Nightmare at the end of the universe

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1:22Adam Riess: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 expanding.

2:13Adam Riess:And then, like some cosmic rubber band, it would start to snap back. Slowly, at first. Then faster. And faster.

2:26Nathalie Palanque-Delabrouille: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.

2:36Adam Riess: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.

3:26Adam Riess: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 Rees was eight years old, He couldn't believe how exciting the sky was.

4:03Adam Riess:I'm looking back in time to the way things were, and that's available to me now, anytime 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.

4:52Adam Riess: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.

5:31Adam Riess: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 before. I'm Noah Massenfeld, and today on Unexplainable, the biggest mystery ever.

6:08Adam Riess: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.

6:49Adam Riess: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. Like, what was going through your mind at that moment? You know, I would love to tell you, you know, 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.

7:27Adam Riess: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. Well, 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, you know, wake him up around 6 a.m.

8:04Adam Riess:Are you done? Because, you know, 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 he 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.

8:38Adam Riess: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.

9:26Adam Riess: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.

10:18Adam Riess: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.

11:01Adam Riess: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. 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.

11:47Adam Riess:So it's a pretty sad, dark, and cold ending for the universe. And, you know, certainly in the late 1990s, when we first came out with this, there was a lot of skepticism. The world of astronomy is in 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, well, that's what that experiment said, but I don't know, to like, yeah, that's what fits everything.

12:30Adam Riess: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 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.

12:57Adam Riess: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 he 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.

13:40But over the last decade, I would say, that tension has grown to an uncomfortable level.

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16:32Adam Riess:A few years ago, this huge group of international scientists got together to work on the Dark Energy Spectroscopic Instrument, or DESI. And they had a pretty ambitious goal.

16:43Nathalie Palanque-Delabrouille:The goal of DESI is to understand what dark energy is.

16:47Adam Riess:Natalie Palanc de la Bruy is a cosmologist who works with DESI, which is based at the Kitt Peak National Observatory outside of Tucson, Arizona.

16:55Nathalie Palanque-Delabrouille: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.

17:14Adam Riess:Just like Adam, the scientists 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 Kitt Peak. and then they aimed the whole contraption at a small patch of sky.

17:45Nathalie Palanque-Delabrouille:Each fiber is held by one robot and maps one galaxy at a time.

17:50Adam Riess: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 allowed Desi to get an absurd level of detail. And then they pointed the entire contraption at another patch of sky.

18:22Nathalie Palanque-Delabrouille:In a very good night, we can measure the distances of up to about 150 ,000 galaxies.

18:28Adam Riess:And they did this over and over, for years, until they created the largest 3D map of the universe ever.

18:38Nathalie Palanque-Delabrouille: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.

18:55Adam Riess:The universe was expanding, but way slower than they thought it would be. It seemed like dark energy was weakening.

19:03Nathalie Palanque-Delabrouille:This was a big surprise. We were really not expecting to be finding something that was in tension with what previous experiments had found.

19:11Adam Riess: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.

19:19Nathalie Palanque-Delabrouille:Yeah, exactly. I think that's a simple conclusion to what I've been saying so far. Yeah.