Unexplainable – "Nightmare at the End of the Universe"

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

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.

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).

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)

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

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.

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.