Measuring cosmic distances is the central bottleneck of cosmology, solved by a layered 'distance ladder': parallax for nearby stars, Cepheid variable stars (via Leavitt's period-luminosity relation) for intermediate distances, and Type Ia supernovae as standard candles for the billions-of-light-years distances needed to trace the universe's expansion history.
Riess walks through why each successive rung was necessary: parallax fails once the baseline (Earth's orbit) becomes imperceptibly small relative to distance, and Cepheid variables, while visible in nearby galaxies, are not luminous enough to be seen at cosmological distances. Type Ia supernovae, billions of times brighter than the sun for a few weeks, are the only objects bright enough and uniform enough to close the gap.
cosmic-distance-ladder
Type Ia supernovae work as reliable standard candles because they are not random explosions but a specific physical trigger: a white dwarf star accreting matter until it crosses the Chandrasekhar limit (about 1.4 solar masses), a mechanism predicted theoretically by Chandrasekhar in the 1930s and only confirmed useful for cosmology roughly 50 years later.
Riess distinguishes this from the far more variable supernovae produced by collapsing massive stars. Because Type Ia explosions are triggered at nearly the same mass every time, their peak brightness is uniform enough to infer distance, though in practice this required additional corrections for dust extinction and small brightness variations tied to how quickly the light rises and falls.
cosmic-distance-ladder
In 1998, Riess's High-Z Supernova team found the universe's expansion is accelerating, not slowing, which directly contradicted the expected result given the gravitational pull of ordinary matter, and forced a revival of Einstein's discarded 'cosmological constant' under a new name: dark energy.
The team set out to measure how much the expansion was decelerating due to gravity. Instead, the data showed acceleration at every level of cross-checking. The only physics that could explain a repulsive form of gravity was Einstein's cosmological constant, which he had called his 'greatest blunder' after Hubble proved the universe wasn't static.
dark-energy
Riess's first reaction to the accelerating-universe result was fear and suspicion of an analysis bug, not excitement, and the result only became credible after weeks of internal cross-checking and independent confirmation from a rival team measuring the same thing with different data.
He describes spending weeks re-checking his simulations for errors and sending incremental results to teammate Brian Schmidt for independent verification rather than trusting his own analysis. Confidence increased further when a competing research group, using its own telescope time and different supernovae, reported the same acceleration within months, which Riess frames as the actual mechanism by which science builds trust in a surprising result.
scientific-epistemics
Quantum theory's naive prediction for the energy density of empty space (dark energy) is about 120 orders of magnitude larger than what is actually observed, which Riess calls 'the worst prediction in all of physics' and treats as evidence of a real, unresolved incompatibility between quantum theory and general relativity.
If the naive quantum calculation were correct, the resulting repulsive force would have prevented gravity from ever forming galaxies, stars, or planets. Riess says physicists have no choice but to admit they do not understand how the quantum vacuum relates to gravity, since neither zero nor the huge predicted value matches what is observed.
dark-energy
The 'Hubble tension' is a roughly 10-percent, statistically significant (about five-times the margin of error) mismatch between the expansion rate implied by the early-universe cosmic microwave background (67 plus or minus 0.5) and Riess's direct present-day measurement via the distance ladder (73 plus or minus 1), and it persisted even after switching to the more precise James Webb Space Telescope.
Riess emphasizes that in physics, a five-sigma-scale mismatch is not an acceptable rounding error the way it might be treated in social science; it signals either an uncorrected systematic error or genuinely new physics. That the discrepancy survived an instrument upgrade (JWST) rather than shrinking is what makes him take it seriously as a real anomaly rather than noise.
hubble-tension
Riess treats persistent unexplained discrepancies as clues rather than problems to explain away, drawing an explicit historical parallel to Mercury's orbital precession: astronomers wrongly inferred a missing planet ('Vulcan') the way they had correctly inferred Neptune from Uranus's orbit, but the actual fix required an entirely new theory of gravity (Einstein's general relativity), not a new object.
He uses this history to frame the Hubble tension as genuinely ambiguous in the same way: it could be a small tweak to the Lambda-CDM model, or it could point toward missing physics as fundamental as the shift from Newton to Einstein. He says his own role is limited to making the best possible measurement, not to guessing which outcome is correct in advance.
hubble-tension
The Lambda-CDM model's assumption that dark energy's density has stayed constant throughout the universe's history is adopted mainly for simplicity and elegance, not because of strong theoretical justification, and some newer observations hint dark energy might instead behave like a decaying field.
Riess compares the 'constant' assumption to Einstein's original static-universe assumption: convenient, but not derived from first principles. He notes that if dark energy is instead the energy of an evolving field (analogous to a magnetic or electric field), its density would change over time, and recent data have started to suggest exactly that kind of variation.
dark-energy
Riess deliberately avoids reading media coverage of the Hubble tension because he believes framing scientific disagreement as a personal rivalry between researchers misrepresents how the field actually resolves disputes: through published data, conference discussion, and repeated independent checking, not personalities.
He describes science as comparatively well-functioning in a polarized media environment specifically because disagreements get settled by data rather than by which scientist is more persuasive or sympathetic. He states plainly that 'the universe doesn't care what we think about it,' and that reporters' narrative framing around personal conflict misses the actual epistemic process.
scientific-epistemics
Only about 4 percent of the universe's total energy content is the ordinary matter that makes up stars, planets, and everything directly observed by humans; the remaining 96 percent is dark matter and dark energy, neither of which interacts detectably with light.
Riess notes dark matter is inferred only indirectly, from its gravitational effect on how fast stars orbit within galaxies, since it does not emit, absorb, or scatter light and has evaded direct laboratory detection despite dedicated underground experiments using materials like liquid xenon.
dark-energy
In the listener-question segment, Levitt advises against uncapped, ascending-bid real estate auctions abroad by minimizing perceived bidding room, avoiding direct competition where possible, and exploiting the real estate agent's own private incentives, since auctions mechanically transfer surplus from buyers to sellers.
His three concrete tactics: seek out properties with no competing bidder, signal falsely or truthfully that you are at your absolute limit so the agent cannot extract more, and appeal to the agent's non-price incentives (a fast, certain, all-cash deal, or in some jurisdictions a personalized letter to the seller) since the agent, not just the seller, has interests that can be worked.
negotiation-strategy
Media referenced
Science Magazine's Breakthrough of the Year (1998) - article - Science Magazine named the High-Z Supernova team's accelerating-universe finding its Breakthrough of the Year, a sign the result was credible enough to take seriously before later confirmation from cosmic microwave background data
Companies
High-Z Supernova Search Team - The research collaboration Riess was part of that discovered, in 1998, that the universe's expansion is accelerating rather than slowing; led by Brian Schmidt, with whom Riess later shared the Nobel Prize
Hubble Space Telescope - The primary instrument behind most of Riess's distance measurements, including the original 1998 result and his later work on the Hubble constant and the Hubble tension
James Webb Space Telescope - A newer, higher-resolution telescope Riess used to re-check the Hubble tension; the discrepancy persisted even with JWST's improved precision, strengthening the case that it is a real effect rather than measurement error
Gaia - European Space Agency telescope Riess cites as part of the modern toolkit, along with Hubble and JWST, used to make increasingly precise measurements of the present-day expansion rate
Techniques and frameworks
Cosmic distance ladder - The layered method astronomers use to measure distance at increasing scales: parallax for nearby stars, Cepheid variable stars for galaxies within reach of large telescopes, and Type Ia supernovae as standard candles for cosmological distances too far for any other method
Type Ia supernovae as standard candles - Riess's core measurement tool; these supernovae are the explosion of a white dwarf star that has accreted matter until it crosses the Chandrasekhar mass limit (about 1.4 solar masses), making their peak luminosity uniform enough to infer distance from brightness
Cepheid variable period-luminosity relation (Leavitt's Law) - Henrietta Leavitt's discovery that a Cepheid star's luminosity correlates with the period of its brightness variation, which let astronomers infer distance from a star's blinking rate; this was the key that let Hubble prove the universe is expanding
Lambda-CDM model - The standard cosmological model built from Riess's 1998 result: Lambda for dark energy (assumed constant, following Einstein's cosmological constant) plus cold dark matter (CDM); it fits most cosmological data but is now strained by the Hubble tension
Trust but verify approach to cosmology - Riess's stated epistemic stance toward the Lambda-CDM model: treat it as a useful, well-supported framework, but take any persistent discrepancy between its predictions and direct measurement seriously as a possible clue that the model is incomplete
Summary
This episode centers on astrophysicist Adam Riess, who twice upended cosmology: first in 1998 as part of the High-Z Supernova team that discovered the universe's expansion is accelerating rather than slowing, and again more recently through his work on the "Hubble tension," a persistent mismatch between two different ways of measuring how fast the universe is expanding today. Levitt structures the conversation as a build-up through the history of cosmological distance measurement (parallax, Cepheid variable stars via Henrietta Leavitt's period-luminosity relation, and finally Type Ia supernovae as standard candles) before getting to Riess's own results, using accessible analogies throughout, a raisin-bread loaf for the expanding universe, a lighthouse for standard candles, an ambulance siren for redshift.
The heart of the episode is the 1998 discovery: measuring the deceleration expected from gravity, Riess's team instead found the universe's expansion is speeding up, a result so unexpected that Riess's first instinct was fear that he had made an analysis error, not excitement. Only after weeks of internal cross-checking and independent confirmation from a competing research team did the finding become credible, eventually earning Riess and Brian Schmidt the Nobel Prize and reviving Einstein's discarded "cosmological constant" under the name dark energy. Riess is candid about how badly quantum theory predicts dark energy's actual magnitude, calling it "the worst prediction in all of physics," a roughly 120-order-of-magnitude miss that exposes real, unresolved tension between quantum theory and general relativity.
The second half turns to Riess's current work on the Hubble tension: a roughly 10-percent, statistically significant discrepancy between the expansion rate implied by the early-universe cosmic microwave background (67 plus or minus 0.5) and his own direct present-day measurement using the distance ladder (73 plus or minus 1). Crucially, this gap persisted even after upgrading from the Hubble Space Telescope to the more precise James Webb Space Telescope, which is what makes Riess take it seriously as a possible sign of missing physics rather than instrument error. He draws a pointed historical parallel to Mercury's orbital precession, an anomaly 19th-century astronomers wrongly attributed to a missing planet before Einstein showed the real fix required a wholly new theory of gravity, framing the Hubble tension as similarly ambiguous between a minor patch and something revolutionary. He also reflects on deliberately avoiding media coverage of the debate, arguing that narratives about personal rivalry between scientists distort how science actually resolves disagreement.
The closing listener-question segment shifts entirely, with producer Morgan Levey relaying a question from a listener navigating an uncapped, ascending-bid real estate auction abroad. Levitt offers three tactics rooted in auction theory and negotiation: avoid direct competition with other bidders where possible, control what signals you send about how close you are to your limit, and remember the real estate agent has private incentives of their own that can be worked separately from the seller's.
Notable Quotes
"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 of your experiment. In fact, it's not just not okay, we get very excited." - Adam Riess
"It's often called, 'the worst prediction in all of physics.'" - Adam Riess, on quantum theory's estimate of dark energy's magnitude
"The universe doesn't care what we think about it. And we shouldn't care what specific people think about it either." - Adam Riess
"Sometimes you pull a loose thread on a sweater and you pull off that thread and other times it could unravel the sweater." - Adam Riess, on whether the Hubble tension requires a small fix or a new theory
"In economics, when there's a 10-percent difference between two sets of estimates, we call that a successful replication and we go out for a beer to celebrate." - Steve Levitt