Unexplainable - Nightmare at the end of the universe

Episode Date: August 18, 2025

Dark energy is the strange stuff that makes up the vast majority of the universe and will ultimately lead to the end of everything. Unless it doesn't exist at all. Guests: Adam Riess, astrophysicist at Johns Hopkins University, and Nathalie Palanque-Delabrouille, director of the Physics Division at the Lawrence Berkeley National Laboratory and member of The Dark Energy Spectroscopic Instrument For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ And please email us! ⁠⁠⁠⁠[e-mail]⁠⁠⁠⁠ We read every email.Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members Learn more about your ad choices. Visit podcastchoices.com/adchoices

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free of charge. BetMGM operates pursuant to an operating agreement with Eye Gaming Ontario. When I was eight years old, I had a very specific recurring nightmare. The Big Bang, the giant fireball that gave birth to our universe. I'd watch these 90s science documentaries, which would start all the way at the beginning of everything. The blast generated so much momentum that it's driven the expansion of the universe for 15 billion years. And eventually they'd make it to the end.

Astronomers have long believed that all the matter in the universe, and all the gravity it exerts should slow the expansion down. At some point, billions of years from now, the universe would slow down so much that it would stop expand. And then, like some cosmic rubber band, it would start to snap back. Slowly, at first, and faster, and faster. And then at some very distant time in the future, you could imagine a kind of big crunch, which is the crunching together of the universe.

The big crunch. I was obsessed with it. I couldn't stop thinking about everything and everyone I knew just getting smushed together until we all couldn't breathe. In the end, in 30 billion years from now, all the matter of the universe would exist as one tiny point. much as it was at the instant of the Big Bang. Maybe you don't regularly lose sleep over something that might happen billions of years in the future. But, like, one, I've never really been the best sleeper.

And two, come on. It's so visceral. The crunching. Still messes with me. I used to be up all night. And my dad was pretty nice about it. He would just keep reminding me about the whole this. isn't going to happen for billions of years thing, which kind of worked sometimes.

But not every kid has this kind of relationship with endless emptiness. When I was eight years old and I would look at the night sky with my dad, he would point out to me that the stars that I was seeing may not be there anymore. And I asked why is that? And he said, well, you know, the light has to travel millions of years to get here. And those stars may have disappeared since then. When Adam Reese was eight years old, he couldn't believe how exciting the sky was. I'm looking back in time to the way things were, and that's available to me now, any time I want. That is amazing.

Just picturing that in your mind, that something you're looking at isn't there. I think it just kind of blows your mind in a way. We're also used to what you see is what there is. But, you know, once you decouple those, then a lot of other ideas are possible. By the time I was eight, Adam had grown up. He'd become an astrophysicist. And he started looking for more things to blow his mind. What I love about cosmology, the area I study, is you start out with these really esoteric questions.

And they really seem like philosophical questions, like you would. ask these questions of a rabbi or a priest or a philosopher or, you know, you'd argue with your friends, you know, understanding there'll never be answers to these things. It's like how many angels dance on the head of a pin. But when you enter the field of cosmology and physics, you learn, actually, these are very practical, specific questions and we could get specific answers. One of those questions Adam was working on was exactly what was keeping me up at night. And to be honest, if I'd known what he was up to when I was a kid, I think it would have helped. Because when I was worrying about the big crunch at the end of the universe,

Adam was trying to figure out if it was even possible. And one day, when he was looking out into deep space, he discovered something big, like really big, bigger than literally anything that's ever been discovered. I'm Noah's Hassanfeld, and today on Unexplainable, the biggest mystery ever. In the 90s, Adam was working with a team of astronomers who were trying to measure how fast the universe was expanding. The idea was, if it was expanding fast enough, it would probably just keep expanding forever. Gravity wouldn't be able to pull it back in. No big crunch.

Eight-year-old me would have been thrilled. So Adam and his team, they started by comparing how fast the universe used to be expanding billions of years ago with how fast it's expanding today. They figured it had gotten slower over time because of gravity. They just wanted to know how slow. At the time, I was a postdoc, so I'm only two years removed from getting my degree. And I would say nobody expected anything particularly exciting at that moment. But after analyzing the data, the answer I got was very surprising.

it wasn't decelerating at all. The answer I got was that it was accelerating. Put me inside your head for a second. What was going through your mind at that moment? You know, I would love to tell you, I had profound thoughts about, you know, implications and all that stuff. But it was much more basic than that.

You know, my first thought was disappointment. Oh, I had made a mistake. You know, here I was hoping to learn something about the universe, and I just learned I have a bug in my code. You know, in physics, we use a fancy word. unphysical. I had gotten what seemed like an unphysical answer, which is when a physicist says, this can't be, I don't understand it. I made a mistake,

and now I'm going to have to go and find the bug. But I spent weeks and then months looking for the bug, and I couldn't find it. Why, you spent months. Months. Oh, yeah. I mean, you have to find the bug, right? Adam started farming out pieces of the calculation to different people asking them to double check his work, but no one found anything.

including his collaborator in Australia. I would call him and wake him up around 6 a.m. Are you done? Because we were dying to know what the answer was. At a certain point, Adam had poked at it from every angle he could find. So we sent the last bit of data to his collaborator, went to bed. And when I woke up in the morning, there was a message. And it was, I checked the last step.

I'm seeing exactly the same thing. And that was really the sort of watershed. moment for me was, wow, this is not a mistake. This is what the sky says. If this wasn't a mistake, how could this be happening? What would make the universe expand faster and faster? This really is one of the biggest puzzles in science. But our best guess is it is the energy of empty space. The energy of empty space. It's kind of hard for me to wrap my head around. And to be honest, scientists have the same problem. We're very uneasy about it.

They don't really understand this stuff, which is why they call it dark energy. Dark energy is kind of this springiness of space. You know, it is sort of repulsive to itself. It pushes things apart. Scientists think dark energy is what's making the universe expand faster and faster. Now, this isn't dark matter

that other weird stuff scientists don't really understand. Dark energy is way weirder. Scientists understand it even less. And there's more of this unexplainable stuff than anything else out there. Dark energy makes up a ridiculous sounding 70% of the universe. You know, we are really the frosting on this cake. And, you know, we've only understood the sort of size and composition of the cake in the last couple of decades. But how did scientists get from spaces expanding really fast to dark energy is the thing doing the expanding? It starts with a mistake. When Einstein came up with general relativity over 100 years ago, his equations weren't really working.

They were showing that the universe was changing, which didn't really make sense, because at the time, astronomers didn't think the universe was something that could change. So Einstein was like, there might be something I don't understand here. What if I fudged this equation a little bit? So in order to balance out the equation and make the whole thing work, he put a little number in. It was called the cosmological constant. A few years later, Edwin Hubble discovered that the universe actually was changing,

that it was expanding. And Einstein was like, crap. He ended up calling the cosmological constant his biggest blunder. But then, like 70 years after that, Adam discovered that the universe was expanding faster and faster. And scientists looked back at Einstein's cosmological constant. This number Einstein had kind of just shoved in there to make his equations work. And they were like, oh, Einstein was right all along. This was the evidence.

Dark energy was the thing making the universe expand faster. Which meant, luckily for me, no big crunch. But it did point to a different kind of ending for the universe. Because the universe would expand forever at an accelerated rate, objects become eventually so far away from each other that there's no longer an opportunity for light to reach us from distant objects. So it's a pretty sad, dark, and cold ending for the universe. And certainly in the late 1990s, when we first came out with this, there was a lot of skepticism. The world of astronomy is in a lot of. an uproar about the possibility of a runaway universe.

There certainly were people who said, you know, this has just got to be wrong. But Adam kept running tests, which kept backing up dark energy, and it started to get harder to argue with these mountains of data. There was a kind of tipping point when it went from, oh, well, that's what that experiment said, but I don't know, to like, yeah, that's what fits everything. I remember it went from Science Magazine, 1998. The breakthrough of the year was our discovery that the expansion was accelerating, but that's when, you know, the interpretation was still a struggle.

2003, the breakthrough of the year for Science Magazine was a consensus on the cosmological picture. And then, I guess, the Nobel? Yeah, that helped, too. But even after winning the Nobel in 2011, Adam wasn't totally sure. about the story he was telling. I would say physicists remain very unsatisfied by this explanation. So we started checking out new data from new telescopes and poking at his theory. There were a number of experiments that began to kind of mismatch or have what we call

tension between what our group's best measurements were of how fast the universe is expanding and how fast it ought to be expanding. Which is normal. that's how science works. People thought, well, you know, those things happen. You know, let's just keep collecting better data. That will go away. But over the last decade, I would say,

that tension has grown to an uncomfortable level. It's all about you. And when you fly with Virgin Atlantic in their upper class cabin, they take the VIP treatment to the next level. With a private wing to check in and your own security channel at London Heathrow, you can glide from your car to their clubhouse, a destination in its own right in 10 minutes or less.

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I'm not sure what that means. A few years ago, this huge group of international scientists got together to work on the dark energy spectroscopic instrument, or DESE. And they had a pretty ambitious goal. The goal of DESE is to understand what dark energy is. Natalie Palank Delabrui is a cosmologist who works with DESE, which is based at the Kit Peak National Observatory outside of Tucson, Arizona. This is an isolated location as dark as possible so that we have a very clear view of the sky. And I don't know, I like the feeling of being in a solitary. You're like almost by yourself and with a beautiful sky above your head and trying to

observe it and to understand its mysteries. Just like Adam, the scientist at Desi wanted to understand how fast the universe was expanding. But they wanted to go bigger, get even more detail. So they decided to make a three-dimensional map of the universe. They started by connecting a series of tiny robots to incredibly thin fibers. They attached these robotic fibers to the telescope at Kit Peak, and then they aimed the whole contraption at a small patch of sky. Each fiber is held by one robot and maps one galaxy at a time.

There were 5,000 of these robotic fibers that each mapped a galaxy. which is kind of nuts, right? Space is so ridiculously huge that just in this one tiny patch of sky, they could map 5,000 galaxies. Not 5,000 stars, 5,000 galaxies. All these robotic fibers allow Desi to get an absurd level of detail. And then they pointed the entire contraption

at another patch of sky. In a very good night, we can measure the distances of up to about 150,000 galaxies. And they did this over and over for years, until they created the largest 3D map of the universe ever. We were basically hoping that we would be confirming what everybody else had seen so far, and that with additional data, we could go beyond just confirming

and finding more precise results. But to our surprise, the results were not the ones we were expecting. The universe was expanding, but way slower than they thought it would be. It seemed like dark energy was weakening. This was a big surprise. We were really not expecting to be finding something that was in tension with what previous experiments had found.

I mean, basically what you're saying is, you know, the thing that makes up the vast majority of the universe, we understand it even less than we thought we did. Yeah, exactly. I think that's a simple conclusion to what I've been saying. saying so far, yeah. Okay. Not only don't we understand dark energy, but it seems to be even more complex

than what we had imagined. If it's true, it's the biggest clue we've gotten about the nature of dark energy since its discovery. It would change the understanding that we have of dark energy, this even sort of basic understanding into being something else.

Adam wasn't involved in Desi's research, but he was impressed, even when they only released limited results last year, which they called a hint. And then just a few months ago, they released three times as much data, having reanalyzed everything, and that result was still there. It had actually grown even stronger. And so, you know, you start to collect an inventory of cracks in, you know, the simplest explanation of the universe.

So based on these new results, how would you describe our best understanding of dark energy now? I would say it's in flux. Okay. I mean, I would say if these experiments are right, then all bets are off in terms of the future of the universe. Scientists had assumed dark energy was constant, you know, because of the whole Einstein thing, which meant that the universe would keep expanding faster and faster, this runaway universe. But if dark energy can change, my nightmare, the big crunch, it's back on the table. If dark energy is weakening, it's still possible that it decays to a point where the universe would recalapse.

Adam says these DESE results show that dark energy is probably a lot more complicated than scientists initially thought. It's flexible, unpredictable, which means one of the main things we thought we understood about dark energy, that it's constant. It's kind of out the window. which makes me wonder whether there might just be a different, better explanation here. When we saw the universe accelerating, there were a number of possible explanations. And physicists tend to go with the simplest one, what we call Oakum's Razor, because, you know, the simplest usually ends up being right. And in a way, as crazy as it sounds, to come up with, you know, 70% of the universe was missing before,

it was easier than, you know, reinventing all of physics. or new ways of thinking of gravity, which nobody had. So as odd as this sounded, everything kind of fit. Even though we barely understood dark energy, it was a simpler, cleaner option than reinventing all of physics. But there might be other ways to explain what's going on out there besides blaming this all on dark energy. Like, scientists think gravity works differently at tiny scales, like inside atoms.

So maybe it works differently at super large. scales, like at the level of galaxies. Or maybe we do need to reinvent all of physics. The question we have on our minds these days sort of is like, are these just loose threads on a sweater? And, you know, as we study them, we'll find, oh, okay, well, there's some small wrinkles there, but, you know, it's nothing really changing the main story. Or, you know, sometimes a loose thread can, you know, you pull on it and it can unravel the sweater. That might mean dark energy doesn't exist at all. It would mean reimagining the most fundamental map of what the universe is made of.

Adam already did this 30 years ago when he discovered dark energy in the first place. He's not sure we're going to need to go that far now and reimagine everything again. But he is toying with that idea. Have you had any conversations or interactions where people are just like, Adam, again, you're an idiot. What are you doing? Yeah, certainly I have. And, you know, I continue to have those. You know, I, unfortunately, it's hard because we're humans. We have emotions. We have feelings. You know, there are advantages and disadvantages sometimes to us personally if this story or this model of the universe is right or wrong. And so, you know, it's hard as humans to put that aside. And it's especially hard to put that aside if there's nothing better to go in its place.

There's not a simple story at the moment, and so we're wandering in the wilderness trying to understand what it means. It's easy to assume science is just about getting the right data, that if we can just make the next observation, we'll finally understand everything. But science is just as much about interpreting that data, making a better model or telling a good story that makes sense of the data. Like the ancient Greeks, they did a pretty good job predicting how the planets would move, even though we're not. a lot of them thought the earth was at the center of everything. Their system was complicated. You know, they needed to include all these weird little circles all over the place. But it did work really well. And then Copernicus came along. He didn't discover anything new. He's not the first person that suggests that the earth went around the sun. To be honest, he was working off old data.

He just told a much simpler, clearer story. And that's the question scientists are facing right now. Do we try to tell a more complicated story about dark energy? Or do we write a new one? Nobody's really sure what to do. And that actually gets Adam excited. It's the same thing that made him fall in love with deep space when he was a kid. Looking at light from stars that might have already died, realizing that what you see isn't what is, that even when you think you've started to crack the biggest mysteries of the universe, there's still so much deeper to go. It's hard to know that there's this massive story and not want to know, why is this all here? How did this all begin? Where is this all going? Though I will say, Adam, if you do,

end up finding out that the answer to where all of this is going is the big crunch, it's totally fine if you don't tell me. I really think I'm good. This episode was produced by me. We had editing from Jorge Just with help from Meredith Haudinut, who also runs the show. Mixing and sound design from Christian Ayala, music from me, and fact-checking from Melissa Hirsch. Julia Longoria is our editorial director, and Bird Pinkerton reached out and slammed the wooden chest shut. I won't do it, she said. Not until I see the octopus. Thanks, as always, to Brian Resnick for co-creating the show.

and if you have any thoughts about our show, any ideas or criticisms or suggestions, send us an email. We're at Unexplanable at Vox.com. We really love hearing from you. We read every email. And we'd also love it if you could leave us a review or a rating wherever you listen.

It actually does help us find new listeners. You can also support this show and all of Vox's journalism by joining our membership program. You can go to Vox.com slash members to get ad-free podcasts. You can get a whole bunch of other goodies, and you'll also be helping us make this show. Unexplanable is part of the Vox Media Podcast Network.

We're off on Wednesday, but we'll be back next week.