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.

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.

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.

This is not far from the Simons Observatory. That's in the northeast.

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.

It has a picture.

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.

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.

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.

And what's the diameter, Marco?

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.

From that, it's less than, say, 18 inches?

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.