Transcript

Brian Keating:
Some of the strongest evidence that the universe is accelerating doesn’t come from one telescope or a single experiment. It comes from a tiny ripple frozen into how galaxies cluster across the cosmos. Today, from the Royal Observatory in Edinburgh, we’re following that ripple with cosmologist Marcos Palheiro to see what it really says about dark energy. I’m Brian Keating, and this is an exclusive tour of the Royal Observatory Edinburgh with cosmologist Marcos Palheiro. We’ll go from this historic telescope, to cutting-edge simulations, to the DESI experiment, one of the most ambitious galaxy surveys ever built, to ask a simple question: is dark energy really constant, or is our entire cosmological model starting to crack? Long before silicon chips, the computers up here weren’t machines, they were people. Often they were women hired to comb through photographic plates measuring every faint smudge of light by hand. Their names rarely made it into papers, but their measurements are literally baked into the datasets we still build our modern cosmological models on. This building was designed as a cathedral for starlight.

Brian Keating:
The telescope sits on a massive pier that sinks into the hill, isolated from the floor so footsteps don’t shake the images. As our cities grew brighter, places like this became less useful for frontline observing, but the engineering mindset behind them is still the same one we still use today to measure the universe’s properties.

Marcos Pellejero:
This is a picture of the family of the Royal Astronomer before this place had no house anymore. Okay. For, for them. Um, uh, good. Uh, yep. And, and, and this is basically like the idea of like in the old times you will have looked through a telescope like this one, but nowadays, uh, in the, here in the lab, they are building things like this, like these robotic arms to basically place fibers. And get some of the light and decompose it and study.

Brian Keating:
This is not far from the Simons Observatory. That’s in the northeast.

Marcos Pellejero:
Okay. And just one more question. Sorry, I know that you have been here for quite a long time. Do you see any weird wall in this room? Yes. Which one? Why do you think it’s weird? There are two reasons.

Brian Keating:
It has a picture.

Marcos Pellejero:
Well, it has a picture. Yes, this one is weird, but this is a door, right? This is not a wall. It’s made out of bricks. Yes, exactly. So welcome to the dome. So do you see something weird in this dome with respect to other domes that you might have seen? It’s not a dome exactly. It’s like a cylinder. And this relates to what I was saying before, that they were not trying to do a functional building.

Marcos Pellejero:
They were trying to do a beautiful building, right? And then they were thinking on building something that was like a cathedral for science. Okay. So the idea, and a cathedral needs towers, right? So this is again, like, this is quite old. And when I was telling you what will we find at the end of that weird wall, the reason is this thing. So this square here goes all the way down and into the hill. And it is separated from the rest of the building because you need to do very precise observations. And if this is connected to the rest of the building, then if the building moves, this moves. And you want to avoid that.

Marcos Pellejero:
So basically what you do is you create a pyramid that goes, that takes its, puts its roots to the, like, deep into the mountain, and it moves at the least you can, right? This specific telescope is from, was built in Newcastle in 19, it’s written here, in 1928. Okay, so it’s not as old as the building. This would not be the first telescope that was here, but it’s quite old.

Brian Keating:
And what’s the diameter, Marco?

Marcos Pellejero:
So this was, I think this is a 40 centimeters one. This is the primary mirror. 40 centimeters, I don’t know in inches. I have no idea.

Brian Keating:
From that, it’s less than, say, 18 inches?

Marcos Pellejero:
18 inches, yeah. Okay, that’s good. If you say so. So actually, so I mean, I guess you know quite a lot about telescopes already. The primary mirror is not here anymore. Okay. The secondary mirror, which is up there, you can still see it. That’s there.

Marcos Pellejero:
And the detector is completely missing, right? This is empty. Now, what’s the reason for this? Well, it’s the same, basically the same reason why the original observatory was completely useless by the mid-19th century, which is the cities in Europe started being lighted with electric lights and not candles as had happened before. So no fire anymore. Okay. So they basically, they were like very, very bright. And if you have an observatory close to the city center, then you couldn’t, you could see nothing, right, of the night sky. So they, they built this one far away from the city, but the city grew, right? So at some point, the light pollution of the city made this observatory useless. Not useless, but in comparison to other observatories, basically useless.

Marcos Pellejero:
Okay. So basically for the last, yeah, no, like for the, for the, now it’s not anymore, but since 1975 or so, this was used for checking the stability of detectors. So they were building detectors in the lab in the other building. They will put them here and then they will shake them basically to see if there were any loose pieces that they had forgotten or something like that. Okay. Yeah. So that’s basically how this worked, but you have to think about like an astronomer in 1895. Okay.

Marcos Pellejero:
Basically coming here with a candle, right? And then do you want me to show you how they will open the dome? Yeah. It’s very cool because it’s just with ropes and there’s no technology, no weird technology happening here. Come with me, let me show you. So do you see the wheels? The wheels are all around the dome. So those would be used to actually rotate the dome. Okay. And then you would have to open this and it is very heavy. It is as simple as this, even though it’s a bit heavy.

Marcos Pellejero:
Yes. But that’s the way one would open the dome. The dome. Yeah, yeah. Very cool, right? It’s raining. Now, what does he want to open it like up there or straight up? Yeah. So the thing is that there’s two, right? So one is this one. The other one is that one.

Marcos Pellejero:
That one will go. Do you see, do you see all of, all of like the levels there? And so you will have to use those. So basically what I was telling you is that the moment, okay, so these finders, okay, so the small telescopes. The ones that basically tell you where, like, in which way you’re looking at, right, are very high, right? So in the moment you tilt this, they are even higher. So to reach them, you will have to go up a ladder, okay? And the Royal Astronomer will have to go up a ladder. And he couldn’t have his Royal Astronomer ass going up a ladder because he was the Royal Astronomer, right? So he asked for this to be built. Okay. Which is an electric chair, but the good kind of electric chair.

Marcos Pellejero:
Okay. So the one in which you will sit down and then you will press the button and then this will elevate you. Okay. And then you can do observations without having to be up the ladder and so on, which is very cool.

Brian Keating:
If you just threw galaxies into the universe at random, you’d get a smooth fog of matter. But when you actually map them, you see a faint preference, a ring, a scale of about 150 megaparsecs left over from sound waves in the early universe. But these patterns are called baryon acoustic oscillations, and they behave like a cosmic ruler for measuring how fast the universe has been expanding.

Brian Keating:
So first of all, what’s a baryon acoustic oscillation, Marco?

Marcos Pellejero:
Well, acoustic oscillations, well, First of all, the way I like to think about them is how galaxies are distributed in the universe. Okay. Because they don’t follow just random patterns. They have like very distinctive patterns. And the first one that catches your eye is basically this cosmic web that you, that you know about. It’s a secondary answer, which is like, yes, closer, but also when you are at a distance of around 150 megaparsecs, then you find again a greater likelihood of finding a galaxy. When you have this kind of patterns, it’s usually the reason why they appear is because you have some kind of border or frontier.

Brian Keating:
Turning BAO into numbers isn’t just about counting galaxies. You have to know what the universe should look like if your dark energy theory is right. That means running huge n-body simulations, and Marcus works on emulators that use neural networks to mimic those simulations in a fraction of the time. So he can explore many more possible universes than ever before.

Marcos Pellejero:
So one of the, okay, one of the main problems to study, um, the distribution of galaxies nowadays is that the gravity formation is very nonlinear. And by this I mean that it comes with loads of complications to solve the equations. So the only analytical solutions that we have are those for the linear, uh, theories, okay, in the linear regime, which are like the regime of the very big scales.

Brian Keating:
In our simplest model, dark energy is just a constant, a fixed energy of the vacuum that never changes. But Desi is starting to whisper something awkward.

Brian Keating:
Is it true, Kyle, that as our colleagues, my friends and colleagues, Suzanne Staggs, Mark Devlin, Lyman Page, have demonstrated, David Spergel, very clearly that the lambda is unavoidable, or, you know, some version of dark energy is unavoidable using the CMB alone?

Brian Keating:
I think that’s true, right?

Brian Keating:
Then is it also Is it true from BAO alone you can derive the imperative of dark energy’s existence?

Kyle Lawson:
Yeah, that is definitely true for BAO alone as well. Our models, if we were to throw out any version of dark energy, would basically be impossible to describe the measurements we see over the redshift range we make the measurements. We see a preference for something like 70% of the current energy contents being dark energy, and that’s hard to get around.

Brian Keating:
The data seem more comfortable if dark energy evolves over time.. And when you combine that with the fact that cosmology wants neutrinos to be almost massless while particle physics insists that they aren’t, you get a serious tension in our best theory of the cosmos.

Marcos Pellejero:
Theoretically, it makes a lot of sense that it’s a constant, right? Because if you think of it as the energy of the vacuum, then the more volume there is, the more vacuum there is, it all compensates, and then you get a constant. So makes a lot of sense. But the latest results from DESI, and being part of the DESI collaboration, I, I trust them. Yeah. Because I know that they have very, very, you know, very picky in how to show the results and so on. That’s— those seems to show that there’s a strong— so yeah, but there’s like strong evidences from DESI to actually departure of this. And I think like the most— okay, to me, the most interesting part is that cosmology has very few predictions that they can make. That can be checked with other kinds of areas in physics, for example, particle physics.

Marcos Pellejero:
And there’s one prediction that cosmology has, which is that we can measure the mass of the neutrinos. Okay. And if you go to the latest results from, from BAO and DESI, and you combine them with other supernovae results and so on, you find out that there’s strong evidence to actually having massless neutrinos in the universe. But particle physics experiments tell you that they cannot be massless., right? They have to have a mass. So there’s a tension here. There’s a paradox here. Like there’s a misunderstanding between these two areas of science and the only— exactly, inconsistency. And the only way of reconciling those two seems to be opening our framework to new ideas on what dark energy could be.

Marcos Pellejero:
And right now it seems that that’s the most compelling way of moving forward. Ah, right. Because there are other ways in which you will actually lose these constraints, but none of them actually move your measurements. They just make them less accurate. But this one actually moves your measurements in the right direction. This one being the dark energy. The dark energy, exactly. So having a dark energy that is not really constant in time but evolves.

Brian Keating:
The Hubble tension, the fact that different models measuring the expansion rate disagree, might end up being systematic error, or it might be a sign of new physics. The only way to know for sure is to redo the key experiments with ever more careful data. That’s part of what surveys like DES and the upcoming LSST are designed to do.

Brian Keating:
So tell me about the Hubble tension. What are the— because you found there was also some discrepancy depending on what values of Hubble.

Marcos Pellejero:
Yeah, so about the Hubble tension, I don’t have a good answer on like what could be happening with the Hubble tension. Everything seems to So that it might be due to systematics, but again, I’m not an expert.

Brian Keating:
Do we need more data for both things? Do we need more?

Marcos Pellejero:
I think we need more consistent data. Maybe that’s the thing, right? Maybe we have, we need to redo some of the things that we have done already. And I know that this is not very attractive and no one really wants to do this, right? Yeah. But sometimes you have to repeat some of the experiments. And this is being done by the DES collaboration, for example, they have like their own set of supernovae. And also the LSST is going to have their own set of supernovae. And again, I know that this is not like very attractive in some way, but it’s the only way forward.

Brian Keating:
To test any model of dark energy, you need to know what the universe’s large-scale structure should look like if your model is correct. That’s what so-called N-body simulations do. They throw billions of particles into an expanding universe and let gravity sculpt the cosmic web, and then we compare it to what we actually see.

Brian Keating:
Explain for a layperson, what is an N-body simulation? How do you actually do it? Do you have a laptop, iPhone?

Marcos Pellejero:
How do you do it? Sure, sure. So they’re usually done in supercomputers, right? The big ones, they’re done in supercomputers. An N-body simulation is basically just a simulation of a very homogeneous universe that evolves with time according to a mixture between Newton’s laws and general relativity laws. Okay. So basically Newton’s laws. On an expanding universe, and it, it evolves only through gravity. Okay. And it tells you what is going to be the gravitational potential.

Marcos Pellejero:
So what is going to be the structures that you expect to see in the late universe? And it solves the equations exactly. Okay. But for a given set of initial conditions, that’s, that’s the—

Brian Keating:
James Clerk Maxwell, Peter Higgs, and the astronomers who built this place were all chasing different versions of the same question. What is the universe really made of and how does it really behave? Marcos does it with simulations, surveys, and machine learning instead of brass and glass, but the mindset is still the same. Take a vague intuition and carve it into something the universe can’t ignore.

Brian Keating:
So what do we need more of? More galaxies, more observations, more simulations, more CPU, GPU?

Marcos Pellejero:
What do we need more of? Well, we need more of everything. I guess, Imani, if you ask me, more of everything. But so simulations, I think we have, I mean, like we have plenty, of course, like the bigger they are, the better, but we have techniques to actually do these embodied simulations quick enough that I think that’s not a bottleneck anymore. It used to be, but not anymore, right? But we also need like machine learning techniques and artificial intelligence techniques to actually use them in the smartest way for that, right? And for that, we need synergies between the computing science departments and the cosmology department. Bottlenecks that we have there in simulations are more related to hydrodynamic simulations, which are the simulations in which it’s not only gravity evolving, it’s gravity plus the pressure from galaxies, explosions of plasma, exactly. So, so star formation and so on, right? Those are very far from being converged. So if you run two similar approaches, the outcomes will be completely, completely contradictory. Okay.

Marcos Pellejero:
So they will, they will basically predict opposite effects, which is, which is something that is very annoying, right? Yeah, exactly. Because then you don’t know in which universe you’re living.

Brian Keating:
Right. So this observatory, uh, there’s the Higgs Center that we’re at right now. Uh, did you meet Peter Higgs?

Marcos Pellejero:
Did you know Peter Higgs? No, I didn’t. I was, I moved here, uh, not very, okay, so basically one year before he died and, and well, he, he was not coming anymore, right?

Brian Keating:
To work. The quick history here, was Maxwell ever here?

Marcos Pellejero:
Was there?

Brian Keating:
Right.

Marcos Pellejero:
Maxwell is one of my like heroes. In history because it doesn’t seem that he was also like a very good scientist. Apparently he was also like a nice person.

James Clerk Maxwell:
Ah, Edinburgh, the city where I first chased light through the mist. At 14, I was already puzzling over the mathematics of curves and colors, scribbling equations. Didn’t pass every exam, mind you. Cambridge nearly said no. But curiosity is a stubborn thing. It carried me from these cobbled streets to the laws that would bind electricity and magnetism forever. Funny, isn’t it? As my friend Professor Brian Keating always says, ABC, always be curious.

Marcos Pellejero:
So he was, he was born here, and you can actually visit the house where he was born. He wanted to come here, but at the time he was, he had not made his most brilliant contributions to physics. And he was actually not accepted in the university. A friend of his was accepted. And at the very beginning I thought like, oh, who would say no to Maxwell, right? But then you realize that they were actually very good friends since they were kids. And they were part of like this club in which they solved mathematical problems together and so on. And then you think like, oh no, actually, I don’t know, probably Maxwell was happy that his friend got a position.

Brian Keating:
And he went on to get a raise. So how does it feel to work in a place like this with all this history, castles, copper, and then you’re doing some of the most modern large-scale simulation ever done, the tundra, and the most mysterious force in the universe, dark energy.

Marcos Pellejero:
I know, I know. I don’t see any big leap in that, uh, in, in, in that sense, right? So in, in, in the end, like, I mean, it, it, it sounds very old when you, when, when you tell these stories and, and, and so on, but they were, they were dealing with the same kind of problem. They, how to make a bigger building, in my case, how to make a bigger simulation. Yeah, exactly. Like bigger telescope, how to, I don’t know, like in the end it’s a very similar mind framework and it’s like a problem solving framework, right? When you have a problem and you want to find a solution, it’s like the brain works in very similar ways. It’s not like music, for example, right? In music it’s different. In music you don’t have a very well-defined problem, right? You just have like an intuition on what could work and what couldn’t, right? Um, and, uh, but yeah, but science is a bit like a mixture of those two, right? Like problem solving and a little bit of inspiration. And, and when you live in a place where there’s so many artistic stuff like around you, then this, you, you, you get, you, you find out that science and arts are not that different and that you need inspiration from both of them, right? Which is, which is the regular cool thing.

Marcos Pellejero:
That’s beautiful.

Brian Keating:
Thank you. From this hilltop in Edinburgh to the edge of the observable universe, baryon acoustic oscillations and DESI are forcing us to ask whether dark energy is really constant or whether our entire cosmological model is starting to bend. A huge thanks to Marcos and the Royal Observatory of Edinburgh for opening their doors. If you want to go deeper into DESI and dark energy, check out my conversations with DESI past spokesperson Kyle Lawson and Nobel laureate Adam Riess. They’re linked right here. See you next time on Into the Impossible. And don’t forget to like, comment, and subscribe.