359 | Solo: Theories of Dark Energy

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 uh 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 know, 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. So I planned out the episode and I realize 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 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 Ten to the minus one hundred and twenty two. 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, etc. etc. 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, you know.

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