Dave, one of my online chessplaying buddies, asked me this question (edited for appropriateness):
Now this speeding up of the expansion of the universe: Do I understand correctly that one of the theories to explain why relates to “dark matter”?
In real simple terms, what in the dark blue blazes is dark matter, and why and how (and I think the “how” is what keeps you boys up late at night) does it cause this increase in speed?
So Dave gives me the opportunity to, first off, make the distinction between dark matter and dark energy.
Dark matter is, quite simply, matter that doesn’t emit light. (Were you expecting a better answer from someone with a Ph.D. in astrophysics?) Seriously, though, it turns out that, as best as we can tell, Suns, stars, and the nuclei of galaxies, the brightest things we know of in the universe, only account for about 2% of matter. The other 98%? Some of it, as far as we know, is normal matter, like protons, neutrons, and electrons, but is just lying around in gas clouds, planet-sized (or smaller) objects, dust grains, or other things that don’t give off light. But most of that matter *isn’t* made up of protons, neutrons, electrons, or any other type of normal matter. But we know it exists, as I’ve already discussed here. I promise I’ll tell you more about in the coming months, as it’s the most important thing I’m researching right now.
Dark energy, on the other hand, is a name we give to the fact that, in addition to all of this unseen mass in the universe, the expansion rate isn’t doing what we’d expect. The way the universe expands is pretty simple, actually; it’s just like a balloon. You can imagine that the matter lives on the surface of a balloon, and from the big bang, this initially very small balloon starts expanding very rapidly. We measure this expansion rate in kilometers/second/Megaparsec (abbreviated Mpc), and it turns out to be about 70 km/s/Mpc today. A Megaparsec is about 3 million light years, and so an object about 1 Megaparsec away should move away from us at 70 km/s, one 10 Megaparsecs distant expands from us at 700 km/s, etc. Now, based on the amount of matter in the universe and how quickly the initial expansion speed is, three things can happen:
- There’s so much matter in the universe that gravity’s going to win. The balloon will keep expanding, but gravity’s eventually going to slow the expansion rate down enough that the balloon will reach a maximum size and recollapse. This is called the big crunch.
- There isn’t enough matter in the universe to cause a recollapse, and the balloon will keep expanding forever and ever, slowing down some, but with everything in it becoming more diffuse and cold. This leads to a fate known as a heat death of the universe.
- There is some “Goldilocks” case in between the first two, where there is just enough matter to balance out the expansion rate, which will asymptote to zero (approaches zero, but never reaches it). This will also lead to a heat death, but only after a very long time.
For a long time, scientists only really thought about the first two cases, and assumed that the third wasn’t even a possibility; any approximate equality that we saw was a reflection only of our inability to measure the expansion rate and/or the density of the universe accurately enough.
But now we do have very good measurements of the expansion rate and the matter density of the universe, and things are more bizarre than ever. It looks like, for the first few billion years of the universe, it was doing the Goldilocks thing, where the expansion rate and the matter density balance each other perfectly. All of a sudden, though, the expansion rate stops dropping towards zero! In fact, based on our current knowledge, the expansion rate of the universe will never drop below about 60 km/s/Mpc!
What this means is that the objects that are expanding away from us now, and hence getting further away, will be expanding away from us *even faster* in the future. That’s why people sometimes call this an “accelerated” expansion. So here’s the million-dollar question: what causes this accelerated expansion? Sadly, we have no idea. So in the meantime, we give it a name, and call it dark energy. I even wrote a paper on one thing that definitely doesn’t cause it, but as for what does cause it, nobody knows. You figure it out and prove it, and I’ll guarantee you’re a shoo-in to win a nobel prize.