358 | Solo: Vacuum Energy and the Cosmological Constant

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**Sean Carroll** (2:04)
Hello everyone and welcome to the Mindscape Podcast. I'm your host, Sean Carroll. In physics, which is a very broad field, includes atomic physics, plasma physics, condensed matter physics, biophysics, particle physics, gravity, cosmology, all these things. Different fields, of course, have different rates of progress, both because of theoretical ideas and experimental input. If you focus just on fundamental physics, and let's not argue about what that means or should mean, I take it to mean figuring out the most fundamental laws of nature the best we can. So not including things like biophysics and condensed matter physics, not because they're not important. It's just a different thing. I once tried to get the field to change the name fundamental physics to the name elementary physics, the most elementary laws of physics being the idea. But no one agreed with that. That's okay. Fundamental physics is here to stay.
In this subfield of fundamental physics, which includes sort of particle physics and gravity, things like that, there haven't been a lot of helpful experimental surprising discoveries in the past several decades, arguably since the 1970s. There have been a few here and there, masses of neutrinos, things like that. Also, things that we expected, like the Higgs boson, gravitational waves. But in terms of true surprises, it's been few and far between. That makes it hard to make progress in fundamental physics because experiments are what drive us to great ideas. And there's one, of course, shining counter example to this idea that there haven't been many surprises, which is the acceleration of the universe. Back in 1998, we were told by results from two competing astrophysics groups looking at Type 1a supernovae, using them to measure the expansion rate of the universe and how it changed over time, that against all expectations, the universe is actually accelerating, not decelerating, in its expansion rate. And of course, this was a big deal. Nobel Prizes were handed out and the whole bit. We still don't understand with complete confidence what is going on with the acceleration of the universe, which is great for fundamental physicists such as myself, for theorists who are trying to figure out what's going on. It would be nice if we had more than that one fact, the acceleration of the universe, it would be helpful to our project of inventing theoretical models to explain it, but it doesn't slow us down. We're going to invent them anyway. There is a leading candidate, the cosmological constant as proposed by Einstein many, many years ago, but there's other ideas as well. So recently, through a set of circumstances that I won't go into, I fell behind personally in my recording of podcasts, and I'm doing traveling and things like that. And so even though I just did a solo podcast on quantum mechanics, not that long ago, I figured I can fill in by doing some more solo podcasts. I figured just one solo podcast, but what you're going to do is you're going to get two, and two separate weeks. What happened was I went to Patreon, where I sometimes turn for advice from my Patreon listeners. If you're ever curious about what it would be like to be in the cool in crowd of Patreon listeners, just go to patreon.com/seanm. Carroll, and you can sign up. And I asked them what would be a good topic for a solo podcast, and I gave them some suggestions. And I think that a pretty clear tendency, even though there's a lot of diversity in the answers, was for the suggestion of theories of dark energy and the accelerating universe. And in some sense, it's shocking. I haven't done that already. This is something that I do know something about. Indeed, my first ever attempt at publishing a trade book, and writing a popular level book and getting it published was I proposed a book on the accelerating universe and dark energy, and no one wanted to publish it.