Dark Energy Has Been Around for the Last 9 Billion Years of the Universe
22 Nov, 2006 03:41 pm
Dark energy is this mysterious energy that seems to make up about 70 % of the universe, but we know very little about it. In an interview with Adam Riess, lead author of the study to be published in Astrophysical Journal, February 2007, we discuss his findings which show that dark energy was already accelerating the expansion of the universe at least as long as 9 billion years ago.
Dark energy is this mysterious energy that seems to make up about 70 % of the universe and its repulsive gravity, as a feature of gravity that Einstein had first predicted in 1916, appears to be accelerating the expansion of the universe. That’s about all we know about it which is very little to know about so much of the universe and that’s why it’s considered one of the most pressing questions in Physics: to understand the nature of dark energy.
Over the past eight years, astrophysicists have been trying to uncover two of dark energy's most fundamental properties: its strength and its permanence. What did you learn with your observations?
What we’ve learned  is that it’s been fairly permanent—if permanent means in this case that it’s been around for most of the history of the universe, so we’ve used the Hubble space telescope to peer even further back in time. We use exploding stars called supernovae as cosmic markers to measure dark energy and its affect on the expansion of the universe. We can see dark energy has been around for the last 9 out of 13.7 billion years of the universe, so it’s a relatively permanent feature, not a new feature of the universe; this is an important clue to help us begin to distinguish between [different] explanations for dark energy.
Why is this important?
Some theories of what dark energy could be, like Einstein’s theory or vacuum energy, predicts that dark energy has always been around and is completely permanent and unchanging in its properties. That’s a prediction we want to test by making observations. Other theories predict that we won’t see dark energy as we go back in time to 9 billion years; it will either not be there or act like other material, like ordinary material such as matter. This is what we do in science: we take competing theories and we look for a difference in their prediction of what the universe will look like and we go out and we make those measurements; so, this is one of those kinds of measurements, but we have many [measurements] to go that will help us begin to, by process of elimination, arrive at the correct—hopefully—explanation for dark energy.
You used Hubble to observe far across the universe back into time and to detect ancient supernovae. Why look to supernovae?
Supernovae are well-calibrated—what we call standard candles. If you were to see a pair of headlights on a car at night on a highway, and if you were to know how bright those headlights are, you could measure how far away that car is by how bright [the lights] appear to you; the dimmer they appear the farther away the car is, but you have to know how bright those headlights are themselves. In fact, it would be really handy if all cars were exactly the same and had exactly the same kind of headlights, then you wouldn’t have any question when you saw a car, and whether that was a particularly bright pair of headlights or not. So this certain class of supernovae explosion are like nature’s gift to us: it’s the headlights that don’t change very much. They are all built to the same specifications which occurs by something called the Chandrasekhar limit, which as explained in the 1930s was the critical mass of this kind of star. If it surpasses that mass, it will have a runaway similar to a nuclear explosion, therefore, this explosion is very homogeneous.
What are your next steps?
We need to make more observations. These clues are not enough to help us rule out all alternative models, so we need more observations. We need more with the Hubble space telescope and more from ground-based telescopes, and it’s very likely that in the future we will need a dedicated space mission such as JDEM, something called the “Joint Dark Energy Mission” which is something that NASA and DOE are working together to develop for launch probably in the middle of the next decade.
 Reiss, A. et al., Astrophysical Journal. (10 Feb 2007)
Interviewed by: Thanh-Tam Candice Vu
Adam Riess is a professor of Physics and Astronomy at Johns Hopkins University.