Thursday, 4 December 2008

Voices Of Authority #1: Dark Energy

When, a few weeks ago, I was ruminating on Big Bang Theory and considering (in my own cackhanded, amateurish way) to what degree it was a more satisfying answer to why we exist than "God did it", it occurred to me that others would be far better at tackling that sort of question than I. Specifically Pause, who is constantly willing to correct my feeble understanding of astrophysics when prompted (and I appreciate that he waits for said prompting, otherwise I'd probably be in tears every other day).

In fact, not long after that post appeared on the intertubes, S. Spielbergo e-mailed me, wanting to know if the all-powerful Pause could enlighten him as to why the universe is apparently not only expanding, but accelerating in its expansion.

Because I care about my readers (at last count, and being generous, you might just be in double figures by now), I asked Pause this very question, and he was good enough to explain it to me: dark energy.

That makes sense, I thought, dark energy. Trouble is, I was thinking of dark matter, and I'm not entirely sure what that is either, but apparently it's very different to dark energy, and I was a crazy, wide-eyed fool for believing differently.

Anyway, this rather rambling introduction is a preamble to introducing a new (and almost certainly tremendously irregular) feature: Voices Of Authority. Within these posts you will find the smartest minds that can be found (read: that can be found in my address book) explaining various complex topics in a way that even I can understand.

We begin with Pause, ably assisted by fellow space-obsessive Cocklick, trying to hammer the basic properties of dark energy into my puny cranium. Jamie was there too, but since we weren't talking about whether Nero ever actually played a fiddle, his turn will have to wait.

SS: Welcome ladies and gentlemen, to the first episode of Voices of Authority. Thanks to Pause and Cocklick for joining us, and to our studio audience for being so well-behaved.
Jamie: You mean me?
SS: Quiet, Jamie. Anyway, since I apparently can't tell the difference between dark matter and dark energy, purely because they both start with the same word, maybe you guys could tell me what the difference is between them?
Pause: Dark matter is still a form of matter, which we can't see, but has gravitational attraction like all other matter.
SS: So how come we can't see it?
P: We don't know: we just can't. Well, we can't detect it. It appears to interact only via gravity, which is the weakest of the four fundamental forces.
SS: The other three being?
P: Weak nuclear, strong nuclear, and electromagnetism.
SS: All right. So we don't know it exists; we haven't detected it.
Cocklick: That's not quite true. We can't observe it in EM radiation, but we can test it by galactic collision. Basically, two galaxies collided, and there was a big explosion in the middle, but the dark matter went straight through.
SS: But you couldn't detect that, surely? Isn't there a problem with inferring its existence in that way having already assumed its existence? Isn't that a bit circular?
C: Not really. The idea was "If we assume dark matter exists, what natural phenomenon would demonstrate its existence?" What would split normal matter and dark matter? And the collision of galaxies is one thing that would do that, so they went out looking for it. And they found... something, by using gravitational lensing.
SS: So there's more to dark matter than just "we need this thing to balance everything out".
C: Well that's how it started, but now we're at the next step.
SS: Proving that it actually exists?
C: Yes.
SS: Right, that'll have to do, since I've already wandered way off what it was I actually wanted to ask. So, dark matter is matter that interacts by gravity but is unobservable. What's dark energy?
P: Dark energy is something completely different. It's a repulsive force which is responsible for the universe expanding. In the loosest possible sense, it's "anti-gravity".
SS: And the universe isn't just expanding, but expanding faster as time goes on, right?
P: Yeah, the expansion is accelerating.
SS: Which confused everyone, since we'd always assumed that the universe would either contract, reach an equilibrium, or just drift apart.
P: And none of those seem to be the case.
SS: I've got to ask the same question I did about dark matter. Is there any reason to believe that dark energy exists, beyond us needing something to explain accelerating expansion? In which case, couldn't I just say "it was God?"
C: Well, not really. Science is about generating models. We have a standard model that explains most things. Now we've found something that it doesn't explain. So we ask ourselves what we can add to model to explain it.
SS: But my point is what makes "dark energy did it" any more compelling than "God did it?"
P: Well, dark energy is just a name for at least two or three different theories. It's more of a label.
SS: So it has nothing to do with actual energy?
C: Maybe not.
SS: So I could call it "space fairies" and be no less accurate?
P: You might find it harder to get your papers published.
SS: Isn't that a little homophobic?
J: You're the one trying to blame universal expansion on gays.
SS: Quiet, Jamie; this is a serious article. Anyway, I could argue that dark energy is actually a racist term.
C: Back to the question at hand, you could call it Unexplained Phenomenon A, if you really wanted. I guess the "dark energy" label came about from attempting to balance Einstein's equation. They didn't want to call it "negative energy."
SS: Because that describes the Mexicans?
P: No comment.
C: The University of St Andrews does not endorse bigotry of any kind.
SS: Yeah, MotCC would like to make clear that the Mexicans are an industrious race, and a wonderful people.
P: We salute our Mesoamerican brothers.
SS: OK, next question. Obviously as the universe expands, the galaxies get further apart and thus exert less gravitational force upon each other. So why isn't this dark energy getting weaker in the same way?
P: Because dark energy, unlike matter, isn't concentrated in galaxies. In fact, maybe it's better to move away from the "anti-gravity" idea. Dark energy actually expands space-time itself. So rather than the "big crunch" idea, where everything collapses back into itself, you get the possibility of a "big rip", in which space-time becomes stretched so far that it just basically snaps. Electrons end up too far from nuclei, and everything stops working. It's one more potential "end of the universe" scenario.
SS: So the expansion of space-time doesn't mean subatomic particles get bigger, they just get further apart?
P: Right. And if this dark energy effect isn't homogeneous, then it's pretty close to it.
SS: OK, well I think that's probably enough for our first session. My thanks to Pause and Cocklick for their co-operation and enlightening answers, and to Jamie for not slowing us down with retarded art-student questions like "What colour is an atom?" See you next time.
(Edited for clarity).

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