359 | Solo: Theories of Dark Energy

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**Sean Carroll** (1:02)
Hello everyone and welcome to the Mindscape podcast. I'm your host Sean Carroll. I do hope everyone listening understands that they are witnessing an historic occasion. This is the first time ever that on Mindscape we've had a two part episode. We always just make history in interesting ways here at Mindscape. I know that in the past I've been happy to just go on at great length in individual episodes, but I do think that this one kind of naturally breaks into two different parts, and you don't have to listen to one part to get a lot out of the other one, so it made sense to do it this way. The overall topic, of course, is dark energy and the accelerating universe, theories of how dark energy works, what it might possibly be.
I planned out the episode and I realized I need to give background about vacuum energy and the cosmological constant, Einstein's old idea. And then, of course, that led to talking about the cosmological constant problem and the particle physics of it all. And so that made its own episode. That was the previous episode. And where we were left off was we have a discovery experimentally that the universe is accelerating. We can explain that by invoking a cosmological constant, much like Einstein did years ago for a slightly different reason. But it raises questions we don't know the answers to. The two big ones are, roughly speaking, the cosmological constant problem, which is why is the value so much smaller than you might expect it to be? If you think about effective field theory, which is an enormously successful paradigm to think about all of quantum field theory and particle physics, the cosmological constant appears as a number in the effective field theory. And we have expectations for how big that number should be. And the actual number is smaller than the expectation by something like 10 to the minus 122 So that's bad and it might be a clue to some interesting physics underlying what's going on. And it certainly wasn't what a lot of particle physicists ever expected. The other problem is, okay, if you think that you've measured the vacuum energy, the energy density in empty space itself, and that's making the universe accelerate, et cetera, et cetera, then why are we so lucky that the value of the vacuum energy is approximately the same as the value of the matter energy density in the universe today? By approximately the same, we mean three or four times or five times as much, something like that, but not 10 to the 100 times as much. This is called the coincidence problem because the relative amounts of vacuum energy and matter density change as the universe expands. The vacuum energy stays constant as an energy density. The amount of matter in the universe as a density goes away as the universe expands. It dilutes away to zero. So if they're approximately the same order of magnitude today, in the past, there was way more matter density than vacuum energy. In the future, there'll be way more vacuum energy than matter. Why is it that we got so lucky to be born at just the right time? It almost suggests that there is some kind of anthropic human-centered explanation for this. And maybe there is, but maybe there isn't. Maybe we can do better in terms of thinking of new physical explanations. So when the idea of the accelerating universe became established in the late 90s, early 2000s, physicists immediately said we can't simply say, it's the cosmological constant and stop there, that is causing the acceleration of the universe. Maybe it's something else. We should be open-minded. We were surprised once. We could be surprised again. And that led people to the idea of dynamical dark energy, something that's not quite the cosmological constant, but looks that way, is a pretty good approximation. And that's something that we can look for experimentally, as well as writing down theoretical models for what it might be. And so that is what we're getting to today. We're going to talk about dynamical theories of what the dark energy could be. And as I said in the previous episode, I'm not doing 100% historically fair and balanced treatment of this subject by any stretch of the imagination. I was involved in writing papers and thinking about this for a long time. So I'm just telling you what I was thinking at the time and how that intersects with what other people were thinking at the time. It's going to cover a pretty decent amount of ground in terms of the different possibilities for what could be going on. We still don't know what is actually going on out there in the world. So in some sense, I'm preparing you to think about new ideas as they get noticed and explained to people, or maybe even come up with some new ideas of your own. So with that, let's go.