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StarTalk Radio

Solving the Crisis in Cosmology with Wendy Freedman

with Wendy Freedman

Published September 23, 2025
View Show Notes

About This Episode

Neil deGrasse Tyson and co-host Matt Kirshen interview astronomer Wendy Freedman about measurements of the Hubble constant and the so‑called "crisis" in cosmology. Freedman explains the history of debates over the expansion rate of the universe, the current discrepancy between local distance-ladder measurements and values inferred from the cosmic microwave background, and why she does not yet consider it a true crisis. She describes her team's James Webb Space Telescope program using multiple stellar distance indicators, discusses systematic errors and the distinction between precision and accuracy, and answers audience questions on dark energy, the future evolution of the universe, and whether the universe is finite or infinite.

Topics Covered

Big Bang Cosmic microwave background Dark energy Dark matter Observational astronomy Hubble constant Standard candles Cepheid variables James Webb Space Telescope Standard model of cosmology

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Quick Takeaways

  • The Hubble constant measures the current expansion rate of the universe, and different methods currently give slightly different values (around 67 vs. around 73), a discrepancy dubbed the "Hubble tension".
  • Freedman argues that the evidence is not yet strong enough to declare a true crisis in cosmology and that underestimated systematic errors in local measurements may explain much of the discrepancy.
  • Her team is using the James Webb Space Telescope to measure distances with three independent stellar indicators-Cepheids, tip of the red giant branch stars, and carbon stars-to better calibrate the cosmic distance ladder.
  • The cosmic microwave background-based determination of the Hubble constant currently has sub‑percent precision and serves as a very strong benchmark for cosmological models.
  • Systematic effects like interstellar dust and uncertainties in stellar physics can make measurements very precise yet still inaccurate, which is why multiple, independent methods are essential.
  • Dark matter and dark energy dominate the universe's contents, but their fundamental nature remains unknown, leaving ample room for new physics even if the current standard cosmological model largely works.
  • Most historical discrepancies in physics have eventually been resolved by better data and error control, though occasionally they have pointed to genuinely new physics; Freedman wants extraordinary evidence before accepting the latter.

Podcast Notes

Opening and framing of the cosmology discussion

Host and co-host banter about checking in on the universe

Neil jokes that the universe "has issues" and there's a family crisis with the galaxy[0:58]
He teases that they may have resolved this crisis in the upcoming StarTalk episode[1:06]

Show introduction and co-host background

Neil formally opens StarTalk and introduces himself as a personal astrophysicist[1:26]
Neil welcomes back co-host Matt Kirshen and references catching him on a cruise ship gig[1:32]
• Matt notes he is performing comedy on a boat and then going on a land tour and club headlining dates
Neil correctly recalls Matt is a host of the podcast "Probably Science"[2:08]

Introduction of guest Wendy Freedman and her role in observational cosmology

Setting up the theme: theorists vs observers in cosmology

Neil says they're going deep into the universe and contrasts theorists with observers who get actual data[2:32]
He emphasizes that theorists ultimately have to "answer to" observers who arbitrate with data[2:32]

Introducing Wendy Freedman

Neil welcomes back astronomer Wendy (transcribed as Freeman), noting she was on StarTalk two years prior, highlighting how fast the field moves[2:44]
Wendy states that data is the ultimate arbiter in science: theories that don't fit the universe are discarded[2:49]
She stresses the need for interplay between data and theory, saying data without theory isn't very useful[3:07]
• Neil jokes she's being diplomatic because she still has to work with theorists at conferences

Wendy's academic position and honors

Neil introduces Wendy as a professor of astronomy and astrophysics at the University of Chicago[3:43]
He specifies her endowed title: the John and Marion Sullivan University Professor[3:51]
Neil notes she has been very busy since her last appearance, mentioning major recognition[3:56]
Wendy has received the U.S. National Medal of Science, described by Neil as the highest science award given by the United States[4:10]
• Neil notes there are parallel national medals for engineering and medicine
Neil says he once served on the committee that selects National Medal of Science recipients via the National Science Foundation and confirms that process still exists[4:31]
He remarks that routing selection through NSF helps depoliticize the award and ensure that recipients truly earned it[4:39]
Wendy states that science has no political affiliation, distinguishing it from most other human enterprises[4:51]

Citation for Wendy's medal: measuring the expansion rate of the universe

Neil notes Wendy was cited for pioneering work measuring the expansion rate of the universe[4:56]
He recalls she worked with the Hubble Space Telescope from early on, using it for exactly the distance-scale work it was named and designed for[5:10]

History of the Hubble constant and early debates

Pre-Hubble Space Telescope debates about universe age

Wendy recalls that before Hubble Space Telescope, astronomers debated whether the universe was 10 or 20 billion years old, a major discrepancy[5:37]
She says Hubble's primary mirror size was set specifically to allow measurement of Cepheid variable stars for distance determination[5:45]
• There had been cost-saving pressure to shrink the primary mirror, but Cepheid distance goals constrained how small it could be
At the time, there was a debate between a Hubble constant of 50 vs. 100 km/s/Mpc, corresponding to very different universe ages[6:08]

Old vs. young universe camps

Neil remembers that in his early career, astronomers informally grouped into "old universe" and "young universe" camps, arguing in coffee lounges about H0 = 50 vs. 100[6:20]
He notes the actual value later landed between those extremes, effectively splitting the difference[6:26]
Wendy points out that while two prominent groups (Sandage et al. vs. de Vaucouleurs et al.) anchored the 50 vs. 100 debate, published values in the literature actually spanned intermediate values as well[6:56]
Neil recalls being at the University of Texas, home base for Gerard de Vaucouleurs, whom he describes as "Mr. Young Universe" with H0 = 100, and feeling pressured not to think outside that box[7:05]
Wendy says she was at Carnegie with Allan Sandage at the time, who strongly disagreed with the high H0 values, illustrating the institutional split[7:26]

Recognition by Time magazine

Neil notes that in the same year as the medal, Wendy was named one of Time Magazine's 100 most influential people in the world[7:33]
Wendy mentions there was a celebratory event in New York that was unlike typical scientific conferences[8:12]
• Neil jokes that she came through New York for that event without visiting StarTalk, but notes his team had tried to schedule something

Definition and discussion of the Hubble tension

Media "crisis" framing vs scientific perspective

Neil says headlines and clickbait have framed the situation as a crisis threatening cosmology and the Big Bang, but suggests Wendy's work indicates the crisis may be overstated[8:41]
He notes her paper seems to show that the situation can be reconciled without abandoning the Big Bang model[8:35]

What is the Hubble tension?

Wendy explains that in the last decade, discrepancies have emerged between local measurements of the Hubble constant and values inferred from the cosmic microwave background (CMB)[9:10]
Locally, astronomers use standard candles such as Cepheid variables, tip of the red giant branch stars, and Type Ia supernovae tied together in the distance ladder[9:13]
The CMB method measures tiny temperature and polarization fluctuations in the relic radiation from the Big Bang and fits them with the standard cosmological model to infer H0[9:26]
The standard model fit to the CMB predicts a present-day Hubble constant of about 67 km/s/Mpc with less than 1% uncertainty, whereas local Cepheid-based measurements with HST give values around 73[9:10]
Wendy highlights that, compared to the old 50 vs 100 fight, the current difference is numerically small but statistically significant because uncertainties are much smaller[9:13]
• Neil remarks that in the 50-100 era, error bars overlapped, allowing the true value to plausibly lie in the middle, whereas now the precise, non-overlapping error bars force a resolution

Historical moment of apparent agreement between methods

Wendy recalls that around 2001-2003, their HST Key Project yielded H0 ≈ 72 ± 10%, and the WMAP satellite measured H0 ≈ 71 from the CMB, suggesting strong agreement[14:31]
She notes that at that time, the accelerating expansion of the universe had been discovered, and the age of the universe was about 13.7-13.8 billion years, consistent across methods[14:58]
Wendy emphasizes how remarkable it was that local stellar measurements and CMB data from redshift ~1100 (380,000 years after the Big Bang) agreed so well, like puzzle pieces fitting together[15:14]

Explaining the Hubble constant and its role

What H0 numerically represents

In response to a listener identifications segment, Wendy explains that the Hubble constant measures how fast the universe is expanding at the current time[19:39]
She states the units as kilometers per second per megaparsec (km/s/Mpc), effectively an inverse time and related to the age of the universe[19:42]
An object 1 megaparsec away recedes at H0 km/s; an object 2 megaparsecs away recedes at roughly 2×H0 km/s, and so on, reflecting Hubble's law[20:03]
Wendy confirms that the Hubble constant today is the slope of the relation between galaxy distance and recession velocity that Edwin Hubble first plotted[20:25]

Hubble parameter vs. Hubble "constant"

Neil notes that Wendy keeps specifying "today's" value of H0, implying it was different in the past and raising the question of why it's called a constant[37:38]
Wendy clarifies that H0 denotes the Hubble parameter evaluated at the present time, t=0, while the Hubble parameter itself changes with redshift and time[38:08]
She agrees that the term "Hubble constant" is somewhat confusing, since the underlying parameter evolves[37:44]

Standard cosmological model, dark matter, and dark energy

Contents of the universe in the standard model

Wendy describes the standard cosmological model as an expanding universe containing ordinary matter, dark matter, and dark energy[21:47]
She states that ordinary matter (baryonic matter) makes up about one-sixth of the total matter content of the universe[21:50]
Roughly two-thirds of the universe's energy density is in the form of dark energy, which causes accelerated expansion[22:27]
Wendy notes that dark matter has not been directly detected despite decades of effort; its existence is inferred from its gravitational effects on luminous matter[22:39]
She says dark matter is probably a particle leftover from the Big Bang, and acknowledges that no physical theory yet satisfactorily explains dark energy[21:47]

Room for new physics vs. astrophysical systematics

Wendy agrees that the unknown nature of dark matter and dark energy leaves room for missing elements in the standard model[22:39]
She presents two main possibilities regarding the Hubble tension: it could signal new fundamental physics, or it could reflect underestimated uncertainties and systematic errors in local measurements[15:25]
Wendy personally suspects that underestimated uncertainties and astrophysical issues (e.g., in Cepheids or supernovae) are more likely culprits at present[15:45]

Measurement precision vs accuracy and the role of systematics

Precision vs accuracy analogy

Wendy uses a coin-flip analogy: flipping a fair coin a few times might yield an imbalanced result, but as you increase the number of flips, the distribution approaches 50-50, improving accuracy[32:19]
She explains that random errors average down with repeated measurements, but systematic errors do not; they persist regardless of sample size[33:03]

Example of a systematic: interstellar dust

Wendy describes how Cepheid variables reside in galactic disks where astrophysical dust is present, analogous to how dust or smoke can redden and dim the Sun or a distant mountain on Earth[32:59]
Dust makes stars appear redder and fainter; if uncorrected, this leads astronomers to overestimate distances because the stars look dimmer than they intrinsically are[34:29]
She emphasizes that no matter how many times you re-measure such dust-affected Cepheids, you'll repeat the same biased result unless you correctly model and remove the dust effect[33:53]

Need for multiple independent methods

Wendy argues that using only one distance indicator (e.g., Cepheids) makes it hard to uncover systematics; independent methods are required to cross-check and reveal hidden biases[34:47]
She notes that historically, systematics have repeatedly "come back to bite" cosmological distance measurements, which motivates her focus on redundant methods[35:21]
Neil summarizes that measurements can be very precise yet still wrong if systematics are not understood, and suggests that may be happening in some local H0 determinations[35:08]

Wendy's James Webb Space Telescope program and distance indicators

Collaborative team and proposal scope

Wendy describes her team as a small, efficient group in which each member brings complementary expertise and works hard on a shared JWST proposal[25:48]

Three distance indicators used with JWST

Their JWST program focuses on three distance indicators: Cepheid variables, tip of the red giant branch (TRGB) stars, and carbon stars[26:08]
Cepheids are rare pulsating stars (roughly 1 in 1000 measured stars) whose brightness correlates with pulsation period, a relation first discovered by Henrietta Leavitt[9:13]
The TRGB method is one Wendy and collaborators, including Barry Madore, have refined over the last decade to improve its precision as a distance indicator[26:20]

Physical explanation of the tip of the red giant branch

Wendy explains that stars like the Sun spend most of their lives fusing hydrogen into helium in their cores; later they exhaust core hydrogen and become red giants[27:42]
In red giants, hydrogen burns in a shell around an inert, degenerate helium core, which gains mass and heats up until a thermonuclear runaway helium flash occurs[27:40]
At a well-defined core mass and temperature, this helium flash ignites, moving the star to the horizontal branch in the Hertzsprung-Russell diagram and setting a characteristic luminosity at the TRGB[26:26]
Because this luminosity is well known, astronomers can use the observed brightness of TRGB stars in external galaxies plus the inverse-square law to determine distances[27:25]

JWST distance ladder strategy

Using JWST, the team measures distances to nearby galaxies with all three methods-Cepheids, TRGB, and carbon stars-to see whether they agree and to quantify overall uncertainties[30:19]
Those nearby galaxies host Type Ia supernovae, whose relative distances are well measured but whose absolute luminosities must be calibrated by such local distance indicators[30:59]
Wendy notes they are partway through the project; they aim to test whether all methods converge, if one is an outlier, and how large the real error bars should be[30:21]

Comparing local measurements with cosmic microwave background results

Planck and ground-based CMB as current gold standard

Wendy says the Planck satellite's all-sky CMB measurements remain the gold standard, with two major ground-based experiments (in the Atacama Desert and at the South Pole) producing consistent results[32:35]
She emphasizes that the CMB-based H0 value from these experiments has a precision better than 1%, setting a very high bar for local measurements[32:08]

Model dependence of H0 from the CMB

Wendy clarifies that to extract H0 from the CMB, one must assume a cosmological model and fit it to the observed anisotropy spectrum[33:21]
Given a model, the CMB predicts today's Hubble constant; testing the model then requires an accurate independent measurement of H0 today[33:17]
She underscores that the challenge lies not in the CMB measurements themselves, which are robust, but in verifying whether the underlying model is complete[33:21]

Is the Hubble tension a true crisis?

Wendy's stance on the word "crisis"

Wendy says she dislikes the term "tension" framed as a crisis and sees discrepancies in science as interesting rather than alarming[14:07]
She views the current gap as an opportunity to refine measurements and understand astrophysical systematics rather than an immediate sign that the standard model is broken[14:49]
She quotes Carl Sagan: "Extraordinary claims require extraordinary evidence" and states she does not yet see such extraordinary evidence for new physics here[31:37]

Historical context and expectations

Neil notes that in the history of science, most discrepancies have been resolved with better data and improved understanding of errors, though occasionally they have revealed genuinely new physics[48:55]
Wendy says she would be excited to discover new physics but insists on being convinced by strong, multi-sigma data before accepting it[48:21]

Example of a disappearing CMB "signal"

Wendy mentions that early polarization data from the Atacama Cosmology Telescope hinted at early dark energy that might explain the Hubble tension[42:02]
With more data, this apparent signal vanished, revealing it was merely noise; had it been real, it would have become stronger with improved statistics[41:27]
She cites the 5-sigma standard (roughly a 1 in 1.7 million chance of being wrong) as the typical threshold for discovery claims in this field, and says H0 discrepancies are not yet at that level[41:41]

Constraints from many other cosmological observations

Wendy notes that roughly 1,500 theoretical papers have proposed solutions to the Hubble tension, but none succeed because each tends to conflict with other well-constrained observations[39:59]
She describes the situation as akin to a Rubik's Cube: changing one aspect to fix the tension often breaks agreement with many other established data sets[39:59]

Is the universe locally different? Bubble and anisotropy ideas

Could we live in a special region?

Neil asks if different parts of the universe could have different Hubble constants, perhaps because we live in a giant bubble or near a mass concentration affecting local expansion[38:21]
Wendy acknowledges such ideas were discussed during the 50 vs 100 debates when mass distribution maps were less complete[39:06]
She explains that thousands of supernovae have now been measured across the sky, and these data show no evidence that the local Hubble expansion varies significantly from region to region at the percent level[39:10]

Cosmic Queries: audience questions and answers

Question on future evolution of dark energy and the universe

A questioner asks how little we know about what will happen to the universe if dark energy changes over time and how we might eventually know its fate[47:02]
Wendy replies that we currently know very little about the universe's far-future evolution because we do not yet know whether dark energy is truly constant or evolves with time[47:02]
She emphasizes that these are empirical questions for now: observations must guide us until a good fundamental theory of dark energy emerges[47:12]
Wendy notes that some recent observations suggest dark energy might be decreasing with time, but she cautions that many more planned experiments will be needed to test this[47:23]

Clarifying telescopes associated with dark matter/energy studies

Neil asks about a so-called dark matter telescope, and Wendy identifies the Vera Rubin Observatory as a ground-based survey telescope that just came online[47:42]
She distinguishes it from the Nancy Grace Roman Space Telescope, which is a NASA survey mission in space; he notes that Rubin was originally nicknamed a dark matter telescope[47:57]

Question: does Hubble tension imply missing physics or measurement issues?

A listener asks whether the gap between nearby and early-universe expansion measurements means we are missing something in measurement techniques or whether the cosmological model must change[48:19]
Wendy says that is exactly the question they want to answer and that she is open to either outcome but must be convinced by data[48:25]
She reiterates that at present she is not convinced that there is a genuine crisis or that the standard model is broken[48:43]
Neil adds that most past discrepancies have been solved with better or more data, and only occasionally have such gaps required new physics; he sees Wendy as not yet ready to invoke new physics[48:55]

Question: navigation in an expanding universe

A question from "Jamie and Sabrina from Transylvania" asks how far-future starfarers could keep track of universal expansion and find their way home when home has moved[49:33]
Wendy says that within our Milky Way or nearby galaxies, we would measure the same expansion parameters and the practical issue is mostly academic for the far future[49:51]
She notes that in about 60 billion years, if accelerated expansion continues and dark energy does not decay, other galaxies will recede beyond visibility, limiting cosmological measurements[50:19]
Neil adds an analogy from early marine chronometers, where clock drift was measured and corrected with an equation instead of reopening the mechanism; similarly, future navigators could use an expansion-law equation to backtrack to home coordinates[50:53]

Question: finite vs infinite universe

A listener asks whether Wendy's work would change if we learned the universe is definitely finite or infinite, and which she finds more plausible[52:37]
Wendy answers that her methods and work would not change; she would continue to observe and measure the universe regardless of its global size[52:56]
She says she has no emotional attachment to a finite or infinite universe and is simply curious, appreciating the process of science that lets us ask and approach such questions[52:37]
Neil calls this lack of emotional investment in outcomes a healthy scientific posture and personally says he tends to lean toward an infinite universe because it is more fun conceptually[53:23]
They briefly reference the film "2001: A Space Odyssey" and its phrase "beyond the infinite" and touch on how calculus forces students to grapple with infinity and infinitesimals, with Matt jokingly siding with Zeno's paradox against motion[54:42]

Closing remarks

Acknowledgments and sign-off

Neil thanks Wendy for returning to the show and congratulates her again on the National Medal of Science, noting that it comes with no money but does include another trip to the White House[55:06]
He comments that bringing science into the White House is always good for the country's health, wealth, and security[55:20]
Neil thanks Matt for co-hosting and jokes about him enjoying the cruise while "getting away with it" as a comic[55:42]
Neil closes this Cosmic Queries edition of StarTalk, emphasizing that observational cosmology provides the understanding of the universe people seek, and ends with his usual exhortation to "keep looking up"[55:48]

Lessons Learned

Actionable insights and wisdom you can apply to your business, career, and personal life.

1

Distinguishing precision from accuracy is critical: you can take many careful measurements and get a very tight number, but if systematic errors are unaccounted for, those measurements can still be wrong.

Reflection Questions:

  • • Where in your work or life are you relying on highly consistent feedback without questioning whether there are hidden biases or assumptions driving it?
  • • How could you deliberately look for potential "dust"-systematic distortions-in the data or signals you use to make important decisions?
  • • What is one important metric you trust today that you could audit this week for possible sources of systematic error?
2

Using multiple independent methods to attack the same problem greatly increases confidence in the result and helps uncover hidden weaknesses in any single approach.

Reflection Questions:

  • • For the biggest question you're facing right now, how many truly independent ways do you have of checking whether your current answer is correct?
  • • In what area of your life or business could you add a second, very different measurement or perspective to validate your conclusions?
  • • What concrete step can you take this month to create a "second distance indicator"-an alternative method-to cross-check a key assumption you rely on?
3

Treat apparent crises and surprising discrepancies as opportunities for deeper understanding rather than immediate proof that everything you know is wrong.

Reflection Questions:

  • • When was the last time a surprising result made you jump to a dramatic conclusion instead of first interrogating the data and methods?
  • • How might your response to unexpected setbacks change if you framed them as invitations to refine your models rather than evidence that your whole approach has failed?
  • • What current "crisis" in your work or personal life could you reframe as a signal to improve your measurement, process, or understanding?
4

Staying emotionally unattached to specific outcomes or models allows you to follow the evidence wherever it leads and reduces the temptation to see what you want to see.

Reflection Questions:

  • • Which beliefs or hypotheses are you most emotionally invested in, and how might that investment be skewing your interpretation of new information?
  • • How could you build habits-like explicitly writing down alternative explanations-that make it easier to change your mind when the data point elsewhere?
  • • What is one area where you could consciously adopt a more curious, less attached stance over the next month to improve the quality of your decisions?
5

Extraordinary claims require extraordinary evidence; before embracing radical new explanations, it is wise to exhaust the more mundane possibilities like better data and error control.

Reflection Questions:

  • • Where might you be too eager to attribute a surprising outcome to a dramatic cause instead of first checking for simple, boring explanations?
  • • How can you incorporate stronger thresholds-your own version of a "5-sigma standard"-before you act on or publicize bold conclusions?
  • • What is one current hypothesis you hold that would benefit from deliberately seeking out higher-quality or additional data before you fully commit to it?

Episode Summary - Notes by Sawyer

Solving the Crisis in Cosmology with Wendy Freedman
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