Podcast Summary

Podcast: StarTalk Radio
Episode: Solving the Crisis in Cosmology with Wendy Freedman
Date: September 23, 2025
Host: Neil deGrasse Tyson
Guest: Dr. Wendy Freedman (University of Chicago, National Medal of Science honoree)
Co-host: Matt Kirshen

Overview

This episode explores the so-called "Crisis in Cosmology," focusing on the ongoing debate in astronomy over measurements of the universe’s expansion rate—a phenomenon known as the “Hubble tension.” Dr. Wendy Freedman, a leading expert in cosmological distance measurements and recent recipient of the National Medal of Science, joins Neil deGrasse Tyson and comic co-host Matt Kirshen. Together, they unpack the latest research, explain key methods used to measure cosmic distances, address the controversy in the field, and discuss what these results may mean for our understanding of the universe.

Key Discussion Points & Insights

1. Setting the Stage: The Role of Observers vs. Theorists

[03:18 - 05:00]

• Tyson points out the fast pace of cosmology, noting that observational data, rather than theory alone, arbitrate what’s true.
• Freedman: “Data is the ultimate arbiter. We can have lots of ideas, but if they don't fit the universe, we throw them out.” [03:44]
• The respectful interplay between theory and observation is foundational to astrophysics.

2. Wendy Freedman's Achievements & Influence

[05:01 - 09:00]

• Recently awarded the National Medal of Science for work on the universe's expansion.
• Celebrated in TIME's "100 most influential people" list.
• Pioneering use of the Hubble Space Telescope to settle debates about the universe's age and size.
• Recounts historic debates—universe age estimates ranged wildly from 10 to 20 billion years, with key camps centered around different values for the Hubble constant (notably between 50 and 100).

3. The Hubble Tension: What Is It?

[09:00 - 16:15]

• Hubble tension refers to a discrepancy between two approaches for measuring the current expansion rate of the universe (the Hubble constant):
  • Direct measurements nearby (using Cepheid variables, red giant branch stars, supernovae)
  • Inferences drawn from cosmic microwave background (CMB) observations and theoretical models.
• Freedman explains Cepheids and their importance, crediting Henrietta Leavitt’s century-old discovery of the period-luminosity relation.
• Tyson: “Not every object serves as a way to know how far away it is... these yardsticks are cherished.” [10:17]
• There’s a notable difference: local measurements yield a higher Hubble constant (~73 km/s/Mpc), while early universe predictions give a lower value (~67 km/s/Mpc).

4. Historical Context & Changes in Cosmology

[16:16 - 24:16]

• Earlier measurements had much larger uncertainties, so disagreement was expected. Today, uncertainties are smaller, making the persistent gap more troubling.
• Freedman notes that both precision and accuracy matter, and the debate now focuses on these.
• Freedman: “Either this is really interesting and we're learning about some fundamental... property of the universe, or we've underestimated our uncertainties.” [24:16]

5. Understanding Standard Candles & Distance Measurement

[26:07 - 31:45]

• The team’s work with three distance indicators using the James Webb Space Telescope (JWST): Cepheids, tip of the red giant branch stars, carbon stars.
• The importance of using multiple, independent methods to identify and correct for systematic errors.
• Tyson highlights the unique perspective astronomers have due to their hands-on knowledge of stars, galaxies, and observational uncertainties.

6. Is There Really a Crisis? Precision vs. Accuracy

[24:48, 31:45 - 35:10]

• Wendy rejects sensationalism: “Would I have used the word crisis? No, I don't believe it's a crisis. Not in my opinion.” [24:48]
• She stresses the need for extraordinary evidence to claim extraordinary physics (quoting Carl Sagan).
• The current discrepancy may be due to underestimated errors—especially those hidden or systematic in nature (e.g., dust in galaxies impacting Cepheid measurements).

7. State of the Field: The Gold Standard and Systematics

[32:38 - 37:45]

• The cosmic microwave background (CMB) measurement with Planck satellite “is the gold standard,” highly precise.
• Local measurements involve complex, less well-controlled astrophysical systematics—stars, dust, supernova calibration—favoring skepticism.
• “It's always the systematics that come back to bite you, and often they're unknown systematics.” – Freedman [35:23]

8. Universe’s Expansion, Terms, and Models

[37:45 - 39:02]

• The Hubble constant refers to the present value (H₀); it was different in the past (the expansion rate is not truly constant).
• Tyson and Freedman joke about astronomical vernacular and misnomers.

9. Could the Universe Be Inhomogeneous?

[38:22 - 42:03]

• Question: Might the universe’s expansion rate differ in different regions?
• Freedman explains that thousands of supernovae measurements across the sky show no evidence for significant local variation.
• Despite over 1,500 papers proposing exotic explanations for the Hubble tension, all break some other well-established observation—so the Big Bang model remains robust.

10. Cosmic Queries: Listener Questions

[46:03 - 54:55]

• On Dark Energy and Distance: Future measurements may reveal varying/evolving dark energy—numerous upcoming experiments aim to clarify this.
• On the Gap in Expansion Rates: Could indicate new physics, but Freedman is not convinced yet: “I have to be convinced by the data, and at the moment I am not convinced...”
• On Infinite versus Finite Universe: Freedman doesn’t harbor a preference; curiosity and scientific process are what matter.

Notable Quotes & Memorable Moments

• "Data is the ultimate arbiter. We can have lots of ideas, but if they don't fit the universe, we throw them out."
  — Wendy Freedman [03:44]
• "Extraordinary claims require extraordinary evidence, and I'm not yet seeing extraordinary evidence."
  — Wendy Freedman [31:45]
• "Would I have used the word crisis? No, I don't believe it's a crisis. Not in my opinion."
  — Wendy Freedman [24:48]
• "It's always the systematics that come back to bite you, and often they're unknown systematics."
  — Wendy Freedman [35:23]
• “In 60 billion years... if the acceleration of the universe continues... we won’t see other galaxies, and we won’t have the chance to make the measurements that we're making today.”
  — Wendy Freedman [49:54]
• "The Hubble constant refers to a Hubble parameter at the current time t equals 0h0... it is the value of the Hubble parameter at the current time."
  — Wendy Freedman [37:55]

Moments of Humor and Tone

• The hosts joke about astrophysical terminology, such as calling all elements heavier than helium "metals," and the misuse of “burning” in stars.
  • "Bigger insult is that we consider pretty much everything heavier than hydrogen and helium to be a metal." — Wendy Freedman [29:28]
• Neil and Matt riff on who “matters” more—ordinary matter or dark matter/energy.
  • "A sixth of you matters." — Neil deGrasse Tyson to Matt Kirshen [22:14]
• Playful banter about finding one’s way home in an expanding universe, featuring a cosmic GPS analogy.
  • "So bring an equation with you for the expansion of the universe and then go backwards along that path and you should be able to get home." — Neil deGrasse Tyson [51:45]

Key Timestamps

| Timestamp | Segment/Topic | |-----------|---------------| | 03:18 | Introduction of topic; the observer’s role in science | | 05:01 | Wendy Freedman’s recent honors and context | | 09:00 | Introduction of the Hubble tension crisis | | 11:04 | The history and importance of Cepheids | | 13:56 | The meaning of “Hubble tension” explained | | 16:16 | Local vs. distant cosmic expansion measurements align | | 19:47 | What the Hubble constant actually measures | | 21:32 | Could our basic model of the universe be wrong? | | 24:48 | Is this a crisis? (Freedman: No) | | 26:07 | JWST project: Measuring distances by multiple methods | | 31:45 | Are the uncertainties underestimated? | | 32:38 | The CMB and the “gold standard” of cosmology | | 35:23 | The true challenge: unknown systematic errors | | 37:45 | Hubble "constant" through cosmic time | | 38:22 | Is the universe homogeneous? | | 42:03 | Sensational headlines and the state of the field | | 46:03 | Cosmic Queries begin: Dark energy, new instruments | | 48:22 | If discrepancy persists, new physics or better data? | | 49:29 | “How will we get home if the universe is expanding?” | | 52:43 | Would the field change if the universe is infinite? |

Summary & Takeaways

• The “Hubble Tension” is a real and curious discrepancy, but not a crisis. The community must be careful about extraordinary claims, as underestimated errors—especially systematics—are often to blame.
• Freedman’s JWST work aims to unify and cross-check distance measurement methods, hoping to clarify if the tension is due to new physics or simply measurement errors.
• The Standard Model of Cosmology remains robust, withstanding challenges from a barrage of new theories, as each new explanation must fit a vast range of constraints.
• Science thrives on questioning and precise measurement. Both Freedman and Tyson exemplify a cautious but open-minded approach: hope for new discoveries, but require strong evidence.
• Humor and humility are woven throughout, from cosmic-yardstick jokes to thoughts on infinity, making the episode accessible and offering a glimpse into how real scientific debates proceed.

For Listeners New to the Episode

This conversation offers a rich, clear window into how upper-echelon cosmologists evaluate and debate the evidence about the universe’s expansion, balancing skepticism with curiosity and the hunt for fundamental new physics. Dr. Wendy Freedman’s expertise—tempered by an insistence on robust, multi-pronged evidence—gives a refreshing and reassuring outlook: the universe may not be done surprising us, but scientific rigor guides the way.

Keep looking up!