use the pulsar as an astrophysical clock that we can monitor as it orbits its companion. We can measure the effects of the companion object's gravity on both the pulsar's orbit and also on the radiation from the pulsar as it propagates to us. Potential departures from predictions made with a particular theory of gravity translate into our ability to put very precise tests on that theory. We've seen pulsars in orbit around other objects but not around a black hole yet, so we know that configuration is rare. We really need to do a complete census of the galaxy looking for pulsars. There are probably about 20,000 radio pulsars in the Milky Way that we could detect, with perhaps just a few dozen being useful for testing gravity. Pulsars were discovered about 40 years ago and since then we've discovered 2,000 pulsars. Some of them are what we call vanilla pulsars, kind of boring. Then there are the others that are in these special situations that are quite interesting, so we want to find the rare few. The "sweet spot" in our galaxy for doing this is the center of the galaxy, because our galaxy has a compact black hole at the center, Sagittarius A*. What we would like to do is find pulsars orbiting Sagittarius A*. That would be pretty much the ultimate thing we could do in terms of using a pulsar to study the properties of the black hole and the environment around it because the black hole has a mass a few million times that of the sun. Q: I think one of the things that's particularly interesting about astronomy is that there's still a certain degree of mystery. There's still so much unexplored territory. A: Right. We have this science case that's based on explicit projects, with goals of learning particular things, but ... we also see this as a discovery instrument. Not only will it have the collecting area—that's a big boost—but the really new thing is the instantaneous wide field of view. The way it works is that the bigger the antenna, the smaller the field of view. So traditionally when we've had high sensitivity we've only been able to study small patches of the sky at once. And vice versa. But what we really want is both high sensitivity and the ability to study wide fields in the sky. The reason that's of interest is that there's much greater appreciation now that a lot of the radio sources up there are time variable. Some of them just go up and down, the intensity fluctuates, whereas the really interesting objects are the ones that go BOOM! You see them and then you don't! We expect that this "transient sky" is where a lot of the discoveries will be made with the SKA. It's far more exciting to find something new and unexpected. For me, that's one of the most interesting aspects of the SKA. Q: What can you tell me about NCSA's contributions to this project? A: The images of the sky that you're going to be able to make with SKA are going to be far, far more sensitive than, say, with the VLA. The fainter you go, the more sources there are, which brings up the challenge of how to construct images with enough fidelity and dynamic range so that you can distinguish the faintest sources from the imperfections from the strong sources that are in the field of view. You've got to calibrate all of the signals from several thousand antennas, and then you also have to process them properly. Doing so requires development of algorithms as well as providing feedback to antenna designers on the performance of the individual antennas. The set of problems associated with calibration and processing is the sub-project led by Athol Kemball at NCSA that also involves about 10 other institutions in North America. Q: So where are we in the timeline of the project? I think I've heard 2020 mentioned. A: Right now there are some pathfinder projects. The Allen Telescope Array is one, there's one in Australia called Australia SKA Pathfinder, and the South Africans have one called MeerKAT. All three of these will be working areas that will produce scientific results. At the same time, their construction requires technology development that is important for the SKA. In parallel with the pathfinder array projects, the U.S. Technology Development Program project is a four-year project that is complementary to and integrated with a European project called PREPSKA. It's so acronym heavy! The TDP combined with PREPSKA combined with these pathfinder arrays will lead to a concept design for the SKA in about 2012 and another few years may be needed to develop an engineering design. The different countries that are involved in this international project will be seeking funding with the idea that construction of phase one of the SKA might begin some time around the middle of the next decade. Then phase two consists of building out to a full SKA whose time frame will depend on funding levels but hopefully will not be too long after 2020.