Date: September 2, 2011
Title: The Dark Energy Survey Project: Q & A with David Gerdes
Podcaster: Chris Lindsay with David Gerdes
Organization: The Ann Arbor Science and Skeptics
Links: http://annarborscienceskeptic.com
For more information about David Gerdes, please see http://www-personal.umich.edu/~gerdes/
For information about the Dark Energy Survey Project, please see http://www.darkenergysurvey.org
Critical Wit Podcast: http://www.criticalwitpodcast.com
Description: This episode consists of an interview with David Gerdes, a participating member on the Dark Energy Survey (DES) Project, a 5000-square-degree optical survey of the southern sky using the 4-meter Blanco telescope at Cerro Tololo in Chile. The DES team will be measuring the acceleration of the universe over time and hopefully determine if the universe’s expansion is due to Einstein’s cosmological constant or some other mechanism.
Bios: Dr. David Gerdes is an Arthur F. Thurnau Professor in the Department of Physics at the University of Michigan. His research field is observational cosmology, and he focuses on the mystery of dark energy, the property of empty space that is causing the expansion rate of the universe to accelerate. He is also a participating member in the Dark Energy Survey Project.
Chris Lindsay is a science enthusiast and podcaster. He’s the organizer of the Ann Arbor Science & Skeptics, and host of the Critical Wit podcast, a weekly 20-minute podcast about science, literature, and the arts.
Sponsors: This episode of “365 Days of Astronomy” is sponsored by the Ann Arbor Science & Skeptics group located in Ann Arbor, Michigan. You can find them at www.annarborscienceskeptic.com.
This episode of “365 Days Of Astronomy” has also been brought to you by Distant Suns 3, the award winning personal planetarium software for the iPad and iPhone. Unleashing your inner astronaut since 2008.
Transcript:
Hello. Thank you for listening to 365 Days of Astronomy. I’m Chris Lindsay, the organizer of the Ann Arbor Science and Skeptics and host of the Critical Wit podcast. The Dark Energy Survey is a project designed to measure the expansion of the universe with a high degree of precision, in an effort to explain the nature of dark energy. I asked Dr. David Gerdes, a physicist from the University of Michigan who is participating on the project to tell me more about it.
Chris Lindsay: David, the 365 Days of Astronomy listeners are likely to be familiar with what dark energy is, but to give our conversation a reference, can you provide a quick synopsis of why we think the universe is expanding due to the influence of this “dark energy?”
David Gerdes: Sure, so dark energy is really just a name for something we don’t understand, but it’s a name that’s given to the observational fact that the universe is expanding more quickly now than it was in the past. And this was discovered by two independent teams of scientists in the late 1990s, who were looking at Type 1A supernovae. These are a type of exploding star that for various reasons tend to explode at nearly the same brightness. Anyone of these kinds of supernovae, anywhere in the universe, will tend to be exploding at the same brightness. So they act, more or less, like standard candles – an object with a known intrinsic brightness, whose apparent brightness you can then use to determine the distance to the supernova. And in essence what these two teams found, by independently analyzing different sets of data, is that the most distant supernovae were fainter than what would be expected if the universe was expanding at a constant rate or slowing down. And uh, so this sort of bizarre inflection in the expansion rate, the idea that it’s been speeding up, is what really came to be called ‘dark energy.’ Since that discovery in 1998, further surveys with better statistics and deeper reach have, in essence, confirmed the data from 1998 and in addition, several new observations have lent independent credence to the idea that the universe is accelerating. This includes data from cosmic microwave background observations from the WMAP experiment, and measurements of large-scale structures – the distribution of galaxies and dark matter in the universe.
Chris: And you are working on what’s known as the Dark Energy Survey, which is going to be a new imaging instrument that can help you make some precise measurements on how fast the universe is accelerating. Can you talk a little bit about what specifically the Dark Energy Survey imager is going to do, and how it’s going to be doing it?
David: So the Dark Energy Survey is a project that will survey 5,000 square degrees of the southern sky in the optical and near-infrared band regions using a new camera. What we’re really doing is building a new camera that will go on an existing telescope – the four meter Blanco Telescope at Cerro Tololo Observatory high up in the Chilean Andes. It’s a 570 mega-pixel camera that consists of an objective lens with a diameter of about a meter, and four other lenses that serve to flatten the field over this enormous focal plane. The focal plane of the camera is populated with 74 chips, it’s kinda like the size of a small pizza. And it has a 2.2 degree field of view, which to kind of put it in terms that everyone can understand, that’s about twenty times the area of the moon on the sky. So every image taken by the dark energy camera encompasses the region of the sky about twenty times the area of the moon – seen through the eye of a four meter telescope. So we’re looking very, very deep. And we’ll be tiling this 5,000 square degree survey region over a period of 520 nights, distributed over five years. So we’ll be building this dataset gradually over time, taking images and different regions of the optical spectrum, in different parts of the sky. But at the end of the day, we will have a dataset that consists of optical images of about 300,000,000 million galaxies, the most distant of which are being observed at a time when the universe was roughly half its present size. In addition to this 5,000 square degree survey area for which every field is going to be visited essentially once, about a ten degree – a ten square degree subset of this survey area is going to be visited repeatedly, every, say, four nights, to look for these Type 1A supernovae – to look for things that are changing in brightness, new things appearing and getting brighter and then becoming fainter in this characteristic way of supernovae. So we expect to discover, in addition to these 300,000,000 million galaxies that we’ll observe once, we ought to observe something like 2,000 Type 1A supernovae that will add richness to this dataset and allow us to see more deeply into the past with that tool.
Chris: Now, is there any concern about atmospheric distortion effects or obstacles to being able to make some of these precise measurements, or be able to see these far, distant regions of the universe?
David: Well, so one reason that the experiment is being done in the Chilean Andes is that these are some of the best and darkest skies in the world. But any telescope that’s on planet Earth has to deal with the effects of the Earth’s atmosphere. And so we need to take those distorting effects into account as you say, and that’s particularly important for one of the measurements we intend to do with this data – which is to measure the very subtle distortions of the shapes of galaxies caused by gravitational lensing. Gravity – matter bends light, as we’ve known since 1919, I think it was, when the deflection of light from nearby stars during a solar eclipse was first observed from Earth verifying Einstein’s prediction of the theory of relativity. The same effect operating on cosmic scales, distorts the shapes of galaxies and gives us actually a very precise way to kind of x-ray the matter distribution of the universe – both the visible matter as well as the dark matter. But in order to separate those effects out from mundane effects like the distortion of the atmosphere, we need to very carefully understand the properties of , not only our instrument, but also the atmosphere on that specific night. And the best way to do that is to just look at what happens to the images of stars. We know that stars in our field are point-like sources of light, and so by looking at the deviations of stellar images in our photographs, from an ideal point-like object, we get some measurement of the combined effects of the atmosphere and the instrument. And so we can, we can then, there’s lots and lots of stars in our field – and we can subtract those effects out and what we have left is the intrinsic shapes of galaxies.
Chris: Now, is there any specific hypothesis or hypotheses that this particular project is trying to confirm or falsify? Or is this more or less, just recording measurements and data?
David: Well, I think as observers, our first interest is assembling what would be the state of the art, best, richest, astronomical dataset ever collected. And so this is going to be incredibly useful, not only for dark energy science, but for all kinds of auxillary bits of astrophysics and astronomy. The data will all be public within a year of being collected and available to the whole community, the whole world, on the web. And so, in the first instance, this is just a treasure trove of astronomical data for whatever piece of science people want to do. Now, on the dark energy survey, our own interest is primarily using this data to understand the history and fate of the universe. There are many theories on the market. We’re in a situation right now, where although certain theories have been falsified already, theories that predict a greater rate of change of the acceleration that has been seen, for example. Some of these theories have already been ruled out. But there’s still a, a sort of, wide-ranging variety of theories that make differing predictions about the – whether or not the acceleration is constant or not constant, and if not constant – what way is not constant. And we will certainly place better constraints on these theories than our currently available, in some units by a sort of a factor of 3 or 4, we expect to improve on existing constraints. It’s difficult, on the other hand, to confirm a theory. We can place better and better constraints on what things can’t be, but I’d say that in general, one of the most interesting things we’re trying to do is understand if the acceleration of the universe can be accommodated within Einstein’s theory of general relativity or if some modified theory of gravity is required. General relativity can accommodate cosmic acceleration through several different mechanisms, including just simply adding a cosmological constant term to Einstein’s equations. But modified theories of gravity would affect the red-shift distance relation as well as the evolution of the volume of space-time over time in a different way than general relativity would. And by doing a set of complementary measurements of supernovae, of the formation of large-scale structures, of the patterns of weak gravitational lensing, of the patterns of grouping of galaxies and clusters in the sky, we hope to be able to sort out some of these distinctions between general relativity and some of the alternatives.
Chris: And you mentioned before that all of the results of the data will be released at one time. Do you anticipate any papers or reports that might come out during the interim – before you’ve reached the end of the project in five years?
David: Yeah, actually we expect – we expect to be releasing our data periodically as we go along as well, not all in one big batch after, after five years are done. We expect to be releasing all of our data publically within a year of when we collect it. So that means that sometime during the second year of the survey, we expect to be releasing the data from the first year of the survey. And so on, throughout the time of the survey. So along the way, we expect to be writing papers that will gradually be including more data and becoming more precise. But there will certainly be early papers coming out of our survey, I hope within a year. Not only do I hope that, but a lot of the graduate students and post docs on the survey are very excited to start looking at the data and start writing.
Chris: Great, well thank you very much for your time, Dr. David Gerdes, I really appreciate it.
David: Thank you very much, Chris.
End of podcast:
365 Days of Astronomy
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