- I don't think there are any physicists in the world who are satisfied with our explanations of dark energy and dark matter.

- Which is we have no idea what they are.

That's the explanation I know.

- Right.

Right.

So that's why it puts you in a realm where experiments, even things where you think you know the answer, are important to do.

- Yeah.

Yeah.

(upbeat music) Adam Riess, welcome to "Particles of Thought."

- Nice to be here.

- Oh man, have I been waiting to talk to you for years.

You are a cosmologist.

You have a Nobel Prize.

You're our first Nobel Prize winner here.

Yeah.

And, 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.

Completely unexpected.

And even crazier, I hear that a big part of your work was done on your honeymoon.

So how do you win a Nobel, how do you do Nobel Prize winning work on your honeymoon?

- Well, you have to do a little work ahead of time, it turns out.

(both laugh) But in our case, you know, it was just a multi-year process of a research experiment, and everything came to a head right at that time.

And so, you know, I left for my honeymoon, but I had the results.

I checked the results.

I'd shared the results with colleagues.

They had checked the results.

And it was just that moment to have the conversation, which started over email.

And because I was away at the actual wedding, a bunch of my colleagues were saying, "Well, this is what it looks like.

"This is what I think we're seeing."

And I had done a lot of the work, so I came back from the wedding packing my bags for the honeymoon and checked the email and I was like, "Whoa, I think I gotta answer some of this."

So I started responding, and got some icy stairs from my wife who was like, "I don't get it.

"This is our honeymoon.

"Is this the way our life is gonna be, "that you're always working?"

I was like, "No, this is a really special email.

"I really gotta get back to these guys."

- Yeah.

- But yeah.

- Did you believe it?

- You know, I don't think you ever believe something in the beginning.

You know, in science, we all have the experience of, I'll say, making so many mistakes.

- Absolutely.

- I mean, you know, science is really, really hard.

You're always doing something that, you know, hopefully, ideally you're getting to a point that nobody's done before.

- Yeah.

- And there's just a million ways to do something wrong.

- Absolutely.

- And so, you know, all your research career, you've been like, "That doesn't make sense.

"Oh, I got a bug here.

"This doesn't make sense.

"Oh, a negative sign there."

I mean, how many times just even running a computer program, it tells you, "This doesn't run."

- Right.

- You know?

And so you're constantly living that.

- Every time.

- Right.

And then, so when you actually, if you actually ever do see something that you can't find out why it doesn't, why it's wrong, but yet it seems surprising, you know, it takes a long time to build trust in that.

- Yeah, I bet it does.

So dark energy could be stuff in space-time.

It could be the intrinsic energy density of space-time, but whatever it is, it's causing space-time, one of the largest scales, to expand ever more rapidly, like a repulsive gravity.

Right?

Is that correct?

- That's correct.

- All right, let's get into the discovery, but I want to unpack it a piece at a time.

- Sure.

- So you basically use a special type of exploding star to measure the size of the universe versus time.

So that type of star is a Type 1a supernova, right?

So tell us about that process of, you know, what those stars are and what leads to them exploding.

- Sure.

So in order to gauge and measure the universe, we need to be able to measure, as you say, how far away things are.

- Yeah.

- And so we have to look out in the universe and see things that we can recognize.

Okay, just like here on Earth, you know, you cross the street, you see the headlights of a car, and you can gauge how far away that car is by how bright the headlights appear.

That requires that you actually understand that headlights are pretty luminous things, right?

- Right.

- So we look out into space and we don't have any of these human made objects where you can go, oh, there's a, you know, a Toyota Corolla with its headlights and stuff.

So instead you have all these lights and you're like, "What are these things?"

And so it took astronomers really a century, I would say, to get to the point where they understood them well enough.

And in the case of what we're discussing, there are certain kinds of stars that explode at the end of their life called a supernova.

- Yeah.

- And it's important because by being so luminous, they're billions of times the luminosity of the sun.

You could see them very, very far away.

- I gotta interject here.

- Yeah.

- When you say it explodes at the end of its life, by definition... - That is the end.

Not more is gonna happen.

That's absolutely true.

But there are different ways that that can happen.

And so this is really the essence of it is there are different flavors of supernova, just like there's different flavors of stars.

And they're not all the same.

And so you would, you know, make a terrible misestimate of distance if you confused, it's like confusing, you know, a motorcycle headlamp for, you know, a Mack truck or something.

You know, they're just very different.

So in the 1930s, it's that far back, Subrahmanyan Chandrasekhar, the famous Indian astrophysicist, explained that a certain kind of star could not exist, could not be stable, couldn't hold itself up against gravity if its mass exceeded the Chandrasekhar Limit, his limit, which is about 1.4 times the mass of the sun.

And so a star just below that will be holding itself up by gravity, but will be so close to the conditions of fusion throughout the entire star.

Like a fusion bomb.

And so we think a friend, and with friends like these who needs enemies, right?

Will be orbiting that star and- - Another star.

- Right.

So two stars orbiting.

And somehow mass will transfer or move over.

And so whether it's, you know, teaspoon by teaspoon, we're not sure, but at some point it crosses that Chandrasekhar Limit.

Whether that happens 'cause the two stars merge, or whether material spilled over, somehow it crosses that limit.

And then you get a runaway thermonuclear explosion.

And the beautiful thing for a cosmologist is that because they always blow up at or around 1.4 times the mass of the sun, they're gonna be very uniform.

And so you see one far away and if you can recognize it's that kind, now you can figure out how far away it is.

- Oh, so essentially, you know how bright they are.

So based on how bright they appear, and this particular star is always around the same brightness sort of thing.

- Correct, correct.

Right.

- So identifying an exploding star sounds easy, but, you know, how often does a star like that explode in a galaxy like ours or any galaxy?

And then how do you find them if you don't know where they're gonna explode?

- Yeah.

Yeah.

That's a great question.

So a supernova explosion is very rare.

And we're grateful about that because if they were blowing up all around us, you know, we wouldn't last very long.

- Yeah.

- And so, you know, we get a nice long window here before that happens.

So about once a century in a galaxy like ours, a supernova of this type will explode.

So if you just picked your favorite galaxy and just stared at it, right, it's gonna take about 100 years.

This would not be a good thesis project for a graduate student, right?

- Not one from this planet.

- That's right.

That's right.

So what you want to do then is you want to stare at many galaxies at the same time.

And so this is like the question, how do you win the lottery?

And the answer is you buy all the lottery tickets, right?

- Be sure to win.

- You buy a lot of lottery tickets.

And so what changed the game was around the 1990s was the development of new cameras on telescopes that had very wide fields of view.

They used electronic detectors, CCDs, wide fields of view.

And so for the first time, you could take a single image that might have 10,000 or 100,000 galaxies- - Wow.

- In one image.

And then you take an image like that, a month later, you digitally subtract one from another.

And you don't know which galaxy is gonna have a supernova, but certainly some of them will because there's just so many galaxies you've been monitoring.

- Yeah.

Yeah.

And so when you do the subtraction, everything that didn't change stays there.

- Correct.

- And then the stuff that changed pops out.

- Right.

- And then you have to figure out, was that what we're looking for?

- Yes, that's right, that's right.

And then a Type 1a supernova will have a certain spectral fingerprint.

So when the star explodes, it's mostly made of carbon and oxygen.

And it will also burn or fuse elements to higher up on the periodic table.

You'll get a lot of silicon and sulfur.

And so when you take a spectrum, when you take the light from the distant supernova and you pass it through a prism, like we split the colors of light, you will get a spectrum.

And you will look for features in that spectrum, which tell you, oh, there's silicon, there's sulfur.

More importantly, the whole fingerprints looks exactly almost the same every time one of these Type 1a supernova goes off.

So not only are they all the same luminosity, they have the same fingerprint.

So you look for that.

- Yeah.

- And then, you know, you have one, and then you wanna measure how bright it is, tells you how far away it is, and you wanna measure how much it's light has redshifted by the expansion of space.

And so because space is expanding, some wavelength of light is traveling to us from the supernova.

And as it travels, space expands, and it stretches those wavelengths of light.

And longer wavelengths of light is redder light.

So we call it the redshift.

And so, you know, the redshift is telling us how much the universe expanded, where the distance measurement is telling us how long ago that supernova exploded, because knowing the speed of light and distance tells you time.

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Tell me what you expected to see versus what you actually saw.

- Right.

Right.

So in the 1990s, astronomers were using these Type 1a supernovae to measure how fast the universe is expanding today.

- Right.

- So that meant observing nearby Type 1a supernovae.

So there's an extra wrinkle to this story, which is really what makes this all possible is it takes light a long time to reach us from these distant supernovae.

And so when we look further and further away with these techniques, we're not actually measuring how fast the universe is expanding today, but how fast it expanded in the past.

- Ah.

- So there's this kind of built in, you know, time delay, if you will, that actually is our superpower because it allows us to look at the past expansion history of the universe just by looking at more and more distant supernovae.

So by the 1990s, we had well measured the nearby rate.

The new game was to look for these ultra distant ones, which were very faint, so they were just at the edge of what you could find telescopes, but they would tell us how fast the universe was expanding many billions of years ago.

- Just as a quick insert, 'cause I wanna get back to that story, what size telescopes were you using?

- Yeah, so we were using four meter telescopes.

Yeah, so they're good size.

I mean, by today's standards, they're not the biggest.

- Right.

- But by those standards, they were just about the biggest.

You know, it was around the time the first 10 meter telescope became available.

Keck in Hawaii.

- Oh.

- The Hubble Space Telescope had first become available.

So while Hubble doesn't have the field of view to find the supernovae, it can follow them up.

- Oh, to get that spectrum.

- To the spectrum, or the light curve, the rise and fall of the light.

So it was kind of all hands on deck, all the best instruments around the world operating at the same time.

We looked for these ultra distant ones.

We would find them, get their spectrum, and use it to measure how fast the universe was expanding then.

- So a quick question about that.

Now that you, it turned out to be a Nobel Prize winning discovery.

We're still gonna get back to what you saw and what you expected.

But, you know, now we realize how important it was.

- Yes.

- But time on these big telescopes is competitive.

- Yes.

- So did they really appreciate?

Were you getting told a lot of nos?

Oh, sorry... - Right.

You know, I think at the time people recognized that this was a very important experiment to do.

And so something very unusual happened actually at the time.

This was the mid 1990s.

For example, the Hubble Space Telescope was the hardest telescope to get time on.

It was still pretty new at that time.

And it was oversubscribed.

And the director at the time, Bob Williams, thought this was such an important experiment that he gave his own special pot of high risk director's discretionary time.

- No, he didn't.

- Yeah, and he gave it to both teams.

- What?

- And so, again, this is very unusual.

Usually you compete with other astronomers to do the experiment on something.

And he said, "Let's have two experiments doing this "just to be able to crosscheck the results."

- Wow.

- So that was pretty- - Bob and his forethought, man.

- Yeah, yeah.

The Hubble Deep Field he did.

Time for these experiments.

So anyway, when we were looking at the data, the prevailing wisdom was that we would see the expansion of the universe slowing down, decelerating.

And why is that?

Because after the Big Bang, the universe is expanding, but there's all the matter in the universe that has attractive gravity that is going to pull back on the expansion.

- Against the expansion.

- Right.

It wants to be back together.

And so, you know, like tossing a rock up in the air, you know, you give it that initial throw, but then the pull of the Earth is going to pull the rock back.

And if you could measure how much that rock is decelerating, you essentially weigh the Earth.

- Wow.

- And so we thought, oh, we're gonna weigh the universe by measuring how much the expansion was slowing down.

And that'll even tell us whether the universe had escape velocity from itself.

Would it expand forever or would it collapse?

Like that rock, if you toss it high enough and far enough and fast enough and the Earth weighs little enough, it will escape the gravitational pull.

So that was the game.

And so just before my honeymoon, I had done the final analysis of the data, and I had what looked like a sine error because it seemed to be showing me that the universe was not decelerating, was accelerating.

And that's like, you know, it doesn't make any sense.

- Makes no sense.

- If you toss that rock up in the air and then it went up like a rocket.

- Yeah.

- You know, you'd go, "What is doing that?"

'Cause attractive gravity doesn't do that.

- Right.

Right.

So when you get a result like that- - Yes.

- I mean, man, I mean, I would be sweating 'cause I know how our colleagues are.

- Yes.

They're rough.

- They're rough.

They're rough.

- They're rough.

- And chances are, if you've gone through everything- - Yes.

- And you're like, "I'm sure this is right," because we have so many and everybody's so smart, somebody's gonna be like, "No, you idiot, it's that."

They know immediately.

- Right.

- So how did your team receive this result?

And how did you feel reporting this result to your team?

- Yeah.

Very nervous.

You know, at first you find something like this, and as I said, you're sure you're wrong.

But, you know, you know the process to go through.

I'm gonna go over all the steps, check everything, do this, do that.

Right?

'Cause you wanna find your own error first.

- Absolutely.

- You wanna find your error and not even admit to anybody that you made an error.

- That's right.

Yeah.

- Your former advisor, your colleagues.

You're just like, "I'll just find it."

- And not to mention you were pretty young.

- Correct.

Correct.

I was just fresh out of graduate school.

- Wow.

- And so, you know, I hadn't had any significant results.

And so yeah, you wanna find your own error.

And then when I couldn't and I had checked everything, then I began working with people on the team saying, "Look, I'm seeing something funny.

"It's probably gonna 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.

- Oh boy.

- So I had a very sweaty night one night when I was like, "I've checked everything.

"This is the final calculation.

"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, you know, the Greek symbol for what we call the cosmological constant or dark energy.

And so he saw the exact same thing.

- Wow.

That is... Well, Brian seems to have had the belief in the result to make such a bold statement.

- Right.

- Yeah.

- It was really interesting, when I look back on the emails of our teams talking about this, they almost skew, I'm gonna say at some level with age, because, you know, the older you are, the more you've seen.

And you've seen things come and go and be wrong, and you probably become more conservative.

I think for me, it was an advantage.

This was like my first rodeo, right?

So I was like, "Hey, maybe we just discovered something about the universe.

"Isn't that cool?"

- Yeah.

Well, what was really surprising is how everybody else accepted it so quickly.

- Yeah.

- Like, the field accepted your result- - Right, right.

- As if it were true, which never happens.

- This is very important.

So there were a couple of reasons why that was.

I mean, there was another competing team that was doing a similar experiment and they were reporting the same results.

That was important.

But also, sometimes when you find something, it fits in the sense that once you show it, you say, "Actually this solves multiple problems."

And so one of the big problems at the time was the age crisis, if you remember.

- The stars versus the universe?

That one?

- Correct, correct.

Right, so it looked like there were stars that were much older than the universe.

And when we say that we think as astrophysicists, we can calculate the lifetime of a star from how much energy it puts out and how much energy it has.

It's like, you know, driving a car, how much gas do I have in the tank and, you know, how many miles per gallon do I get and when will I run out, right?

There's that side of the calculation.

But the other was the universe itself, depending on how fast it's expanding, you can run this movie backwards in time and say, when was the Big Bang then in that case?

And so based on an expectation that the expansion was slowing down, right, we thought, oh, this is a slow rate for the universe right now, so therefore when we run the universe back in time, it would be relatively young, okay?

But if instead the universe is accelerating, and we go, oh, this is actually a faster rate than it normally would have.

This means the universe was suddenly older.

Older than the oldest objects in it.

So there's kind of a breath of relief from other parts of the cosmology community.

Another element was there was this puzzle that about 70% of the universe was missing.

- Yeah.

- So there's a deep theory about the universe called inflation, you know, that says how the universe propagated after the Big Bang, and it expects that there's a certain amount of matter and energy in the universe.

And astronomers had only found about 30% of the matter.

And we didn't even know there was gonna be energy.

- This other stuff.

Yeah.

- So yeah, so what we found actually fit in the, it was the missing, you know, puzzle piece, if you will, there too.

So although there was a lot of skepticism, there was also a lot of, well, you know, this does fit a lot of things.

- This is pretty convenient here, even though it's crazy.

- Right, but even after our results came out, it still took years of additional confirmation.

So from the standpoint of supernova observations, we continued to push back further in time, and we were eventually able to see the universe change over from decelerating to accelerating, which is a very important signal because I mean, if the universe is really made of dark matter and dark energy, when it's compact, it will feel its matter more and it will decelerate.

It's only once it dilutes and thins out by expansion, that dark energy is waiting in the wings and pushes it apart.

So we could eventually by the early 2000s we were seeing that with supernovae.

And then the cosmic microwave background radiation measurements came along.

- Right.

- And they confirmed this picture.

- First with WMAP.

- Right.

Some ground based stuff, then WMAP, then Planck.

And so, you know, by the mid-2000s, my confidence went way up in this.

And my confidence peaked in 2011 because we won the Nobel Prize.

- Ah!

- And I thought that's, this is probably- - Validated.

- Probably true.

- Validated.

- Probably true.

- I wonder if there have been any Nobel Prize discoveries where they discover later, oh, actually not.

- Right.

There are a few things that don't hold up that well, let's put it that way.

- Right, right.

Your discovery goes back to year 1998.

- Yes.

- It's now 2025.

- Yeah.

- Did you think that the answer to what dark energy is would be known by now?

- Wow.

That's a good question.

I don't know that I ever contemplated at the time.

You know, I think it was mostly a focus of, is this right?

Is this right?

- Right.

- And then after that, I think that I do think these missions that are coming up have a good chance to tell us what it is.

I'll be totally honest with you.

I think the way science like this works is observers give clues, which comes from doing experiments.

So nature showing you how things go.

But I would say the ultimate answers generally come from theorists, you know?

People like an Einstein who sit there and synthesize this and say, "Wait, here's what's really going on."

And so I don't ultimately think we come to understand dark energy just through experiments, but rather theorists synthesizing experiments.

- I see.

- And unfortunately, you know, it's hard to predict when a theorist will make a breakthrough, you know?

And I mean, I've asked people this before, when Einstein did his development of general relativity, did people see that coming?

Was that like on the horizon?

And the answer is usually no.

You know, there were some clues, but nobody would've derived general relativity because the orbit of Mercury was processing around the sun, which is one of the anomalies of Newton's physics.

So it's hard to predict theoretical breakthroughs.

- Okay, so let's go back then to, you mentioned Lambda.

- Yeah.

- 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, you know, that, you know, 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?

- Yeah.

I mean, you know, 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.

- A universe of one galaxy.

- 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.

I mean, this isn't like, if Newton had had this problem, he would be stuck because, you know, 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.

- Exactly.

That's what I was gonna say.

You could have positive energy and positive pressure, which would be attractive or positive energy and negative pressure- - Right.

- Which would be repulsive.

- 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- - With no physical interpretation?

- Yeah.

With initially not a lot of physical interpretation.

It was just what we would like to call boundary condition.

Well, the universe is static, therefore a term is there.

- Wow.

Yeah.

- 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.

You know, this question of whether he called it his biggest blunder or not is more, you know, anecdotal, apocryphal, you know, who is 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, you know, once that toothpaste was out of the tube, right, there was no putting it back.

- Right.

- 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.

- Right.

- And so we have always been in this situation of this ambiguity.

Is it going to be there or is it not?

You know, and every couple of decades somebody says, "I think I see it."

And then others were like, "No, I don't."

- Yeah.

- But it wasn't really until 1998 that we saw the direct consequence of it that is irrefutable.

- Irrefutable.

Now in 2025 has even become stronger and stronger, right?

- So I'm gonna ask you a question, man, that, you know, we're gonna get back to dark energy, but I gotta take a adjacent detour because there are things we say all the time, and we're comfortable saying them.

And for me, when I see it in mathematics, I understand the math, but the physical manifestation doesn't, it isn't always intuitive to me.

And one that does that for me is the statement, "Space-time is expanding."

Like, what?

Like, I see the math.

We can explain it in math, but intuitively, like how do you deal with that?

Like, what does that mean?

- Right, there's a third way besides the math and the intuition, which is the observation, really.

And so as an observer, you know, it's a fact that you look at things- - Yeah.

- And every aspect of them tells you that the universe is expanding.

You go, "Wow-" - What do you think is happening in space microscopically?

- Yeah, I mean, I think that at, I think on every scale, things are expanding.

However, there is also the opposition to expansion, which can be attractive forces.

So you get to the scale of, you know, the atom, and it's not expanding because the electromagnetic forces or the strong nuclear force are holding things together.

You get to the scale of the Earth, and the Earth is not expanding.

There's electromagnetic force and there's gravity that are holding things together.

So, you know- - And the galaxies aren't expanding.

- The galaxies aren't expanding because there's attractive gravity holding things together.

It's sort of like, I don't know, imagine, you know, skaters on ice where they want to be, the ice wants to be pulling them apart, but they're holding together, they're holding hands.

So you can overwhelm the expansion which is going on because it's not all that strong.

It's just that when you get out into intergalactic space, everything is so diluted at that point that you are now experiencing the expansion of space that's being kicked into higher gear by dark energy.

- Got it.

Got it.

- So it's really, you know, it's really the competition of two titans, you know, going in opposite directions and who wins where depends on how diluted one of them is relative to the other.

- Ah.

- So deep extra galactic space is really more and more the playground of dark energy, and, you know, galaxies and clusters and that's really the playground of dark matter now.

- Ah.

Ah.

That's really good.

So back to dark energy, precisely.

So I haven't been deep into dark energy for about a decade.

And, you know, I remember there were competing models.

You know, just like with dark matter, it had its competing models.

- Yeah.

- And then a lot of 'em got kicked out.

- Right.

- Right?

What remains for dark energy?

What are the possibilities for- - Right.

- Describing what dark energy is.

And I remember everyone was looking to measure this W parameter that the- - Correct.

- Yeah, yeah.

The equation of state of the universe to identify it.

So has things changed?

- Right.

So remarkably, you know, Einstein's cosmological constant remains the sort of in pole position for the last 25 years or so because in a way it's the simplest, and, you know, physicists love simple and elegant.

And so if you tell them two stories, one of them is simple and one of them has extra, you know, features and chance occurrences going on.

They don't like that one nearly as much.

- But this one has a chance occurrence that we call the cosmological coincidence.

- It does.

And I'm glad you brought that up, because to me, I'm one of those people who often pushes back on my own community and says, this is not a contest of something simple and something complex.

This is a contest of something complex and something else complex.

- Right.

- That we, you know, and I don't really see a big advantage of one or the other.

But I will say this.

This is what's important is for dark energy, we have models that are static where we say dark energy is just, it's always been in the universe.

It's like a constant of nature.

It will always be there.

It acts like this cosmological constant, and the universe will accelerate forever, okay?

And then an alternative is that dark energy is instead temporary energy due to a field in space.

So you could think of any field you know of like the electric field or the magnetic field.

This would be a different field.

And a field has energy.

I mean, just take a, you know, take a compass and play with it in a magnetic field.

And you'll see there's some energy.

It's able to shove that needle around, right?

So there's energy there.

And that energy could be the dark energy.

And if that's the case, then it is probably temporary.

It probably changes over time.

And so the important test right now is to see, has our dark energy been changing over time or not?

A third possibility we have to keep in mind is that we've broken Einstein's theory of gravity and that it only kind of looks like there's dark energy.

But it's really because we've, if you go to the scale of the whole universe, right, that Einstein's gravity doesn't continue to operate correctly.

It's sort of like Newton's theory of gravity broke down as you get close to the sun because gravity's strong there.

And so, you know, you could try to invent structures and epicycles and things that will work.

And so, you know, we have to keep in mind that, you know, it's possible we've broken Einstein's theory of gravity.

And so the test there would be that you wouldn't find a story about dark energy that actually worked at all scales.

You'd say, oh, I need one kind of dark energy to explain the accelerating expansion of the universe.

But a different kind to explain how structures are growing in the universe.

- By structures you mean galaxies?

Galaxies- - Correct.

Correct.

Right.

So that's gravity on a different scale.

- Right.

- And so right now we are doing a lot of new experiments with new facilities.

You know, the new Rubin telescope, the new NASA Roman telescope due to launch next year, Euclid, this new DESI experiment.

- So that's the DESI result.

So the DESI result shows that dark energy maybe have been observed to be changing with time.

- Right.

- How big are those error bars?

- Yes, so that was a great surprise to people when this first came out now almost two years ago, that all the experiments up till now had been kind of zeroing in on, well, it looks pretty static.

It looks kind of like the cosmological constant.

And then DESI, which was the best experiment up till that time, was like, "Wait, not so fast.

"We see evidence that it has changed the dark energy "over the last several billion years."

And if that is true, that would be the biggest clue we have about the nature of dark energy, I would say, since it was discovered.

- Yeah.

Yeah.

The expansion rate, or dark energy becoming weaker.

Right?

Is that what it it suggests?

Is dark energy effective weakening?

- That is a face value interpretation of it.

I think another interpretation of it is also that our best cosmological observations don't all fit together, that there's some conflict between them.

And so it's unclear whether the conflict you are solving it correctly by allowing dark energy to vary, or if it's that model is breaking in some other way.

- And what does DESI stand for?

Dark energy survey?

- Spectroscopic.

- Spectroscopic.

- Instrument.

- Instrument.

Oh.

- Yeah, yeah.

Yeah, so it was very interesting when it came out, DESI is the best measurement of a feature in the distribution of a galaxy, it's called the baryon acoustic oscillation, which is kind of a standard measuring length.

So we talked earlier about- - Oh, is that what DESI used?

- Yeah, yeah.

So we talked earlier about using the brightness of a supernova to measure distances.

You could also, in principle, if you look far away and you see a big object looking tiny, right, that tells you how far away it is.

Like, I look out on the highway and I see little bitty cars and I know those aren't really little bitty cars.

- Right.

- They're really regular cars far away.

- Right.

- And so, likewise, DESI looks at this feature called baryon acoustic oscillation, which was supposed to be originally 150 megaparces across, but it looks itty bitty tiny.

And they can use it to measure how far away it is.

So we use baryon acoustic oscillation.

We use supernovae.

We use the radiation left over from the cosmic microwave background.

We bring them all together in the context of something called Lambda CDM, which is our story.

It's our standard model.

It's everything we know that how it fits, all the physics and the inventory of the universe, and we apply it to this newest data, and it doesn't all fit.

- It doesn't all fit together nicely.

When we look at these different cosmological techniques, okay, you have the cosmic microwave background radiation measurements, which are sampling the universe 13 and a half billion years ago.

You have your supernova cosmology measurements, which are sampling the universe over some numbers of hundreds of millions of years up to today.

Then you have your baryon acoustic oscillation measurements, which are probably over a similar time scale, but I would guess half bigger error bars.

- Yep.

- And three of these are giving you different answers.

So the question is, two parts, how do you go from data to physical model?

And when these physical models disagree 'cause, you know, we're gonna get to this other thing called the Hubble tension in a second.

- Yeah.

And that's real disagreement.

- Yeah, so how do you go from measurement to physical model theory and theory, and how do you, when they disagree, how do you resolve these disagreements?

- Right.

I mean, first of all, this is wonderful because this is the process of science, is you say, I look at a lot of phenomenon.

I understand some things.

I develop an understanding based on what I have seen.

And then I use that understanding, which we will call a model, okay, to predict things I haven't yet seen.

And I will test whether my understanding is right.

And a, you know, good, robust, closer to correct model has power beyond where it was learned.

It has power to predict the future and other experiments, okay?

But sometimes the things don't, the theory doesn't match the experiment.

I think they said in the movie "Oppenheimer" over and over, right?

Theory will only get you so far.

- Right.

You know what, man, I've not been able to get into "Oppenheimer."

I've started it like three times and I've not been able to- - Push on, push on.

- Push on.

Okay.

- But anyway, and so this is our process.

It's the science process.

And sometimes the data doesn't match the theory.

And if it's good reproducible data, then you have to revisit the theory.

- So here's the question.

Is it just fitting the equation to the data?

Is that how you get to the theoretical?

- Pretty much.

Pretty much.

I mean, you know, you start with this model today called Lambda CDM, which is the model that really after that late 1990s work that we talked about became the standard model.

And it literally says, Lambda CDM literally says there's dark energy.

That's what the Lambda stands for.

There's cold dark matter.

That's what the CDM stands for.

That's a lot of the stuff.

But there are more elements in there.

Usually people are saying the universe is flat- - Yeah.

- Geometrically.

So that's sort of in there.

They're saying how many neutrinos there are and what their properties may be.

There are a number of ansatz in it.

So it's really a package, which is really like the, it's the physics of the universe and it's the inventory of the universe.

And then given that, you could predict the outcome of an experiment, okay, like DESI.

- Yeah.

- Or of multiple experiments.

And then generally these theories, these models actually have free parameters in them in the sense that you say it's still the same model, but there could have been more matter and less dark energy, or vice versa.

How much do we have?

And so you actually allow the model to best fit the data, okay, to learn those parameters.

Now, that might work out well, or you might find, hey, when I compare the model to this data, I get a different set of parameters than when I compare it to this other one.

That means they're not agreeing very well.

Or you might even say, I can't get this model in any form to fit.

I have to add some new physics.

And so most recently with these new results from DESI was combining DESI, cosmic microwave background, and supernovae, and saying, you know, the model fits great any one of these data sets, but once I start to require it to fit two or three at the same time, it's starting to fail.

Now why is that?

These experiments are probing different points in the history of the universe.

And so just like if you, I don't know, draw some complex curve, you know, on a piece of paper, I could fit that curve with a line over little portions of it, right?

But if I try to make that line work for the whole thing, now I'm gonna start to see some discrepancies.

And so that's kind of a perspective of when things break down, it may be that there's more wrinkles to the model that's not traveling the straight line.

The story is more complicated, you know?

- You know, one of the most amazing results I've ever seen come from our friend who just passed away, George Smoot, and JWST PI, who was also on COBE, what's his name?

- John Mather.

- John Mather.

Right.

Smoot and Mather, when they showed their COBE black body curve, and they had the error bars on the line that was the... You know, so the data was dots with error bars.

And the theory was the black body curve.

And then you read, it said, oh, the error bars have been multiplied by 400 so that they are visible.

- Just you could see them.

Yes, I know.

I know.

Right, that is actually the best, as I understand it, the best measure of the energy coming from a hot object that exists in nature- - Yeah.

- That's ever been measured.

And it's actually of the Big Bang.

- Yeah.

So here's what I'm getting, here's where I'm going.

When you made this measurement in '98, there were 42 supernovas.

The error bars were visible.

Now, you know, a quarter century on, how many more data points do you have and how small have the error bars gotten?

And when you compare it to these other observations like baryon acoustic oscillation and CMB.

I know CMB has smaller bars.

- Sure, sure.

Yeah, so compared to, you know, in the 1990s, we had dozens of supernovae, and now we have thousands.

And Rubin, this new facility, is just about to turn that thousands to tens, hundreds, thousands, and millions, okay?

- Wow.

- So we're even now still just taking off.

But to answer your question, what was your question?

- So the question is, we're comparing these three different- - Yes.

- Data sets, but I'm assuming they're not all of similar quality.

- That's right.

But what's I'll say ingenious is that as long as you can state what that quality is, then you could still use it to test- - Weight.

- Yeah, you weigh it correctly.

So we're factoring that in, I would say.

Now that isn't to say you'll meet some cosmologists who will say, "Well, I prefer this data, "and I'm less confident about that data."

But, you know, I would say in terms of the actual quality of the data, like do I have a lot or a little, we're pretty good at understanding how that propagates into our knowledge about the model.

- Okay.

So in terms of certainty, I would love to hear you say three statements (laughs) clearly, because I think that, you know, the average person who may be listening to this, and I've heard people say this in casual conversation, that think that, oh, this Big Bang model is just a model.

We don't know that that's reality.

Dark energy.

How certain are the existences and the happenings of the universe expanding, the Big Bang happening, dark energy existing, dark matter existing.

- Right.

So the important thing, and I sort of teach this in my Astro 101 course, is to distinguish between the observation and the conclusion or explanation of the observation.

So for example, in our case, we saw that the universe is not just expanding but accelerating, okay?

That's closer to the reality of the situation.

It's what we actually see.

Okay?

And it's also a statement about an action that the universe is doing.

The interpretation then becomes, well, what would do that?

Well, if Einstein's theory of gravity is true, then it requires this term to be active.

And that term represents the background energy, so we'll call that dark energy.

But I've now walked down a road where I'm using theory now to explain, and I'm not maybe completely comfortable with all the theory.

Right, we know that general relativity doesn't work or is not compatible with quantum theory.

So you have to keep in the back of your mind if this is the right theory.

- So now let's get into the Hubble tension.

- Ah!

- The Hubble tension.

Now, when I think about the Hubble tension, you know, sometimes, man, you know, I'm gonna be honest, I love me some nerds.

- Yeah.

- But I don't always vibe with the nerds.

And why do I not vibe with the nerds?

'Cause sometimes I feel like they're making too much out of a topic.

And so the classic example to me is whether or not black holes have hair.

Right?

I'm just like, is it really that deep?

You know?

- Right.

Right.

- So the Hubble tension, where does that fall?

- Much bigger deal to me.

- Much bigger deal.

- Well, okay, so going back to this idea- - Define it.

What is it?

- Yeah, yeah, yeah.

Well, so what is the Hubble tension?

Okay.

So we reach a point today where we say, we kind of think we have a pretty good model of the universe, but, you know, has big areas of ignorance.

95, 96% is in the form of dark matter and dark energy that we have kind of cartoon-like explanations for.

But that's fine, okay?

And then we say, all right, let's really test this model.

Let's see if it's really right, right?

So what I've been calling the best end-to-end test of the model, right, you do an end-to-end test when you really want to know does my thing, you know, operate as I expect.

The best end-to-end test of the universe is to look at the cosmic microwave background, which tells you the state of the universe shortly after the Big Bang, okay?

And it allows you to predict how fast the universes should be expanding today.

It's like if you had a kid who was two years old, you could predict what height they will grow to based on growth charts and your understanding of human physiology, okay?

But the end-to-end test of that story is to actually measure today how fast is the universe expanding?

A number called the Hubble constant.

That would be like measuring that kid's height when they are fully grown.

- Right.

- If you really understand things, the two will match within the error bars, within the uncertainties.

And so over the last decade, we've seen this mismatch growing and growing in significance.

First it was one or two times the error bar is away.

Then it was three, then it was four.

- Really?

- Then it passed five.

Now in physics, five's considered the kind of gold standard for like going from, don't bother me with that to like, this doesn't make sense.

- Yeah.

Yeah.

- And now we're probably up to six or six and a half.

- Wow.

- And the reason that it has grown is because the data is getting so much better.

We've had the Hubble Space Telescope, now we have the James Webb Space Telescope.

Previously we had crude parallaxes.

Now we have the ESA, European Space Agency Gaia mission measuring parallaxes.

You know, the cosmic microwave background data has gotten better.

First it was ground-based, then it was WMAP, then it was Planck.

And now it's also these ground-based experiments with high resolution like ACT and SPT.

- Yeah.

- And then we have many techniques for making these measurements.

So when we measure the Hubble constant locally, we build what is called a distance ladder.

- Right.

So speaking of that, I just want to define a term.

You mentioned parallax, which is a geometrical way of determining distances, and it works best for nearby objects, but you can cross calibrate more distant objects where they overlap.

- Correct.

- Something of that nature.

- Yeah, yeah, yeah.

And that process, we call the distance ladder.

I mean, ideally you would look at some distant galaxy and you would measure its distance from us geometrically by looking at parallax.

The parallax is when the Earth goes around the sun, and your perspective on a nearby object changes with respect to something distant, you form a triangle in space, and you can measure how far away it is.

The problem is things are far away.

They're really far away.

So that shift in position becomes imperceptibly small.

- Yeah.

- And so, like you said, you can only measure it for stars in the Milky Way.

But you calibrate the luminosity of a certain kind of star called the Cepheid variable, whose period, the rate at which it pulsates correlates tightly to its luminosity.

And so then you see one, you calibrate one that has a 20 day period with parallax, then you look in a supernova host galaxy and find more 20 day Cepheids, and you go, "Ah, it's the same kind of object just further away."

Now I know how far away it is.

Now I've calibrated the luminosity of the supernova.

And this is called a distance ladder.

- You build it up from nearby objects that you get a very precise distance to and move further and farther.

- So at the turn of the last millennium astronomers use this technique to get the Hubble constant to about 10%.

And that was great.

But over the last 20 years, we have been improving that, now approaching about 1%.

- Oh, wow.

- And ever since we got to about 5%, we started seeing this tension of- - So different techniques give you different answers?

- Well, really that the cosmic microwave background route, starting from the early universe and using the model gives you a lower Hubble constant than when you measure locally around us with many different techniques.

So that, you know, more and more it starts to look to people like the problem isn't in the cosmic microwave background measurements, they've been duplicated and replicated.

The problem isn't with the local measurements, they've been duplicated and replicated.

The problem might be with the story we tell ourselves that connects the two, this Lambda CDM model.

Maybe there's something else going on that we haven't yet understood.

Maybe dark matter and dark energy are more complicated.

Maybe there's been more episodes of dark energy than inflation at the beginning and dark energy at the end.

Maybe there's been a in between dark energy.

And so these are the things that people are thinking about because otherwise we don't know how to explain what we're seeing.

- So the cosmic microwave background radiation comes from 13 and a half billion years ago.

- Correct.

- This local measurement that you're making with the- - Comes from now, essentially.

- Now essentially?

- Close.

Close.

I mean, you know, redshift 0.5.

- Maybe half.

- No, not even.

No.

- Not even?

- No, I would say maybe 200 million years back.

- Oh, wow.

- Yeah, so it's really now.

- That's a big gap.

- Yeah.

Yeah.

- Oh boy.

- Yeah.

- Okay, so what's the solution to filling in that gap?

- Well, what you would like- - The observational solution.

- Yeah, well you would like is you would like to be able to measure something in between- - Yeah.

- Where you had an absolute knowledge of distance.

And so this is always the big rub in our field is I wanna start out with something that's absolute, that it's like running a tape measure out and going, "This many inches."

- Yeah.

- But instead we get lots of these standard canals or standard rods or things where we go, "Well, it's uniform at least.

"So if I see it here and I see it there, "I could tell how much further away it is."

And so this clash really comes down to the clash between parallax, which is the geometric sort of starting point for distances nearby and at the other end is physicists' theoretical understanding of something called the sound horizon, which is, it's like, it's the distance that a fluctuation in the early universe can travel from the moment of the Big Bang until the universe becomes transparent a few hundred thousand years after the Big Bang.

- Yeah.

- And so this is like a standard ring for them.

They could calculate it from first principles.

And so each of us are starting with these absolute references at opposite ends.

It's like, you know, those famous, that famous meter stick in France that was kept in a refrigerator.

You know, it's like, this is the meter.

- Right, right.

- So we each have our, this is the meter.

- Right.

- And then we use tools to try to bring them closer together.

But when we hold them up next to each other with these other tools, they're not agreeing.

- Oh geez.

That's tough.

So- - No, it's great actually.

- Well, it's great from your perspective.

- It's great, yeah, because this is how we learn things in science.

It's the opportunity that we get.

I mean, 1998, things didn't fit either.

- Right.

Yeah, yeah.

- They didn't fit the conception at the time.

So to me, this is what makes science so much fun.

- So there's a discovery waiting in resolving this tension?

- I think so.

- Potentially.

- I think so.

- Yeah, yeah, yeah.

So, man, that is amazing that it's only 200 million years of really solid acceleration of, excuse me, expansion rate of the universe data.

That's mind blowing to me.

I didn't... Yeah, yeah, I thought that if you have a Hubble diagram- - Well, you don't want to go too far back because then you actually do start- - Oh 'cause it's changing.

- It's changing.

And you would have to account for that, and it would become, you know, that would go more on the model than the measurements.

- So speaking of changing, do you have enough data now to measure the expansion rate of the universe as a function of direction to see if it actually is uniformly expanding?

- Right.

That's a good question.

So I would say not to probably the satisfaction of everybody.

- Okay.

- So, you know, once you take these thousands of supernovae and you distribute them around the sky, and now you want to make a map of how fast is the universe expanding this way, this way, this way, this way, right?

- Yeah.

- Previously, we assume it's probably the same in all directions.

- Right.

- Theory sort of says, well, what we call the cosmological principle- - Yeah, yeah.

- Which says everywhere should be the same- - Yeah.

- Says it should.

But if you really want to test that- - Yeah, exactly.

- Then you're gonna need a lot more supernovae.

And so things like this Rubin Observatory that will collect a million will allow us to slice and dice the data in different directions.

- Okay.

That's good.

So it's sort of like, you know, when you say, well, theory says.

Theory also said it should be decelerating.

- Absolutely.

So what we need to do in all times is question our basic precepts of theory, you know?

- Right.

- That you go, great, that's a great idea.

I will trust, but I will verify.

- Right.

- Because again, you have to remind yourself, we're in a state right now where there's a lot of ignorance in terms of some big pieces in the cosmological model.

So if everything was understood, if this was like, you know, Maxwell's equations or Newton's laws or whatever, you'd go, I don't need to roll every ball down every incline plane.

I'm convinced.

- Right, right, right.

- But when you're dealing with the universe where you, you know, lots of times we've done a new experiment and we discovered something else is out there, you always have to be concerned, do I understand that thing well enough?

No, I need to do a new experiment.

Am I sure that this is the unique model of the universe?

Could there be another story?

You know, the Greeks held onto the, you know, Earth centric model long after they should because they kept inventing other elements to add onto the model, epicycles and- - Right, right.

- And stuff.

And, you know, at some point you have to say, "This isn't right."

- Yeah.

- You know, and hopefully you'll come up with a new model.

So you just, I'm only saying, you have to be open-minded, and the best thing to do is do the experiments, which have not been done before.

- Right.

- Especially the ones where the model tells you what you should see, and then see what you do see, because, you know, in the hierarchy of truth, I think experiment always has to sit above theory.

- You know, this reminds me of my PhD advisor.

So he pioneered the telescopes that take pictures of the sun's hot atmosphere.

Corona EV.

Solar dynamics observatory.

And when he got his first images, the coronal loops on the sun, right, these loops of plasma.

- Sure.

- The theory according to the theorists was that, well, if you look at a magnetic field, as you move away from the magnet, the field puffs out.

It spreads out.

But these loops in your images are constant cross section.

They're not spreading out.

So there's something wrong with your optics.

- Right.

- Yeah.

And he was like, "Well, I'll show you."

So his first flight had three telescopes.

His next three had 16 to 22.

He's like, "I'm gonna image the same wavelength of light-" - Right.

- With four different- - Right.

They can't all be wrong.

- They can't all be wrong, right?

Then finally, people accept it, like, yeah, they really are that.

- And that's the right thing to do.

I mean, you know, in this constant tug between experiment and theory, right, I mean, data can be wrong and theory can be wrong.

So at least for data, I know how to handle that problem, which is, it needs to be reproducible, it needs to be done many ways.

It needs to be... And if you replicate it enough, right, at some point, your conviction is strong.

- You have corroboration.

- This is absolutely a fact about the universe.

But at that point then, you know, you start to point the finger at theory and you go, "How good is this theory?"

And by good, I mean, is everything derivable from first principles or are there leaps in there where we say, and then there's some stuff here and it'll have these properties, but eh, we can't really say at a microscopic level what it is.

I mean, I don't think there are any physicists in the world who are satisfied with our explanations of dark energy and dark matter.

- Which is we have no idea what they are.

That's the explanation I know.

- Right.

Right.

So, you know, that's why it puts you in a realm of where experiments, even things where you think you know the answer, are important to do.

- Yeah, yeah, yeah, yeah.

Absolutely.

So what's next, man?

So you have 25 years of observations.

You have Rubin coming on, which is gonna give you these millions of supernovae.

And the data is, am I correct?

It is open access.

Anybody gets the data in real time.

- Right.

Right.

- So is there like another competition you're a part of, or are you all kinda coming together?

- No, I think, you know, well, I mean, you know, it's, I mean, I would say it's less competition because simply you don't have to compete for time because it's very democratic.

The data will be available to everybody.

I think we're gonna be drinking from a fire hose for a while.

You know, so much data coming in that in the beginning we're gonna get limited by the rate of computer processing.

It's just, you know, that much information.

So, you know, it'll be interesting to see how much we learn, not in the first 10 minutes when we get going, but, you know, within a couple years when Moore's law catches up a little bit.

I remember the old days, we used to get a telegram, you know, the astronomical telegram, you know?

A supernova has appeared in this galaxy.

We don't know what kind it is.

And we're like, one at a time, you'd go, who's getting the spectrum tonight?

And we'd talk about it in the morning.

What does it look like?

And now like, who has time for that?

We have to be doing like hundreds at a time.

Thousands.

- Hey, everyone, if you're loving this podcast, please go ahead and like us or leave a comment and also make sure to subscribe, so you never miss an episode.

Your support means everything and helps us reach more curious minds like yours.

Now back to the show.

Let's talk about winning a Nobel Prize.

Now I'm gonna tell you the one story, so you know how it is.

I tell people this all the time.

By being an astrophysicist, you know Nobel Prize winners.

I've known several.

And, you know, it's like if you're in the NBA, you know LeBron right?

You could be a bench warmer, end of the bench on the worst team, you still know LeBron, right?

So I remember when I was in graduate school, there were four Nobel Prize winners in our department in a row year after year.

And one of 'em was Doug Osheroff, who is a buddy.

And when I told, you know, I remember waiting for everything to die down.

And when everything died down, I'm like, now I'm gonna approach you and give you your congrats.

Doug said, "It was about damn time.

"I did the work 22 years ago."

(both laugh) How did you feel when you, like were you expecting the call that year?

Like, you know, you had to know it was coming.

Everybody did, I feel.

I felt like I knew it was coming.

How did you feel when the call came?

Were you expecting it and- - Right.

- Yeah.

- I mean, it was, you know, up until just a few years before it, so we did the work in 1998.

And we won it in 2011.

But there was a lot of confirmatory results and work that occurred over, I would say, the next 10 years after the discovery.

- Yeah.

- But it's true by the, you know, late aughts, you know, I was getting a lot of, you know, statements from people, you know, in the news, whatever.

You know, most likely to win on lists.

You know, this is so embarrassing to tell you this, but your university PR department comes to you years before and requires that you sit down with them and develop a press release and you go- - Before?

- This is ridiculous.

Why are we doing this?

They go, "Well, what if you win the Nobel Prize "and then we're not prepared of what to say?"

And I'll be like, that would be a good problem to have, just figure out something.

And they go, "No, that's not the way we work."

So you're like, you're really commenting on this thing, and you're like, this doesn't even feel right, like looking at something.

But anyway, so there's a lot of kind of hoopla.

And there are some of these other prizes that are coming in that are kind of- - Like the Copley.

- Right, right.

But the reality is you have no idea because the reality is there's no official shortlist.

I mean, there is in the Royal Swedish Academy, but not anything that anybody is privy to.

So everything is just speculation.

And look, to be honest, some people wait 30 years.

- Yeah.

- And some people you say, "Why didn't they get a Nobel Prize?"

Why didn't Vera Rubin get a Nobel Prize?

- Exactly.

- Why didn't Hubble?

Well, he died early, but, you know.

- Well, you know, what I thought- - So you really don't know.

- What I thought about in your case and Vera Rubin case, and I think that the Nobel Prize Committee is sort of like coming to grips with this, is that it is not a Nobel Prize in astronomy, it's in physics.

Astronomy is physics.

It's astrophysics.

That distinction doesn't exist the way it used to.

- That's right.

That's right.

- So, you know, this is about nature.

- Right.

Right.

I think in our case, because what we were learning about was new physics in the universe, particularly we were learning that there's something that accelerates the expansion.

So something like dark energy.

So, you know, it's not like finding another moon around Jupiter or something that you'd go, that's cool, but it doesn't change our understanding of not astronomy, but physics.

And so in this case, I think it was that that made it, you know, of interest to them.

But yeah, physics is a broad tent.

It covers a lot of different things.

I mean, we've even seen things like what, AI get, you know, a Nobel Prize.

- In physics?

- Yeah.

- Oh, I didn't realize that.

- Am I having that right?

Yeah.

Cut that if I'm wrong.

But that's what I remember.

Yeah.

- I remember the giant magneto resistance, which led to like tiny- - Right.

And sometimes you read the things you're like, blue LEDs really?

I had no idea.

And then other times you're like, it's about time that that thing, you know.

Gravitational waves is one of my favorite.

- Oh yeah.

Tell me this.

Or as they say, answer me this.

(laughs) If dark matter had not been discovered first, what would we call dark energy?

'Cause I think the only reason we call it dark energy is because of dark matter preceded it.

What would it have... What would it have been called?

- I don't know.

The accelerant.

(Hakeem laughs) - The accelerant.

- Or, you know, my colleague Sean Carroll says we should call it smooth tension because he thinks that's closer to the physical description of what the stuff is that accelerates the universe.

It's kind of a smooth tension in space.

- Right.

- But anyway, I don't know.

This is our tradition is we call things dark when we don't see them directly.

- Right, right.

- But, you know, nobody won the Nobel Prize for dark matter.

- Right.

- And, you know, a lot of these things come down to, for better or for worse, you know, can you isolate the work?

- Yeah.

- When it was done, who did it?

And then the peculiar rules of the Nobel that it can only be three people.

- Right.

- And so, you know, some things just don't conform to that, and of course, you want to be very sure about it too.

If you put yourself on the Nobel Prize Committee, right, you're like, first do no harm.

Don't pick something that's wrong or silly.

We don't want to embarrass the prize.

- Well, that's the Ig Nobels.

- Right.

That's right.

That's right.

So anyway, all I can say is there's a bunch of factors that must come together.

- You were there at the ceremony.

- Yeah.

- Were you like the youngest dude there?

- I was.

I was.

Yeah, yeah.

So I was 41, which, you know, in the modern era is very, very young.

You know, back in the 1920s- - Oh boy.

- You know, people were winning at like 25 or 30 years old.

You're like, oh my god.

But, you know, they didn't know that much then either.

- Right.

- So you could get to the top of your field pretty quickly.

- Yeah.

- And it was a smaller world then.

- That's right.

- And so now I think I've read that the average time between the work and the actual prize is about 30 years.

- Wow.

- And the average age that people win it is around 70.

- Wow.

- So winning it at 41 was, it wasn't the youngest ever, but it was one of the.

- So here's a question for you.

After all the hoopla died down and you could get back to some sense of normalcy, have you noticed a permanent change?

Like, do you get like the best seats at the restaurant now?

(laughs) - There's definitely a lot of permanent change.

There's no question about it.

You know, and it goes in all kinds of directions.

You know, the best to me is that I want to keep doing science.

And people who treat me, react to me, or work with me like I'm a scientist, like I always was, you know, science doesn't, you know, have, you know, shouldn't have a hierarchy of person.

- Well, I would imagine that the people you work with directly, it's about the same every day.

- Correct.

Correct.

- But outside of that.

- So outside of that, you know, I get kind of, I would say a lot of, to be honest, non-linear responses.

You know, I get anything from the people who are sort of adoring because of the glow of the Nobel Prize.

- Yeah.

- And then other people maybe who get some sort of chip on their shoulder about it.

But, you know, sometimes you feel like people aren't really reacting to you.

- Right.

- They're reacting to this thing that you represent.

- Right, right.

- And I think that just comes with the territory.

- So one thing that all of us professors have to deal with are these emails from just random people.

I have figured out the- - However many you get, I get 1,000 times more.

- That's exactly where I was going with that.

I can't imagine.

- And they're growing with time.

Have you noticed this too?

Oh my gosh, I get multiple daily.

I have a theory about the universe.

And I'll tell you, I think AI is only making it easier because it allows people to take their crazy idea, run it through ChatGPT, and make it look almost respectable.

Like, almost like, you know, it smooths the rough edges.

And so that you go, wait, is this?

It beds it in a legit looking paper.

- Right.

- And then you have to look through and go, wait a second, there's nothing here.

- Right.

Yeah.

- And so, yeah, it's proliferating.

And I would say to people who are listening to this, is that when you have a great idea, and people can have great ideas, you know- - Absolutely.

- You should send them to a journal.

- Right.

- And not to individual people.

Right, we are not the gatekeepers.

You know this.

- That's right.

- We are not the gatekeepers.

You could give a talk somewhere.

You could submit a paper.

But coming up to people like us, and we are not the gatekeepers that are barring or accepting ideas.

- Right.

That's not how it works.

- It's not how it works.

So it's not helpful to them either.

- So one of the thing that happens, especially if you have success in a career as a scientist, people pull you away from your science.

- Correct.

- And they put you in leadership.

- That's right.

That's right.

- And so, one of the things that I do admire about you that I didn't understand.

I'm just like, how has he avoided that?

You've continued to be a scientist.

- Yeah.

I've learned there's a two letter word that you have to say a lot, which is no.

- But then it comes with, you know, quadrupling your salary.

- I know.

I know.

So, you know, you have to, if you love science, which I do, and, you know, you're 41 when you win the Nobel Prize- - Yeah.

- So that, you know, people understand, you know, if you stop doing research to do some high administration at a university or something, you're not going back.

- It's hard.

- It's very hard.

So it's kind of a one-way valve.

- Yeah.

I've done it a couple times.

- Yeah.

Yeah.

But you know it's hard.

- It's so hard.

- So hard.

So hard.

- Like, in my case, it takes like a 18 to two year, 18 month to two year just focus to get in this one problem.

- Right.

That's right.

- You know, to publish something.

- And just the reality is most colleagues I have, you know, once they've gone on that direction, they have not come back at all.

- Right.

Most don't.

Yeah.

Very rare.

- So I knew that if I wanted to keep doing science with all these requests, I would have to really be pretty strict with saying, "Look, I like doing science.

"You know, I'm better at that "than I am at running your university or your department "or, you know, your foundation or whatever that is.

"And so, you know, it's really better for everybody "if I keep doing science."

- And, you know, what's really awesome being a leader of a scientific research group is that everybody that works for you tends to be highly motivated and very, you know, their mind state is about being productive and constructive.

You don't deal with as many, 'cause, you know, I've been presidents of organizations.

- Sure.

- And I've had these roles where you're dealing with humans and their conflicts.

- And you spend most of your time on the very small percent of people with conflicts.

And that's fine.

But as a scientist, it's not my skillset.

- And it's not satisfactory.

- Right.

- You don't get the same satisfaction.

- So I think, you know, I mean, to be honest, I think for a lot of people, I understand that the ego of being asked to lead an organization or something is very appealing.

But, you know, you have to ask yourself, you know, are they asking me because I'm really good at that or because I have a Nobel Prize?

- Right.

- And if it's the latter, you know, then you have to remember, oh, but what I'm good at is this science.

- Yeah.

Yeah.

That's a tough thing, you know, when people... For lack of a better phrase, we all want to be loved.

We all want the positive feedback from our fellow human beings.

But there's a difference between people actually know you, people actually respect you for the work that you do.

- Sure.

- And, you know, one of my, you know, I'll tell you this.

This is gonna sound crazy.

Every time I tell it to a scientist they're like, "Hakeem, you're outta your mind."

I always tell them like, you know what, I don't even care to publish.

Let someone else publish my work because I just love doing it.

I don't really, you know, none of, and I feel the same way when I teach.

I feel like, you know, it's really a shame that I have to give grades.

I wanna give you assessments.

I want you to get better.

Right?

But the fact that I have to give you this grade at the end that follows you throughout life.

You know, I don't, you know, why are you putting me in that position?

I just want to have the teacher student feed your mind interaction, right?

So you've been able, so how big is your group?

Do you have lots of mentees, postdocs?

- I mean, you know, I also run a kind of a, I would say lean and mean group because I'm a very hands-on, I'm a data guy.

So I like to look at data, not just my data, whatever the student is working on their data and, you know- - So you get in there with them.

- Yeah.

Yeah.

'Cause, you know, ultimately having been through this experience, right, you're only willing to publish things that you think are right.

- Absolutely.

- And how do you know something is right?

And the the answer is, 'cause many people check it, and I need to be one of those people too.

- Right, right, right.

- And so I run a group that's small enough so that we can all sort of work together and check each other's thing.

So, you know, a couple of postdocs, a few graduate students, that's about it.

- You keep it like that.

- Yeah, yeah, yeah.

- That's pretty good.

- And also if your group becomes too big, you spend all your time fundraising and then you're not doing the science.

You're not fun raising, you're fundraising.

- Yeah.

(laughs) But you know, that's that other email sets you get, not just the people who figured out the universe, but all the people who want to be your mentee, right?

- Yeah.

Of course.

Yeah.

- It must be tough.

- Yeah.

Yeah, but you just have to be disciplined about it.

- You gotta be disciplined about it.

- You just have to be honest with yourself.

You know, what is it that I like?

What is it I'm good at?

And what makes the hours fly by on the clock?

You know, like, you know, is it filling in forms?

No.

- Oh, heck no.

- You know?

Is it like really thinking about a problem?

I mean, the best time I ever have is when I have, you know, a difficult chewy math science problem and I carry it with me throughout the day.

And then like, you have a eureka.

You're like, "Wait!

"I could do, I could try this."

I mean, it's so exciting.

- Oh, absolutely.

So tell me this, man, do you ever see yourself retiring?

- I don't know what that would look like because it feels like I'm playing most of the time and having fun.

So, you know, how do you retire from something that's stimulating your brain, you're having fun doing.

- Yeah.

- You know, you might do some things in a different way, but, you know, for the most part, why would you stop that?

- Right, exactly.

That's how I've always felt about myself.

Like, I can't even see it.

I remember our buddy Gerson Goldhaber, I remember chatting with him once at Berkeley, and I mentioned my father's age.

And he goes, "Oh, he's quite a young lad."

And I'm going, "How old are you, Gerson?"

And he said some, you know, some crazy super- - 88 or something.

- Yeah, exactly.

I was like, wow.

You know?

And I have to think that keeping your mind active in that way helps you as a human being.

- For sure.

And being truly interested in what you're finding, in what you're searching.

And to be able to see over the decades, wow, when we got started on this, we didn't know hardly anything and look how much progress we've made.

That's a very compelling sort of endeavor as well.

So, you know, for anybody, I tell them, you know, find the thing that you get in a kind of flow state about it.

For me it's been science.

- All right, so here's gonna be my last question for you, man.

You're about to get all this data.

You say that's what you're looking toward.

Is it gonna resolve the Hubble tension?

- I hope so.

You know- - That was non-committal.

- I mean, you know, I would have to have a crystal ball to know.

- Right.

- But, you know, if you tell me I have a certain amount of data and it's showing me this kind of puzzle, and then you go, but what if you had 100 times as much data?

- With higher precision.

- It would be surprising that that didn't have a pretty positive impact on the problem.

So I can't guarantee it.

You know, it's always a question of where does nature hide her treasures or her puzzles, right?

It's like, I always look at it like, if you ever as a kid dug in your backyard, right?

And you're like maybe six inches deep and you found a cool rock.

Maybe you went like two feet deep and you found something even cooler, right?

And like what's out there in your backyard?

How deep do you have to go?

And so, you know, these experiments are like giant excavators, but are they deep enough, you know?

And so I'm excited about it, but, you know, then the ultimate question is, and how deep was the answer?

And I don't know.

- Man, if you ever need a postdoc, I'll clear my schedule.

- All right, sounds great, Hakeem.

- Adam, this is awesome, man.

I've waited decades for this conversation.

I've been in a room with you many times.

Never had a time to chat 'cause I was too shy.

But thank you, man.

- Oh my pleasure.

- You turned out to be such a kind, warm, brilliant guy.

I really appreciate you coming.

- I enjoyed it thoroughly.

Thank you.

- Thank you.

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