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?