Dark energy: Q&A with Steve Kuhlmann
Why do we care about dark energy in the first place?
One of the most fundamental questions that scientists face is to try to figure out what the universe is made of. We know that dark energy is the largest component of the universe, accounting for about 74 percent of the mass and energy that exists combined. If we want to understand the components of the universe, we have to understand dark energy.
Dark matter makes up another 22 percent, and that's not well understood, either. In a very real sense, we only understand four percent of the universe. To basic scientists like us, that's a crime—that's not allowed.
Sometimes we think we're so smart, because we have all these great tools, but we only understand four percent. That's not good. Dark energy has shaped the history of the universe ever since the Big Bang, and if we want to understand the history of the universe, how it evolved and where it's going from here, dark energy plays a big role in that.
Lastly, dark energy is responsible for the biggest discrepancy in modern science. The best explanation we have for how dark energy works generates predictions that are off by one hundred and twenty orders of magnitude. That's a mind-bogglingly huge number – 10^120. For the people who are making those predictions – high-energy physics theorists and particle physics theorists – it's incredibly daunting.
Now, we've just found the Higgs Boson over at CERN, and at some level that completes the particle physics model. We think we know the subatomic particle list, and that makes for a nice package. But when you think you have everything all sorted out and you're still missing something fundamental by 120 orders of magnitude you know you're missing something big.
What is the Dark Energy Survey?
At the most basic level, the Dark Energy Survey is a combined Department of Energy and National Science Foundation project to replace an existing camera on a telescope in Chile. There's a four-meter telescope that sits on a mountaintop that has gotten a new camera built by DOE and NSF. Now that we have what's called "first light," – meaning that the camera's open to the sky—that step is mostly complete. In exchange for building the camera, DOE and NSF get 525 nights of observing over the next five years. So we get to do whatever we want to help ourselves understand dark energy on those nights.
The dark energy survey will, in its most basic form, discover hundreds of millions of new galaxies. One of the things we can do with these galaxies is called a lensing study. If you have a clump of dark matter and you have a galaxy behind it, the light from the galaxy is bent around the dark matter. So you can make a three-dimensional dark matter map by looking at the lensing from all of these galaxies. That's one of the most powerful tests we'll be doing.
So what's Argonne's role in the DES?
We helped to build the camera, known as DECam. We contributed about 20 percent of the mechanical engineering and design of the camera, and that includes the camera control systems. The shutter is controlled by an Argonne-designed system, and likewise the cooling system that maintains the charge-coupled devices (CCDs) – the heart of a digital camera – was built by Argonne.
We have very special CCDs, and so we did tests on them here at the Advanced Photon Source. We used the X-ray beam as a scanning tool to test for the uniformity of the CCDs. So I'd say those are our main contributions to the camera.
What does the future of the project look like?
Now, we just have our first image from the telescope. They'll do hardware commissioning for the next month or two. Once that's done, they'll do more detailed mini-surveys of the sky to determine how dim a star you can see; that's a critical element in this kind of an instrument. That's especially important in doing supernova calculations, because we want to get the most distant supernova we can possibly find, and we're limited by how far the telescope can see a star. Supernovas can also tell us information about dark energy.
The nice thing about this camera is that even though we're getting 525 nights over five years, that's only one-third of the time. And so, the scientific community gets more than two-thirds of the time on this great new camera.
During our nights, we're going to be mapping out 5,000 square degrees of the southern sky, by marching along in some fixed pattern taking pictures. Like any astronomy camera, it takes multiple filters and then moves to the next spot in a very simple pattern. Interspersed with that are special filters for supernovae.
What's the ultimate outcome of the Dark Energy Survey?
The best model we have for dark energy is still off by 120 orders of magnitude. What the dark energy survey can do is to either further pin down that original hypothesis, even though it can't resolve the theoretical problem, or we could find out that the model we have is wrong and the data point in a completely different direction. And I think a lot of theoretical physicists would like that because they're stuck on an idea they really don't know how to solve.