What is Dark Energy?

You’ve all heard these words before. Dark Energy. But what is it, and why are we stuck with it? Let me start by telling you a story.

Imagine, for a minute, that you have a candle. You know everything about this candle, including how bright it is and how far away it is from you. Like so:

Now if I move this candle twice as far away, I know it’s going to be one-fourth as luminous. If I move it three times as far away, I know it’s going to appear one-ninth as luminous. And if I move it a thousand times farther away, I know what I see is going to be one-millionth as luminous as the original candle.

Now in space, of course, we don’t have candles. But we do have a special type of event that has, as far as we can tell, the same intrinsic brightness (to within a few percent) everywhere in the Universe. And this special event is known as a type Ia supernova. When our Sun, and for that matter most stars that we know of, burns up all of its fuel, it will eventually become a white dwarf star. Our Sun will be made out of mostly carbon and oxygen, but white dwarfs can also contain helium, neon, and silicon. Here’s an image of one:

Now in our Solar System, there’s only one star. But many star systems have two or more stars. If one of those stars is a white dwarf, it can start to steal mass from one of the other stars. When this happens, it starts to grow in mass. Now, there’s a critical limit to how much mass a white dwarf can hold up before the very atoms themselves collapse. And when the atoms do collapse, that causes an explosion so violent it’s known as a type Ia supernova. Take a look at this movie, simulating one, and notice at the end how the other star gets kicked out of the star system by the violence of the explosion:

Well, when we see these supernova in different galaxies, we can measure their brightness, and we know their intrinsic brightness, so we can figure out how far away they are. We can also measure their redshifts. We can use that information to figure out how the Universe is expanding. Now, you can probably imagine three different possibilities for what the Universe can do, starting with the big bang. You start of with a bunch of matter and energy expanding away from one another, but gravity is trying to pull it back in on itself. Here’s what can happen:

  1. There’s so much matter and energy, and hence so much gravitational force, that gravity wins, and can eventually turn the expansion around, causing the Universe to recollapse on itself. (I.e., a closed Universe.)
  2. There isn’t enough matter and energy to overcome the expansion, and the Universe keeps on expanding forever. (I.e., an open Universe.)
  3. There is just enough matter and energy to counteract the expansion but not enough to turn it around, and so the Universe asymptotes to some state where the expansion rate drops to zero, but never recollapses. (I.e., a flat Universe.)

So now we look at the supernova, and see what they tell us the Universe is doing. Guess what? It isn’t doing any of those three things! It looks like it was doing the flat Universe thing for a while, but then all of a sudden the expansion rate stopped dropping, and now will not only never drop to zero but will become a constant at about 85% of its present value! Why is that? Well, to be honest, we have no idea. But there has to be some new physics going on to make this possible, and we give it the name “dark energy,” since if the Universe were full of a new type of energy that had a repulsive pressure, it would cause the expansion rate to speed up again. But it’s weird, it’s definitely happening, and we don’t know what the right explanation for it is. And that’s dark energy!

Comments

  1. #1 Chris C
    April 9, 2008

    Sorry for being slow here, but if it looked like the expansion rate *was* dropping to zero, but now isn’t, then doesn’t that mean that the expansion rate is still > 0 and therefore a open universe? The expansion rate must calculate to some kind of figure (whether we have that figure right or not) and it must be one of three logical possibilities, x 0 Or is there some other possible value – or is my tiny brain just totally missing the point? :O)

    Many thanks for the incredible blog btw, every post is an awesome read – highlight of my RSS list!

  2. #2 Chris C
    April 9, 2008

    Apologies, comment system munched my ‘less than/greater than’ symbols – should read, …three logical possibilities, x less than zero, x equals zero, or z greater than zero…

  3. #3 ethan
    April 9, 2008

    Chris C,

    I’m going to have to get a little more technical to answer your question, I hope I don’t lose you! The expansion rate, in a flat Universe, goes to zero as the density of matter in it goes to zero. In an open Universe, the expansion rate won’t be zero when the matter density goes away, but it will continue to decrease as the Universe grows; the bigger the Universe gets, the slower it continues to expand. What we find instead is that the expansion rate goes to a constant. So right now, when we measure the Hubble expansion rate, the Universe is expanding at 71 km/s/Mpc. But if we were to measure it 100 billion years from now, the expansion rate would be about 60 km/s/Mpc, whereas in an open Universe, it would have dropped to about one-hundredth of that value.

  4. #4 Kendall
    April 10, 2008

    I thought the expansion was accelerating? Aren’t you saying that it is on its way down to 85% of its current rate? Sounds like expansion is slowing, but still leaves us with an open universe…

    Great blog, btw! You have a nice progression of topics that lay out current cosmological thinking in an understandable way.

  5. #5 ethan
    April 10, 2008

    People say the expansion is accelerating, but the expansion rate isn’t speeding up. It’s just that when the Universe gets bigger and the expansion rate stays constant, that causes an acceleration. That’s worth its own post, Kendall, and I’ll make it later today. Thanks for the suggestion!!

  6. #6 Chris C
    April 10, 2008

    Thank you for the clear reply, I think I understand now – there must be *something extra* that is keeping the Hubble expansion rate from falling off as expected with the lowering of matter density – and that extra something is dark energy?

  7. #7 ethan
    April 10, 2008

    Yes; at least, dark energy is what we call it! For all we know, it might not be energy in the conventional sense at all!

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