Billions of years from now, anyone still in our cosmic neighborhood might look up at night and see inky blackness where galaxies once dotted the sky. Anything not in the vicinity of our own Milky Way galaxy will have sped far away, expanding ever faster outward.
“Things would get more and more diffuse and spread out, and eventually the expansion of space would be faster than the speed of light,” explains recent Nobel Prize winner Adam Riess, an astrophysicist at Johns Hopkins University and the Space Science Telescope Institute. “Not any one thing is moving faster than the speed of light, but the relative expansion or separation between galaxies will grow that way.” Eventually, those galaxies would disappear from view when “their light would have to travel too fast to actually overcome the expansion of space.”
In October, Reiss was awarded the Nobel in physics, along with fellow High-Z Team member Brian Schmidt of Australia, and Saul Perlmutter of Lawrence Berkeley National Laboratory’s Supernova Cosmology Project. They made the discovery simultaneously that the expansion of the universe is actually accelerating. That discovery led, in turn, to the now accepted theory that a mysterious “dark energy” fills most of the universe.
Air & Space Associate Editor Heather Goss talked with Riess about his research.
Air & Space: Tell us about the methods you used to make this discovery.
Riess: My colleagues and I looked for supernovae – a particular class of supernovae called Type 1a supernovae. They all blow up at about the same size, and you can use the brightness to determine how far away they are. You can also measure the redshift, the shift toward the red colors due to the expansion of space. By measuring those two things together, you can measure the expansion rate of space. And by measuring far away supernovae you can measure back in time, so you can measure the past expansion and compare it to the current expansion. That’s what we did, and we found that the expansion rate was increasing, not decreasing as expected.
Air & Space: And you found that this changed at some point, that the expansion was slowing down, then began speeding up. Can you explain that?
Riess: When the universe was younger and smaller and more compact, the attractive gravity that it feels – from the mass of the objects in it – is stronger than when it becomes bigger. As it became bigger, the gravity became weaker and dark energy therefore became ultimately dominant. We think it was about seven billion years ago when that changeover occurred.
Air & Space: You originally studied these Type 1a supernovae for your doctoral thesis. Was your intention to ultimately use them to study the expansion of the universe?
Riess: When I started my thesis, I was not thinking in those terms. But by the end, it became something we were all thinking about doing. I just thought it was so interesting, to be able to make measurements of the universe. And to address basic questions, like how old is the universe and what is it made of. It just seemed like an interesting project.
Air & Space: Are you focused on figuring out the fate of the universe or are you also investigating dark energy?
Riess: Mostly at this point we’re working on dark energy. Understanding what will happen to the universe requires us to understand dark energy.
Air & Space: How far along are you in that research?
Riess: We’re making progress by making more observations, more measurements, to more and more constrain the properties of dark energy, but we’ve got a long way to go before we have a very strong constraint on the nature of dark energy.
Air & Space: What sort of obstacles do you face?
Riess: One thing is, we need telescopes that are largely dedicated to this problem. Most of the telescopes we use, their time is divided amongst the whole community looking at all the other things that people want to look at. In order to really make headway with dark energy, we probably need a dedicated space mission that would spend most of its time or a large fraction of its time studying dark energy.
Air & Space: Are some of those in the works?
Air & Space: Could the James Webb Space Telescope help?
Riess: The Webb telescope will help, but it’s not right in its wheelhouse. It was designed to do other things before we even knew about dark energy. You really need wide-angle capability to be able to make a lot of inroads. So Webb can do some things towards it, some investigations, but it’s not going to be able to give us the answers we’re looking for. That would require a dedicated dark energy mission.
Air & Space: We’re a long way from understanding dark energy, but are there any properties you can talk about?
Riess: We’re talking more or less about its strength, how much push you get on the universe for, let’s say, a cubic centimeter of dark energy. That’s a property we’re trying to measure called the ‘equation of state.’ If we measure that better, we think it’ll give us a better idea of dark energy.
Air & Space: After you were awarded the MacArthur Foundation “genius grant” in 2008, I read that you hoped to use the money for a new kind of telescope filter. Did you?
Riess: Yeah, they’re specialized kind of filters that would enhance the light of supernovae when you saw them and help you indentify supernovae faster. I’m working with a graduate student on that project right now.
Air & Space: Where do you hope to see this research in 10 years?
Riess: I’d like to see it make a lot of progress. If we can make as much progress studying dark energy as we did in the last 10 years I think that’ll be pretty impressive. Hopefully we can get to the point where we’ve measured some of the basic properties of dark energy to about one percent precision, and I think that’s within reach if we end up using a dedicated facility like a space telescope.