Particles of Thought

HAKEEM: Man, you discovered dark energy. You're a co-discoverer of one of the biggest paradigm shifts in the history of astrophysics, right? Since Hubble discovering that the universe is expanding, discovering that it is accelerating, So when you get a result like that, I mean, man, I would be sweating because I know how our colleagues are.

ADAM: Yes. They're rough.

HAKEEM: They're rough. They're rough.

ADAM: They're rough.

HAKEEM: And chances are, if you've gone through everything and you're like, "I'm sure this is right," because we have so many and everybody's so smart, somebody's going to be like, "No, you idiot, it's that." And they know immediately. So how did your team receive this result? And how did you feel reporting this result to your team?

ADAM: Yeah. Very nervous. At first you find something like this, and as I said, you're sure you're wrong, but you know the process to go through. I'm going to go over all the steps, check everything, do this, do that. Because you want to find your own error first, right?

HAKEEM: Absolutely.

ADAM: In fact, you want to find your error and not even admit to anybody that you made an error. Your former advisor, your colleagues, you're just like, "I'll just find it—"

HAKEEM: And not to mention, you were pretty young.

ADAM: Correct, correct. I was fresh out of graduate school.

HAKEEM: Wow.

ADAM: And so I hadn't had any significant results. And so yeah, you want to find your own error. And then when I couldn't and I checked everything, then I began working with people on the team saying, "Look, I'm seeing something funny. It's probably going to go away, but can you just check step B to C? And can you check step C to D? Can you reproduce?" And it was just a series of farming out. And then Brian Schmidt was, who's my colleague, he was leading the team, did the final check on the last step. So I remember he had moved to Australia and I was in California. So when I would send him an email, it would take like 12 hours before I'd get a response.

HAKEEM: Oh boy.

ADAM: So I had a very sweaty night one night when I was like, "I've checked everything. This is the final calculation is, do you see what it favors?" And he wrote me back the next morning an email that said, "Well, hello Lambda." Okay. And now Lambda is, as you know, the Greek symbol for what we call the cosmological constant or dark energy. And so he saw the exact same thing.

HAKEEM: Wow. Well, Brian seems to have had the belief in the result to make such a bold statement.

ADAM: Right.

HAKEEM: You mentioned Lambda.

ADAM: Yeah.

HAKEEM: So Lambda shows up in Einstein's equations as a constant that he inserted. And there's that historical story that I hear people misquote, that people often say Einstein said that was his biggest blunder. But I saw another science communicator, Sabine Hossenfelder, I think, who said, "No, he didn't say that about that. That's not what that was." But I don't know the truth historically. But I do know that when I look at that equation and I attempt to interpret what it means, it can be read in one of two ways. One way is that dark energy is some stuff in space time. And the other way is that dark energy is the intrinsic energy density of space time. What is your interpretation?

ADAM: Yeah. I mean, sometimes it depends on, in this discussion, which side of the equation you put it on. Because basically what you're trying to balance, sorry, what Einstein was trying to balance was that he thought that the universe was static, that it wasn't expanding or contracting. That's what astronomers of the day told him because astronomers of the day didn't know that what we call galaxies were actually outside the Milky Way. So they thought everything was in the Milky Way. Nothing is really expanding in the Milky Way.

HAKEEM: A universe of one galaxy.

ADAM: Right, right. So they said, "Hey, nothing's really expanding or contracting." And he was like, "Wow, that's a puzzle," because this term in the equation, which looks like kind of like Newton's gravity, will cause things to pull together. There must be something pushing the other way. So he made an amazing discovery, which is that although the gravity of stuff, of matter is attractive, that the gravity of empty space could be repulsive, could go the other way, and that these two could be in balance. And so he called this the cosmological constant, and he saw that it could exist as an extra term in his equation. And this is important, there was a place for it. If Newton had had this problem, he would be stuck, because in his gravity there is no option for something to be repulsive. But the curious thing is, Einstein's gravity recognizes different forms of matter and energy as having different gravity. And so energy itself can be repulsive. It has a curious property that we call negative pressure.

HAKEEM: Exactly. I was going to say, you could have positive energy and positive pressure, which would be attractive, or positive energy and negative pressure, which would be repulsive.

ADAM: Correct, correct. And so that is how we attempt to understand it today, as though the universe is filled with this kind of energy that has this property of negative pressure, and so it has repulsive gravity. I think Einstein didn't even go that far. I think it was a term in his equation that he—

HAKEEM: With no physical interpretation?

ADAM: Yeah, with initially not a lot of physical interpretation. It was just what we would like to call a boundary condition. Well, the universe is static, therefore a term is there.

HAKEEM: Wow.

ADAM: And of course, once he learned about a decade later from Hubble and others that the universe was expanding, he certainly thought this was a mistake and removed it. This question of whether he called it his biggest blunder or not is more anecdotal, apocryphal. Who was he talking to? Did they remember it correctly? But the sentiment is certainly true, that he thought, "Well, if the universe is expanding, I don't even need this. Why did I come up with it?" But once that toothpaste was out of the tube, there was no putting it back. I mean, he demonstrated it could exist, it may exist. There's even a physical interpretation of it. It could be the energy of the vacuum that quantum theorists wouldn't know how to get rid of if they wanted to. And so we have always been in this situation of this ambiguity. Is it going to be there or is it not? And every couple of decades, somebody says, "I think I see it," and then others were like, "No, I don't." But it wasn't really until 1998 that we saw the direct consequence of it that is irrefutable.

HAKEEM: Irrefutable. Now in 2025 is even become stronger and stronger, right?

ADAM: Yeah.