Don Lincoln: uncertainty

Transcript context

…So dark energy is either energy of space or energy in space. The most common statement is the energy of space, and it is essentially a repulsive form of gravity. And we believe this is real, and the reason we believe this is real is from observation. This is one of those things where we talked about a while ago where I said that, you know, you can think about things up this theoretical stuff and try to come up with a measurement, or you can make measurements and see where they disagree with predictions and lead that in a direction. So back in the late 1990s, some astronomers were looking at the expansion rate of the universe. So the Big Bang occurred, the universe is expanding, the universe is full of matter, matter attracts, so the gravity due to the matter of the universe should slow the expansion of the universe. And the only question was how much? There were three possibilities. The possibilities were there was so much gravitational force that the expansion of the universe would slow, stop, and be pulled back together in a big crunch. Number two was that the universe would continue expanding, slowing down, but never really stopping. And then the third possibility was the exact critical case, where expansion would slow forever and approach zero only at infinity, never quite stopping or reversing. So those were the possibilities, door number one, two, or three. So they did the measurement, and what did they found? It was door number four. The universe was not only expanding, but the expansion was speeding up, and the only way that could happen, given that gravity slows it down, is there was a repulsive force, and the name we give to that repulsive force is dark energy. This is something that Einstein postulated early on in his development of general relativity, but then because at the time, he knew that his theory predicted that the universe would collapse but he believed the universe was eternal and not unchanging, and so he needed something to counterbalance that collapse, and so he invented dark energy. He didn’t call it that. Called it the cosmological constant. But then a few years later, Edwin Hubble discovered that the universe was indeed expanding, and so since the universe was no longer static, Einstein said, “No need for cosmological constant,” took it back out. Thought it was a dumb idea that he put it in, and was embarrassed. However, in 1998 it became clear that his original idea that there should be some sort of repulsive form of gravity was real, and it’s put back in the theory. a that he put it in, and was embarrassed. However, in 1998 it became clear that his original idea that there should be some sort of repulsive form of gravity was real, and it’s put back in the theory. And so that’s what it is. We are pretty confident at this point that the expansion of the universe is speeding up, and the thing driving it is dark energy. Now, what is dark energy? I don’t know. As I said, the most common thought is that it is the energy of space itself, but it is at least conceivable that there is a field in space where space exists- … and that field is pushing space apart. That’s another conceivability that I’m not sure that we have the instrumentation to distinguish, but that’s not what normally people think. People think it is literally a property of space. But, but there is the what—what you call the worst prediction in physics, which is a nice—…

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