StarTalk Radio
Episode: "Origins of Dark Energy" with Adam Riess
Host: Neil deGrasse Tyson
Guest: Adam Riess, Nobel Laureate Astrophysicist
Co-host: Paul Mercurio
Date: December 16, 2025

Episode Overview

This episode of StarTalk dives into the enigmatic concept of dark energy—the mysterious force driving the accelerated expansion of the universe. Neil deGrasse Tyson is joined by comedian Paul Mercurio and Nobel Prize-winning astrophysicist Adam Riess, whose groundbreaking work unveiled this cosmic puzzle. Together, they trace the scientific journey that led to the discovery of accelerated expansion, explore the ongoing “Hubble tension” debate, and discuss how new telescopes may further revolutionize our understanding. The tone is witty, curious, and occasionally irreverent, making complex topics lively and accessible.

Key Discussion Points & Insights

1. Setting the Stage: Why Dark Energy Matters

• Tyson and Mercurio kick off with light banter, agreeing that cosmology’s “cutting edge” is where real scientific friction (and breakthroughs) happen.
• Adam Riess is introduced as a Bloomberg Distinguished Professor at Johns Hopkins and Nobel Laureate for the discovery of the universe’s accelerating expansion.
  • Notable Quote [05:52] - Adam Riess: "You are co-discoverer of a cause... we always knew that there was this term in Einstein's equation... But no one had a negative. What is that?"

2. History and Discovery of Accelerating Expansion

• Einstein’s “cosmological constant” and its dismissal after the universe’s expansion was discovered by Hubble (the man, not yet the telescope).
• By the 1990s, the big question: Is there enough matter (regular or dark) to slow or halt the expansion?
• Riess’ team—using “standard candles” (Type Ia supernovae)—measured the rate of expansion across time.
  • Making Distance Measurements: From geometry (parallax) to Cepheid variables, to supernovae as the ultimate cosmological “mile markers.”
  • Notable Analogy [12:17] - Adam Riess: "If you're a ship captain…a lighthouse, which you know is very luminous, looks very faint, and assures you, wow, I have to be far away."
• The breakthrough was realizing Type Ia supernovae are remarkably consistent and visible at cosmological distances.
  • Notable Quote [16:52] - Adam Riess: "...when it reaches that Chandrasekhar limit, it's like boom. A thermonuclear explosion runs through the star...So this is a standard candle...It has a certain spectrum, it has a certain chemical fingerprint."

3. The Accelerating Universe and "Dark Energy" ([20:41] – [30:30])

• The team expected to measure how much the universe’s expansion was slowing due to gravity. Instead, they found it was accelerating!
  • Discovery Process:
    • They input data, expecting it to show deceleration, but got nonsensical results (negative mass)—until considering Einstein’s cosmological constant again.
    • Notable Moment [27:39] - Adam Riess: "The answer it spit back was negative mass...And I was like, that isn't right. ...And then...Einstein once had suggested something that could go the other way. You put that into the equations and it like, fit like a glove."
• Dark Energy: The catch-all term for whatever causes this repulsive effect—still not understood at a fundamental level.
  • Quote [29:33] - Adam Riess: "...the universe is accelerating, there's no question about that. But what is causing it? ...we don't understand the physics of that at all."

4. Measurement Techniques and Challenges

• Cepheids and Supernovae: Building the "distance ladder" is fraught with challenges—effects of dust, calibrations, and the need for ever-brighter standard candles.
  • Dust Correction Insight [24:13] - Adam Riess: "...dust makes things dimmer, which would fool you into thinking it's further away...But it does something else too. It makes light look redder when the light passes through."
• Only by resolving these could astronomers confidently measure cosmic acceleration.

5. The Hubble Tension & the Quest for New Physics ([36:16] – [47:58])

• Lambda-CDM Model: The prevailing cosmological model assumes cold dark matter and a cosmological constant (dark energy), but 96% of the universe remains “unknown stuff.”
• Hubble Constant Discrepancy:
  • Measurements of the universe’s expansion today (“local” methods) don’t match predictions from the cosmic microwave background (“early” methods)—a disagreement called the Hubble tension.
  • Notable Quote [39:49] - Adam Riess: "We are measuring opposite ends of the universe, and we are using our story of the universe to connect them. And so disagreeing in this case has the potential to teach us something profound about the universe..."
• Is Dark Energy Changing?
  • Riess discusses models where dark energy wasn’t always constant—potential “early dark energy” episodes could have given the universe extra “kicks.”
    • Quote [42:00] - Adam Riess: "We may be discovering that what we call dark energy is a general phenomenon that happens all the time in the history of the universe."
  • Other explanations: unseen particles, theoretical changes to gravity, or subtle errors in fundamental assumptions.

6. New Tools and the Road Ahead

• Upcoming and current observatories (James Webb, Nancy Grace Roman Telescope, Vera Rubin Observatory, Simons Observatory, LIGO, Gaia) will enable sharper measurements and possibly solve the Hubble tension.
  • Measurement Confidence [46:22] - Adam Riess: "...the fact that James Webb is confirming what Hubble is confirming is confirming that this is a real problem."

7. The Scientific Process and Cosmic Humility

• Tyson and Riess reflect on historical moments when unexpected data forced paradigm shifts (Newton to Einstein, Copernicus’ heliocentric model, discovery of planet Neptune, etc.).
  • Notable Quote [52:24] - Adam Riess: "We're not done because everything isn't fitting...we have this wealth of new facilities...that really should help us answer these questions."
  • Tyson’s Perspective [66:12]: "One of the more fascinating dimensions, the moving frontier of science, is when you don't have an answer to questions that have been posed..."

Notable Quotes & Memorable Moments

• Adam Riess [05:52]: "You are co-discoverer of a cause... we always knew that there was this term in Einstein's equation... But no one had a negative. What is that?"
• Adam Riess [16:52]: "...when it reaches that Chandrasekhar limit, it's like boom. A thermonuclear explosion runs through the star...a standard candle..."
• Adam Riess [27:39]: "...The answer it spit back was negative mass...And then...Einstein once had suggested something...You put that into the equations and it like, fit like a glove."
• Adam Riess [29:33]: "...the universe is accelerating, there's no question about that. But what is causing it?...we don't understand the physics of that at all."
• Adam Riess [39:49]: "We are measuring opposite ends of the universe, and...disagreeing in this case has the potential to teach us something profound about the universe..."
• Adam Riess [42:00]: "We may be discovering that what we call dark energy is a general phenomenon that happens all the time in the history of the universe."
• Neil deGrasse Tyson [66:12]: "...when you don't have an answer...you got to scratch your head and say, do I have to give up some prior expectations...?"
• Paul Mercurio throughout: Provides comic relief, analogies, and the “audience” perspective—grounding the theoretical in relatable terms.

Timestamps for Key Segments

• Discovery & Nobel Prize context: [03:31] to [07:12]
• Distance measurements & standard candles: [09:12] to [14:39]
• Type Ia supernovae breakthrough: [15:01] to [18:23]
• Cosmic acceleration discovery & significance: [22:04] to [29:33]
• Dark energy & fate of the universe: [33:31] to [34:36]
• Modern cosmology, Lambda-CDM model: [36:20] to [37:00]
• Hubble tension explained: [39:09] to [41:37]
• Potential resolutions/new physics: [41:38] to [44:46]
• New telescopes & future prospects: [54:29] to [56:49]
• Scientific humility & history: [48:20] to [51:02]

Final Thoughts

The episode balances history, humor, and groundbreaking science. It highlights the process of scientific discovery—not as a straight path, but a winding road with surprises, wrong turns, and paradigm shifts. The Hubble tension is framed as an opportunity for breakthrough, not just a problem.

As Riess puts it, science is still assembling the universe “using IKEA instructions”—and dark energy (whatever it is) is that mysterious, leftover part that won’t fit. The search continues.

Listen if you:

• Want a lucid, entertaining account of how astronomers discovered the universe is accelerating
• Are curious how “dark energy” went from Einstein’s math folly to a real cosmic mystery
• Are interested in why scientists are hopeful (and humble) about coming breakthroughs in cosmology

Closing Quote [66:12] - Neil deGrasse Tyson:
“One of the more fascinating dimensions, the moving frontier of science, is when you don't have an answer to questions that have been posed or you have data you can't make sense out of... then you got to scratch your head and say, do I have to give up some prior expectations, some prior assumptions that went into this understanding of the universe?... At each turn of those discoveries, we had a deeper understanding of the operations of nature. And that's what makes it all so beautiful. That is a cosmic perspective.”