Why wasn't "Dark Energy" obvious?

The wife & kids are out of town this week, so I’ve had some extra free time, and I’ve filled it with pseudo pop-science shows on the Science Channel. They’ve been going on about mysterious dark energy & how it was surprising. Now, IANA physicist by any stretch of the imagination, but why is the accelerating expansion of the universe surprising? The singularity preceding the big bang would have been the most gravitationally stable state, would it not have been? Why is there a presumption that the expansion should have slowed since then?

Or maybe it’s not really surprising & they’re just milking it for the purposes of the show?

For something to accelerate, there has to be a force applied. The only known force that works on an intergalactic scale is gravity, which is purely attractive. So what scientists were expecting to see was that the expansion of the universe was slowing down as the tug of gravity pulled on everything to hold it together. The big mystery was whether there was enough mass in the universe to stop the expansion completely and have the whole process run in reverse (“The Big Crunch”) or whether the expansion would continue forever, getting slower and slower but never reaching zero. No one expected the experimental results to show the expansion speeding up, because that would imply the existence of a hitherto-undiscovered force that worked on an intergalactic scale like gravity but repelled instead of attracted. But that’s just what the experiments showed. “Dark Energy” is the placeholder name for whatever this mystery force is.

The very fact of the big bang seems inconsistent with your explanation. I would expect the singularity to be the most gravitationally stable state. Given that the universe started expanding, I don’t see any reason to expect gravity would slow or stop said expansion. Some sort of external “push” seems built into the theory from the git-go. I don’t understand why we’re surprised to find it’s still there.

There are ‘usual suspects’ for forces that repel matter from other matter at short distances, and the big bang started with the shortest distances conceivable. Gravitationally, the ‘superatom’ preceeding the big bang might have been stable, but you could have electrostatic repulsive forces easily overwhelming gravity and even providing enough ‘push’ to get the nascent universe expanding into current intergalactic distances - at which point electromagnetic effects are infinitesimal.

But none of those other forces would explain why galaxies are STILL pushing each other away.

Does that help shed any light on the issue? :slight_smile:

When was this dark energy discovered? This is actually news to me. The last time I read something related to the physics of the universe it was A Brief History of Time, and what I remember taking away from that book was that the universe was only expanding to a certain point where it will eventually start to collapse back into itself. Has this discovery negated that theory?

Yes. Evidence for the accelerating expansion of the universe was only first observed about a decade ago.

I think you may not be differentiating velocity and acceleration.

Think of this: If you throw a baseball straight up, the fastest it’s ever going is at the moment it leaves your hand. Your hand (and arm) applies a force to the ball that accelerates it, but the moment the ball leaves your hand the force stops and the acceleration stops. From then on, gravity tugs at the ball, slowing the ball’s velocity. So if you throw a ball up really really hard, you expect to see it zoom away from your hand, and then continuously slow down.

Compare this to a rocket: The force of the rocket’s engines applies for some long period of time as the rocket climbs, continuing to accelerate it (the rocket’s velocity increases) as it rises.

Up until recently, the universe’s expansion was thought to be like the baseball: some kind of force was applied in the beginning, sending the universe expanding outward, but that force lasted only momentarily. Ever since then, gravity has been tugging at everything, slowing everything’s velocity.

Recent observations indicate that “everything” in the universe is not slowing down after all, but actually speeding up. It is as if you threw a baseball straight up, and instead of slowing down and eventually falling, it continued to go up, faster and faster. Like the rocket.

If you observed a baseball doing that, you would have to conclude that there was some heretofore unknown force acting on that baseball – like the engines of the rocket – that caused it to accelerate upward, in “defiance” of gravity. You might even be inclined to call this unknown force “dark energy”.

True, but other forces were at work besides gravity, as **chrisk **points out.

Some forces pull, others push. But all the pushy forces we know about only work over short distances: electrostatic repulsion, magnetic repulsion, etc. When the universe was tiny they would have a large effect on its behavior. But once it gets huge and spread out, they’re just too weak. The only force we know that works over very long distances is gravity, and all it does is pull.

But for the expansion to be accelerating now, there needs to be a pushy force that works over long distances. That long-distance pushy force is dark energy.

Incidentally, wouldn’t it be better to refer to it as the dark force? Force and energy are connected, but they aren’t the same. You do need to have energy to drive force, but the energy driving the dark force might be something that we already understand, just applied in a new way.

Of course, put the words ‘dark’ and ‘force’ together and the Star Wars geeks begin to gather around. :smiley:

Yeah, “dark energy” is a stupid name. I think it was intended to play off “dark matter”, the other big cosmological mystery right now, but it really doesn’t make any literal sense.

It might not even be a force. It might be some weird property of the fabric of space-time at large scales. We don’t know why the universe went through an inflationary period, or why, after inflation started, it stopped again. This might be something similar. A better name would be something like “the acceleration anomaly”.

Well, also “Dark” as in “We can’t observe it.”

The current interpretation of what’s causing the expansion is that it’s a perfectly ordinary gravitational force which obeys all the same rules as any other gravitational force, but that it’s produced by a very weird (by our standards) sort of substance. The key is that gravitational fields, in general relativity, aren’t just produced by mass, but also by pressure. With ordinary materials, the pressure part is far, far weaker than the mass density part, so we usually just ignore the gravity produced by pressure. But the dark energy has a pressure that is not only very large (comparable to its density), but negative, so that the gravity produced by the negative pressure is able to overwhelm that produced by the positive density.

Now, it’s also possible that it is a separate force from gravity, and not a “substance” after all, and that was the way it was viewed for most of the past century (even though it was only proven recently, it’s been hypothesized for a long time). We don’t know enough about it to really tell the difference. I think the reason that it’s now viewed as a substance is because modern theories of particle physics predict the existence of a substance which is qualitatively very similar to what the dark energy should be like (though quantitatively extremely different), and there’s a hope that the vacuum energy predicted by those particle physics theories might be reconciled with our observations of the Universe.

Back in the 70’s the 200 inch Hale reflector let us see about 6 billion years into the past, and up to that distance Hubble’s law held reasonably well.
During the 90’s and the aughts telescopes improved and we started to see things out to 10 or even 12 billion light years. Hubble’s linear relation between redshift and distance appears to break down as you get that far out. That breakdown of a law constitutes a canonical scientific ‘surprise’.