Exploring the dark universe: Dark energy

>>MORDECAI-MARK MAC LOW: My name is Mordecai-Mark Mac Low, and my position is curator of astrophysics. In the last century, we have learned that the universe is expanding and that the stuff that it is made of is primarily not normal matter. It's primarily something that we have called "dark matter" and something else that we are calling "dark energy." What do we know about dark energy? So, the entire universe is expanding. [WALTZ MUSIC] MAC LOW: There's a lot of mass in it. If the density is above some threshold, the universe should go out and come back in and the universe will end in fire. Conversely, if the density is under that threshold, the universe will keep going out ever decelerating and expanding and it will end in ice. So people have tried to determine what the behavior of the universe actually was. Was it decelerating slowly, or so quickly that it would turn around and crunch? By the 1990s, by looking out to great distances and measuring the velocities of the galaxies they found there, people had found a way of measuring the deceleration and they did this by using a certain kind of supernovae called "Type 1A Supernovae." These Type 1A Supernovae always have the same brightness. Or at any rate, can be calibrated to measure their brightness. So, looking for these supernovae out across the universe allowed a measurement of how fast the universe was decelerating. And a graduate student whose- this project was his dissertation, finally said, "Let's put them together and find out how fast the universe is decelerating." So he takes all his measurements, calculates them, measures the rate of deceleration, and must've got something wrong. Because he gets a negative deceleration, otherwise known as acceleration. Which nobody asked for, nobody expected, what's going on here? Well, obviously the first answer was, "Oh, I made a mistake in my calculations." So, he goes and shows it to his thesis adviser and the whole group, and they all busily repeat the calculations, say, "There, there. We'll take care of this." But of course it doesn't go away and eventually they have to say, "Look, the universe isn't decelerating. It's accelerating." And it turns out that the equations of general relativity naturally admit an accelerating universe filled with energy, and that energy got a label stuck on it, called "dark energy." There are about six different measurements that all independently measure the strength of the dark energy. So, we know pretty well that it does exist and now we're trying to measure was the strength of the dark energy always the same as the universe expanded? Because if it changed over time, that would be a big indicator of what kind of thing it might be. Satellites like WFirst in the U.S. and Euclid in Europe, ground-based telescopes are going to measure that fairly precisely. Then we'll step back and see what we've got. [BUZZ, MUSIC]