Adam Brown — Bubble universes, space elevators, & AdS/CFT

**Dwarkesh Patel** (0:00)
Today, I'm chatting with Adam Brown, who is a founder and lead of the BlueShift team, which is cracking math and reasoning at Google DeepMind, and a theoretical physicist at Stanford. Adam, welcome.

**Adam Brown** (0:12)
Delighted to be here, let's do this.

**Dwarkesh Patel** (0:13)
Okay, we'll talk about AI in a second, but first, let's talk about physics.

**Adam Brown** (0:16)
Okay.

**Dwarkesh Patel** (0:17)
First question, what is gonna be the ultimate fate of the universe? And how much confidence should we have?

**Adam Brown** (0:23)
The ultimate fate is a really long time in the future, so you probably shouldn't be that confident. About the answer to that question. In fact, our idea of the answer to what the ultimate fate is has changed a lot in the last 100 years. About 100 years ago, we thought that the universe was just static, wasn't growing or shrinking, was just sitting there statically. And then in the late 20s, Hubble and friends looked up at massive telescopes in the sky and noticed that distant galaxies were moving away from us and the universe is expanding. So that's like big discovery number one. There was then a learned debate for many years about the universe is expanding, but is it expanding sufficiently slowly that it will then re-collapse in a big crunch, like a time reverse of the Big Bang, and that'll be super bad for us? Or is it going to keep expanding forever, but just sort of ever more slowly as gravity pulls it back? But it keeps, it's fast enough that it keeps expanding. And there was a big debate around this question. And it turns out the answer to that question is neither. Neither of them is correct. In possibly the worst day in human history, sometime in the 1990s, we discovered that in fact not only is the universe expanding, it's expanding faster and faster and faster. It's what we call dark energy or the cosmological constant is this, just a word for uncertainty, is making the universe expand at an ever faster rate, accelerated expansion as the universe grows. So that's a radical change in our understanding of the fate of the universe. And if true is super duper bad news, it's really bad news because the accelerated expansion of the universe is dragging away from us lots of distant galaxies and we really want to use those galaxies. We have big plans to go and grab them and turn them into vacation destinations or computronium or in any other ways extract utility from them. And we can't if the cosmological constant is really constant, if this picture is correct, because anything close enough, we can go out and grab it obviously, but if it's further away than about a dozen billion light years, the expansion of the universe is dragging it away sufficiently rapidly that even if we send probes out at almost the speed of light, they will never make it. They will never make it there and make it back. They'll never even make it there if it's sufficiently far away. And that means that there's a finite amount of free energy in our future. And that's bad. I mean, that means we're doomed to a heat death, if that's true. But is it true? Is it? I mean, that was the second half of your question. And first of all, we keep changing our minds about these things over the last century or so. So on first principles grounds, you may be somewhat suspicious that we'll change our minds again. And none of this has settled physics. And indeed, it may be that the cosmological constant is not constant. And you should hope with all your heart that it's not. It may be that it naturally bleeds away. It may be, in fact, that our fate is in our hands and that our distant descendants will go and bleed the cosmological constant away, will force it to go to zero.
They will be strongly incentivized to do it if they can, because otherwise we're doomed to a heat death.

**Dwarkesh Patel** (3:25)
How would they bleed this away?

**Adam Brown** (3:27)
Oh, okay. This obviously depends on physics that we're not totally sure about yet. But it seems pretty consistent with the known laws of physics, that the cosmological constant, what we perceive it as being a constant, this dark energy quantity that's pushing the universe apart from each other, in many very natural extensions of the known laws of physics, that is something that we have the ability to change. In fact, it can change, can take different values. It is not just totally fixed once and for all. That in fact, you have what's called different vacuum, different regions of parameter space that you can transition between in which the cosmological constant can take different values.