Nightmare at the end of the universe

Summary

Unexplainable – "Nightmare at the End of the Universe"

Date: August 18, 2025
Host: Noam Hassenfeld | Guests: Adam Riess, Natalie Palank Dellabrui

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Episode Overview

This episode dives into one of the most profound and unsettling topics in cosmology: the fate of the universe. Spurred by host Noam Hassenfeld’s childhood nightmares of the "Big Crunch," the conversation delves into our evolving understanding of cosmic expansion, the discovery and mystery of dark energy, and the deeply uncertain future that new evidence now suggests. Through candid conversations with Nobel laureate Adam Riess and cosmologist Natalie Palank Dellabrui, the show explores both the scientific puzzles and the human emotions behind unraveling the universe’s fate.

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Key Discussion Points and Insights

1. Childhood Nightmares & the Big Crunch

• Noam shares how 90s science documentaries seeded his recurring nightmare of the universe ending in a catastrophic "Big Crunch," where everything is pulled together and destroyed.
• Adam Riess recalls being awed as a child by his father's explanation that the stars he could see may have already disappeared—a mind-bending revelation about the limits of our knowledge.
  • Quote (Adam Riess, 03:53):
    “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 all so used to what you see is what there is. But, you know, once you decouple those, then a lot of other ideas are possible.”

2. The Discovery That Changed Everything: Accelerating Universe

• By the 1990s, Adam Riess and his team began measuring how fast the universe was expanding, expecting gravity to slow it down—possibly ending in a Big Crunch.
• Instead, their data showed the rate of expansion was accelerating, not slowing, which pointed to something fundamentally unknown at work.
  • Quote (Adam Riess, 06:33):
    “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.”
  • Riess describes his initial disbelief, thinking it was a mistake or "bug in my code" until repeated checking confirmed the data (08:09).

3. Introducing Dark Energy and Revisiting Einstein

• The team concluded this acceleration must be due to "dark energy," a mysterious force comprising about 70% of the universe, yet completely unexplained.
  • Quote (Adam Riess, 09:00):
    “Dark energy is kind of this springiness of space. You know, it is sort of repulsive to itself. It pushes things apart.”
• The concept echoes Einstein’s old “cosmological constant”, which he had discarded as his “biggest blunder.” Adam’s discovery renewed interest in Einstein’s fudge factor as a real physical phenomenon (10:00–11:26).
• The new model: the universe would never crunch, but expand forever into darkness and cold.

4. Mounting Doubt: Is Dark Energy Even Constant?

• For years, the explanation held, and even won Riess the Nobel Prize (12:40), but physicists still felt dissatisfied, as measurements didn’t always align.
  • Quote (Adam Riess, 12:55):
    "I would say physicists remain very unsatisfied by this explanation."
• As telescope technology improved, discrepancies grew: new results didn’t quite fit the tidy story (13:06–13:43).

5. The DESI Findings: A Universe Full of Surprises

• Natalie Palank Dellabrui introduces the DESI project, mapping unprecedented numbers of galaxies with robotic fibers at Kitt Peak (17:09–18:42).
• Surprising finding: the universe’s expansion appeared slower than expected—as if dark energy is weakening, not constant (19:09).
  • Quote (Natalie Palank Dellabrui, 19:32):
    “Not only don't we understand dark energy, but it seems to be even more complex than what we had imagined.”
• These results imply the fundamental nature or role of dark energy may be in flux or even up for revision (20:32).
  • Quote (Adam Riess, 20:37):
    “I mean, I would say if these experiments are right, then all bets are off in terms of the future of the universe.”

6. Return of the Nightmare: Could the Big Crunch Come Back?

• If dark energy weakens, the nightmare of a collapsing universe becomes plausible again (21:06).
• The previously held belief—that dark energy is constant and guarantees endless expansion—may no longer apply.

7. The Meta-Question: Is it Time to Rethink Physics?

• Maybe “dark energy” isn’t the best explanation. Perhaps gravity works differently on cosmic scales, or entirely new physics is needed (22:13).
  • Quote (Adam Riess, 22:42):
    “Are these just loose threads on a sweater… or, you know, sometimes a loose thread can, you know, you pull on it and it can unravel the sweater.”
• Riess shares both the skepticism he faced and the emotional ups and downs of working in unsettled science (23:30–23:53).

8. Interpretation vs Data: The Never-ending Story

• Noam reflects that science is as much about interpreting and storytelling as it is about collecting data, likening the current uncertainty to the time before Copernicus revolutionized our view of the solar system (24:06).
• Adam expresses the inherent drive to seek meaning amid the mystery.
  • Quote (Adam Riess, 25:34):
    “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? What? Where is this all going?”
• Noam closes on a humorous note, telling Adam he doesn’t need to wake him if he learns the Big Crunch is truly back on the table (25:45).

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Notable Quotes & Memorable Moments

• “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.”
  – Noam Hassenfeld (02:26)
• “We're very uneasy about it.”
  – Adam Riess on dark energy (08:53)
• "We are really the frosting on this cake."
  – Adam Riess on humanity’s small place in the cosmos (09:37)
• “It’s in flux… if these experiments are right, then all bets are off.”
  – Adam Riess on new DESI data (20:32–20:37)
• “It's hard as humans to put that aside.”
  – Adam Riess on the emotional investment of scientists (23:30)
• "We're wandering in the wilderness trying to understand what it means."
  – Adam Riess on the current state of cosmology (23:58)

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Important Timestamps

• 01:12: Noam introduces his Big Bang nightmare
• 02:17: Adam describes the Big Crunch
• 06:33: Adam recounts the discovery of accelerating expansion
• 08:09: Realization that the unexpected result isn’t a mistake
• 09:00: Adam explains "dark energy"
• 11:26: Adam outlines the new, cold, lonely ending for the universe
• 16:57: Natalie introduces the DESI project
• 18:36: Mapping 150,000 galaxies per night
• 19:09: DESI’s shocking finding: slowed expansion
• 21:06: Big Crunch possibility returns
• 22:42: Adam’s analogy: loose thread unraveling the sweater of physics
• 24:06: Noam reflects on interpretation vs discovery in science
• 25:34: Adam on the desire to know the universe’s story

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Tone and Style

The episode combines a sense of cosmic awe—sometimes tinged with existential dread—with humor and candor about the challenges of science. The tone is approachable, curious, and honest, markedly driven by personal reflections (especially from Noam and Adam), making the deep science relatable and emotionally resonant for listeners.

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Summary for New Listeners

This episode of Unexplainable courageously confronts the grandest cosmic mystery: does the universe end in a whimper or a crunch? Host Noam Hassenfeld’s childhood fears and astrophysicist Adam Riess’s groundbreaking discoveries guide us through the shifting landscape of cosmology. From the paradigm-shattering detection of dark energy to the latest mapping of galaxies that challenge everything we thought we knew, the episode shows science as an ever-evolving story—where even today, “all bets are off in terms of the future of the universe.” It’s a journey that proves knowledge is always provisional, the unknown always vast, and the universe’s final fate still very much unexplainable.

Transcript

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[Adam Riess]

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[Noam Hassenfeld]

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.

[Adam Riess]

The blast generated so much momentum that.

[Noam Hassenfeld]

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. And then, like some cosmic rubber band, it would start to snap back, slowly at first, then faster and faster. And then at some very distant time.

[Adam Riess]

In the future, you could imagine a a kind of Big Crunch, which is the crunching together of the universe.

[Noam Hassenfeld]

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.

[Adam Riess]

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.

[Noam Hassenfeld]

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, 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.

[Adam Riess]

When I was 8 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.

[Noam Hassenfeld]

When Adam Reese was eight years old, he couldn't believe how exciting the sky was.

[Adam 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 all so used to what you see is what there is. But, you know, once you decouple those, then a lot of other ideas are possible.

[Noam Hassenfeld]

By the time I was 8, Adam had grown up. He'd become an astrophysicist, and he started looking for more things to blow his mind.

[Adam Riess]

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.

[Noam Hassenfeld]

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 before. I'm Noam Hassenfeld, 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.

[Adam Riess]

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.

[Noam Hassenfeld]

Put me inside your head for a second, like what was going through your mind at that moment?

[Adam Riess]

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. 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 gonna have to go and find the bug. But I spent weeks and then months looking for the bug, and I couldn't find it.

[Noam Hassenfeld]

Why you spent months?

[Adam Riess]

Months? Oh, yeah. I mean, you have to find the bug, right?

[Noam Hassenfeld]

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.

[Adam Riess]

I would call him and, you know, wake him up around 6am Are you done? Because, you know, we were dying to.

[Noam Hassenfeld]

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.

[Adam Riess]

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.

[Noam Hassenfeld]

If this wasn't a mistake, how could this be happening? What would make the universe expand faster and faster?

[Adam Riess]

This really is one of the biggest puzzles in science. But our best guess is it is the energy of empty space.

[Noam Hassenfeld]

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.

[Adam Riess]

We're very uneasy about it.

[Noam Hassenfeld]

They don't really understand this stuff, which is why they call it dark energy.

[Adam Riess]

Dark energy is kind of this springiness of space. You know, it is sort of repulsive to itself. It pushes things apart.

[Noam Hassenfeld]

Scientists think dark energy is what's making the universe expand faster and faster. Now, this isn't dark matter 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.

[Adam Riess]

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.

[Noam Hassenfeld]

But how did scientists get from space is 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 a hundred 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 that. 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.

[Adam Riess]

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, 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.

[Noam Hassenfeld]

About the possibility of a runaway universe.

[Adam Riess]

There certainly were people who said, you know, this has just got to be wrong.

[Noam Hassenfeld]

But Adam kept running tests which kept backing up dark energy. And it started to get harder to argue with these mountains of data.

[Adam Riess]

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 the interpretation was still a struggle to 2003. The breakthrough of the year for science magazine was a consensus on the cosmological picture.

[Noam Hassenfeld]

And then I guess the Nobel.

[Adam Riess]

Yeah, that helped too.

[Noam Hassenfeld]

But even after winning the Nobel in 2011, Adam wasn't totally sure about the story he was telling.

[Adam Riess]

I would say physicists remain very unsatisfied by this explanation.

[Noam Hassenfeld]

So he started checking out new data from new telescopes and poking at his theory.

[Adam Riess]

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.

[Noam Hassenfeld]

It ought to be expanding, which is normal. That's how science works.

[Adam Riess]

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.

[Noam Hassenfeld]

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[Natalie Palank Dellabrui]

What'S the problem?

[Noam Hassenfeld]

Well, okay, she's got a really dark energy. A real dark energy. I'm so sorry, can you clarify dark energy? 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 desi, and they had a pretty ambitious goal.

[Natalie Palank Dellabrui]

The goal of DESI is to understand what dark energy is.

[Noam Hassenfeld]

Natalie Palank Dellabrui is a cosmologist who works with desi, which is based at the Kitt Peak National Observatory outside of Tucson, Arizona.

[Natalie Palank Dellabrui]

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 an observatory. You're like almost by yourself and with a beautiful sky above your head and trying to observe it and to understand its mysteries.

[Noam Hassenfeld]

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.

[Natalie Palank Dellabrui]

Each fiber Is held by one robot and maps one galaxy at a time.

[Noam Hassenfeld]

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.

[Natalie Palank Dellabrui]

In a very good night, we can measure the distances of up to about 150,000 galaxies.

[Noam Hassenfeld]

And they did this over and over for years until they created the largest 3D map of the universe ever.

[Natalie Palank Dellabrui]

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.

[Noam Hassenfeld]

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

[Natalie Palank Dellabrui]

This was a big surprise. We were really not expecting to be finding something that was in tension with what previous experiments had found.

[Noam Hassenfeld]

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.

[Natalie Palank Dellabrui]

Yeah, exactly. I think that's a simple conclusion to what I've been saying so far. Yeah.

[Noam Hassenfeld]

Okay.

[Natalie Palank Dellabrui]

Not only don't we understand archaeology, but it seems to be even more complex than what we had imagined.

[Adam Riess]

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 a dark energy, this even sort of basic understanding into being something else.

[Noam Hassenfeld]

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.

[Adam Riess]

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. Even stronger. And so, you know, you start to collect a inventory of cracks in, you know, the simplest explanation of the universe.

[Noam Hassenfeld]

So. So based on these new results, how would you describe our best understanding of dark energy now?

[Adam Riess]

I. I would say it's in flux.

[Noam Hassenfeld]

Okay.

[Adam Riess]

I mean, I would say if these experiments are right, then all bets are off in terms of the future of the universe.

[Noam Hassenfeld]

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.

[Adam Riess]

If dark energy is weakening, it's still possible that it decays to a point where the universe would recollapse.

[Noam Hassenfeld]

Adam says these desi 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.

[Adam Riess]

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 Occam'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.

[Noam Hassenfeld]

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.

[Adam Riess]

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, 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.

[Noam Hassenfeld]

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?

[Adam Riess]

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.

[Noam Hassenfeld]

And it's especially hard to put that aside if there's nothing better to go in its place.

[Adam Riess]

There's not a simple story at the moment. And so we're wandering in the wilderness trying to understand what it means.

[Noam Hassenfeld]

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 make sense of the data. Like the ancient Greeks, they did a pretty good job predicting how the planets would move, Even though 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 to suggest 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. And 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.

[Adam Riess]

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? What? Where is this all going?

[Noam Hassenfeld]

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, Noam Hassenfeld. We had editing from Jorge, just with help from Meredith Hadnutt, 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 doctopus. Thanks as always to Brian Resnik 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@ unexplainableox.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.commembers to get ad free podcasts, you can get a whole bunch of other goodies. And you'll also be helping us make this show. Unexplainable is part of the Vox Media Podcast Network. We're off on Wednesday, but we'll be back next week.