#17 Adam Riess - Modern Cosmology, Hubble Tension, New Physics

Episode Overview

• This episode features an in-depth discussion with Nobel laureate Adam Riess, who discovered the accelerating expansion of the universe.
• The central topic is the "Hubble tension," a major crisis in modern cosmology where measurements of the universe's expansion rate from the early universe disagree with direct measurements of the late universe.
• The conversation explores the role of the James Webb Space Telescope (JWST) in verifying and validating previous measurements from the Hubble Space Telescope, thereby strengthening the case for the tension.
• After years of failing to find a measurement error, the discussion concludes that the Hubble tension is increasingly likely to be a sign of new, undiscovered physics rather than a simple mistake.

Key Concepts

• Accelerating Universe & Dark Energy: The 1998 discovery that the universe's expansion is accelerating, not slowing down due to gravity. This phenomenon is attributed to an unknown force or substance called "dark energy."
• The Hubble Tension: The core conflict in cosmology where "early universe" probes (like the Cosmic Microwave Background) predict a Hubble constant of ~67 km/s/Mpc, while "late universe" direct measurements (using Cepheids, supernovae) consistently yield a higher value of ~73 km/s/Mpc.
• Early vs. Late Universe Probes: The tension is a fundamental disagreement between two different ways of measuring the expansion rate: inferring it from the physics of the very early universe versus directly measuring it in the nearby, modern universe.
• Role of the James Webb Space Telescope (JWST): JWST is not yet statistically competitive with the Hubble Space Telescope due to smaller sample sizes. Its primary current value is in using its superior resolution to verify Hubble's data and rule out potential systematic errors like stellar "crowding."
• Data Consistency: When comparing the same galaxies, measurements from JWST and Hubble show excellent agreement, which increases confidence that the high value for the Hubble constant is not due to an error in the Hubble data.
• Independent Confirmation: New data from other projects, such as the DESI survey's analysis of the "fundamental plane" of elliptical galaxies, independently supports a higher value for the Hubble constant, reinforcing that the tension is a pervasive issue across multiple methods.

Quotes

• At 0:12 - "frankly, one of the most decorated scientists alive today, I think it's fair to say." - The host emphasizes the distinguished status of his guest, Adam Riess.
• At 1:26 - "And that was that the expansion rate of the universe was found to be not just expanding, but actually accelerating in its expansion rate." - Kipping summarizes the core of Adam Riess's Nobel Prize-winning discovery.
• At 3:01 - "Why it is that different people, different telescopes, different methods find different answers for how fast the universe is expanding. Not everybody agrees." - Kipping provides a concise definition of the "Hubble tension," the central topic of the podcast episode.
• At 26:46 - "No. It's still well behind... the uncertainty in the Hubble constant coming from JWST is a factor of two and a half times worse than Hubble's." - Riess explains that despite its power, JWST has not yet "caught up" to Hubble's decades of data collection in terms of statistical precision.
• At 27:09 - "JWST has certain capabilities that allow us to check the Hubble measurements for particular ideas that people had of something that could be a problem." - Riess clarifies that JWST's immediate strength is not in replacing Hubble but in testing its results for potential systematic errors.
• At 28:24 - "it matches what we saw with Hubble for the same galaxies that we observed with Hubble. So when you compare, you know, apples to apples, you get the same thing." - Riess highlights the excellent agreement between Hubble and JWST when observing the same targets, strengthening confidence in the original Hubble measurements.
• At 33:14 - "The difference between 70 and 72 is not even one sigma... So those are not statistically different. That difference is not meaningful." - Riess clarifies that slight variations between late-universe measurement methods are within the margin of error and do not represent a separate tension.
• At 35:40 - "There is no significant difference yet detected between JWST and HST, no matter which method of measuring distances you use. They are all statistically consistent." - Riess summarizes that the new JWST data consistently confirms the results previously found by the Hubble Space Telescope.
• At 37:11 - "It's another way of saying the Hubble tension, but now you're seeing it with a completely different tool, not supernovae, but this fundamental plane from DESI." - Riess explains how a new, independent result reinforces that the tension is a pervasive issue across multiple measurement techniques.
• At 49:06 - "I think something interesting is going on, and I don't consider an error in measurements 'interesting'." - After a decade of intense scrutiny, Riess expresses his growing conviction that the Hubble Tension likely points to new physics.

Takeaways

• The Hubble Tension represents a genuine and deepening crisis, not just a simple disagreement between two experiments, but a fundamental conflict between our models of the early and late universe.
• Instead of resolving the tension, the powerful new JWST has so far only increased confidence in the Hubble data by ruling out previously suspected systematic errors.
• The growing body of evidence from diverse and independent sources all points towards the same discrepancy, making a simple measurement error an increasingly unlikely explanation.
• After a decade of intense investigation has failed to uncover a "smoking gun" error in the data, the scientific community is being pushed to consider more exotic explanations.
• The solution to the Hubble tension may require a fundamental revision of the standard model of cosmology, potentially altering our understanding of dark energy or the physics of the early universe.
• The ongoing mystery highlights that even foundational concepts in cosmology, like the rate of universal expansion, are still subjects of active and potentially revolutionary research.