Podcast Summary: “Unravelling the Universe, Again”

People I (Mostly) Admire, Episode 164
Host: Steve Levitt
Guest: Adam Riess, Nobel Prize-winning Astrophysicist
Date: August 16, 2025

Episode Overview

This episode explores the career and discoveries of Adam Riess, the astrophysicist whose work has dramatically shaped modern cosmology—first by revealing that the expansion of the universe is accelerating (leading to the theory of dark energy) and more recently by uncovering the so-called “Hubble tension,” a discrepancy in our measurements of the universe’s expansion rate. Host Steve Levitt guides a conversation that touches on the history of cosmology, the detective work behind Riess’s breakthroughs, and what recent findings might mean for the future of physics.

Key Discussion Points and Insights

1. Pre-Riess Cosmology: How We Understood the Universe’s Expansion

• Historical Consensus (02:11)
  For most of the 20th century, scientists believed the universe was expanding due to the Big Bang, but debate remained about its ultimate fate. Gravity, from all the mass in the universe, was expected to slow the expansion over time, and the big question was whether it would ever be enough to reverse it.

  Quote:
  “By the 1990s, the big question was, is there enough matter in the universe to stop the expansion in the future?”
  —Adam Riess (03:18)

• Edwin Hubble’s Breakthrough (04:21)
  Hubble measured galaxies’ distances using “Cepheid variable” stars and observed their receding speeds (“redshift”), establishing that more distant galaxies are moving away faster.
  Analogy: Raisins in rising bread all move apart; distant raisins move away faster, just like galaxies.

• The Challenge of Distance Measurement (10:07)
  Direct methods (like parallax with Earth’s orbit as baseline) work only nearby. Astronomers rely on “standard candles”—objects of known brightness like Cepheids—to estimate distances across the cosmos.

• Einstein’s “Blunder” and the Cosmological Constant (15:56)
  Before expansion was observed, Einstein included a fudge factor (cosmological constant) to keep the universe static, only to regret it when Hubble’s discovery rendered it unnecessary—until it reemerged in dark energy.

2. The Discovery of Cosmic Acceleration

• From Cepheids to Supernovae (18:49)
  To look farther and deeper in time, Riess and others used Type Ia supernovae, stellar explosions that are so bright they can be seen billions of light years away. They were found to be remarkably consistent (“standard candles”) due to the theory of Chandrasekhar: These explosions happen when a white dwarf hits a critical mass.

  Quote:
  “That’s the kind of supernova that’s very uniform because it’s always blowing up at or near about the same mass.”
  —Adam Riess (22:09)

• Finding Distant Supernovae (23:06)
  The advent of CCD cameras and massive telescopes let astronomers scan thousands of galaxies at a time, regularly catching the rare events.

  Quote:
  “It’s sort of like winning the lottery because you bought all the lottery tickets.”
  —Adam Riess (24:20)

• Technical Hurdles (25:38) & Data Reduction
  Catching a supernova at its peak, classifying its type, correcting for dust and galaxy light, and extracting signals from noisy data required both technological and algorithmic innovation.

  Quote:
  “We are almost by definition dealing with data that’s just barely of the quality needed to answer the question... The noisier and rougher your data is, the more creativity is required.”
  —Adam Riess (28:17)

• 1998 Breakthrough: Accelerating Universe (32:19)
  Riess’s team expected the universe’s expansion to be slowing due to gravity. Instead, they found it was speeding up—strongly suggesting repulsive gravity or “dark energy,” essentially Einstein's constant reborn.

  Quote:
  “When I made that measurement and cross checked it with my colleagues, we found...it was not slowing down at all. It was actually speeding up. The universe is expanding faster and faster.”
  —Adam Riess (32:41)

  • Reaction from peers ranged from skepticism to excitement, but independent confirmation quickly followed.

    Memorable Exchange:

    • “Who'd you tell first?” —Levitt (35:20)
    • “Particularly Brian Schmidt... I told him, ‘I've clearly made a mistake because the sign's wrong, but I can't find it.’”
      —Adam Riess (35:21)

• Naming “Dark Energy” (38:17):
  Levitt notes that the name makes it sound more solid than it is—really, it’s a placeholder for something unknown physically.

  Quote:
  “It's often called the worst prediction in all of physics.”
  —Adam Riess (39:55) (on quantum theory predicting a value for vacuum energy 120 orders of magnitude too large)

3. The Hubble Tension: A New Cosmic Puzzle

• Lambda-CDM Model and Its Limits (42:06)
  The standard cosmological model—Lambda-CDM—features dark energy (Lambda), cold dark matter (CDM), and assumes dark energy is constant.

• Hubble Constant Discrepancy (44:56)
  Measurements from the early universe (cosmic microwave background) predict the Hubble Constant as ~67; Riess’s local measurements give ~73.
  5σ Significance: Much greater than can be attributed to chance—suggests something’s wrong with the model, measurements, or assumptions.

  Quote:
  “In the laws of physics, things really should match. It's not okay for things to be off by five times the margin of error... In fact, it's not just not okay. We get very excited.”
  —Adam Riess (46:45, and again at 01:22)

• Possible Explanations

  • The simplest model (constant dark energy) may be wrong, as there’s no fundamental reason why dark energy wouldn’t evolve over time.
  • Other explanations: new physics, a previously unknown particle, or even a modification of gravity itself.

  Quote:
  “Sometimes you pull a loose thread on a sweater...other times it could unravel the sweater.”
  —Adam Riess (50:19)

4. The Nature of Scientific Debate & Implications

• Media Dramatization vs. Scientific Process (52:51)
  Riess avoids media coverage that personalizes scientific disagreements:
  Quote:
  “Science operates pretty well...We take data, we publish it, we discuss it at conferences. We don’t always agree...but that’s part of the process.”
  —Adam Riess (53:08)

• What If the Standard Model’s Wrong? (53:48)
  If the Standard Model (and underlying laws like general relativity and quantum theory) needs revision, huge new practical breakthroughs might follow, just as Einstein’s relativity turned out to be crucial for GPS.

  Quote:
  “If there’s something that is wrong about the model, there’s the potential that it’s actually teaching us... Even some of these physical laws, which, if that’s the case...there’s a lot we could potentially learn.”
  —Adam Riess (53:52)

• Definitive Results and New Experiments (55:08)
  Gravitational wave telescopes, new generations of space/ground observatories and particle experiments may soon probe the “dark sector.”

  Quote:
  “The universe and our understanding of it is always under the microscope... Every time somebody takes a new observation, there’s the potential [for revolution].”
  —Adam Riess (55:08)

• Hunting for Dark Matter & Precedent
  Particle physics and underground detectors are still searching for the “dark matter” particle. Similar puzzles—like Uranus's orbital oddities—once led to new planets, and Mercury’s led to new physics (Einstein).

  Quote:
  “Sometimes things don’t match...sometimes it’s a clue that we’re missing stuff...sometimes it’s a clue that we’re missing the right theory...”
  —Adam Riess (59:18)

Notable and Memorable Moments

• Levitt’s Modest Economics Comparison:
  “In economics, when there’s a 10% difference between two sets of estimates, we call that a successful replication and we go out for a beer to celebrate.”
  —Steve Levitt (59:38)

• On Theory vs. Data (throughout):
  Riess repeatedly emphasizes that empirical results, no matter how odd, must be held above personal beliefs or preferences—a theme essential to science but rarely heard so clearly.

• Human Side of Major Discoveries:
  Riess’s worry, anxious validation, and collaborative checking after the unexpected 1998 result highlight both humility and rigor in science.

• History Loops:
  The narrative loops from Einstein’s “blunder” to his being proven partly right, and onward to current mysteries that may again require rewriting the laws of physics.

Key Timestamps

• [02:11] – Pre-1990s cosmological consensus
• [04:21] – How Hubble proved universe’s expansion
• [10:07] – The “distance problem” in astronomy
• [13:35] – Henrietta Leavitt and Cepheid variables
• [15:56] – Einstein’s cosmological constant
• [19:53] – Discovery and use of supernovae as standard candles
• [23:06] – Techniques for finding distant supernovae
• [27:43] – Data reduction and algorithmic “artistry”
• [32:19] – The 1998 discovery: an accelerating universe
• [35:20] – Personal recollections of the 1998 breakthrough
• [38:17] – The limits of dark energy as a concept
• [42:06] – Lambda-CDM and its simplicity
• [44:56] – Local vs. early universe Hubble constant estimates; the Hubble tension
• [46:45 & 01:22] – “It’s not okay for things to be off…”
• [49:06] – Could dark energy be dynamic and evolving?
• [50:19] – The “unraveling the sweater” metaphor
• [53:08] – Riess on media, debate, and science culture
• [53:48] – Potential ramifications of a new model
• [55:08] – Nature of empirical science and future tests
• [59:18] – Theories failing as clues for discovery

Takeaways for the Listener

• Cosmology remains in fundamental flux:
  Discoveries made by Adam Riess and others proved our universe’s expansion is accelerating, requiring a new kind of “dark energy”—a placeholder for an unknown force or law.

• Today’s “Hubble tension” is a sign:
  A 10% difference in expansion rate estimates—far more than the experimental uncertainties—signals a growing mismatch between model and reality, possibly foreshadowing a major scientific revolution.

• History echoes, and humility rules:
  Overturning scientific consensus is emotionally fraught, often greeted with skepticism, but remains central to how science progresses.

• What’s next could be revolutionary:
  Advances in instrumentation (telescopes, gravitational wave detectors), as well as persistence in data analysis, may soon reveal what 96% of the universe—the “dark sector”—truly is.

Final Words

“This is a great adventure that I think we all get to be a part of ultimately as humans on this planet. This just amazing ambition we have to look out into space and try to figure out what everything is, even if most of it’s not like us.”
—Adam Riess (59:24)