118 | Adam Riess on the Expansion of the Universe and a Crisis in Cosmology

Episode Summary

Executive Summary: Sean Carroll interviews Adam Rees about cosmology, focusing on how the universe expands, how distances are measured, and why the Hubble tension matters. They revisit the 1998 discovery of cosmic acceleration, then explain the modern discrepancy between local measurements of the Hubble constant and values inferred from the cosmic microwave background. The conversation emphasizes that this is an active, unresolved problem that may point to new physics or hidden systematics.

Main Topics: Expansion of the universe and observational evidence (Priority: 5/5): Rees explains why cosmologists know the universe is expanding: redshift, time dilation in distant supernovae, and the hot, denser early universe inferred from observations. Distance ladder and standard candles (Priority: 5/5): The discussion covers parallax, Cepheid variables, and Type Ia supernovae as successive rungs in the distance ladder used to measure cosmic distances and the Hubble constant. Discovery of accelerating expansion (Priority: 5/5): Rees recounts how supernova data in the 1990s unexpectedly implied a repulsive component consistent with a cosmological constant, leading to the discovery of dark energy. Cosmic microwave background as a predictive tool (Priority: 5/5): They explain how CMB fluctuations constrain early-universe parameters and, through the Friedman equation, predict the present-day expansion rate. The Hubble tension (Priority: 5/5): The main modern anomaly is that local measurements of the Hubble constant are around 72–73 km/s/Mpc while CMB-based inference gives about 67 km/s/Mpc, with the gap now several sigma. Possible resolutions: early-universe new physics vs systematics (Priority: 4/5): The speakers discuss ideas such as extra neutrinos, early dark energy, or modified early-universe physics, while noting that systematic errors remain possible but increasingly strained. Broader cosmological tensions and future observations (Priority: 4/5): They also discuss the sigma-eight clumpiness tension, black-hole dark matter ideas, and upcoming data from Hubble, JWST, CMB experiments, and standard sirens.

Key Arguments: The universe is expanding, and this is supported by multiple independent observations, including redshift and supernova time dilation. Distance measurement in astronomy is difficult, so cosmologists use a carefully calibrated ladder from parallax to Cepheids to Type Ia supernovae. Type Ia supernovae are useful standardizable candles because white dwarfs near the Chandrasekhar limit produce remarkably uniform explosions. The 1998 acceleration result was a major surprise, but it fit immediately into an existing theoretical framework via the cosmological constant. The Hubble tension is more troubling because there is no obvious, widely accepted theory that explains the discrepancy. CMB measurements do not directly measure today’s Hubble constant; they infer it by fitting early-universe physics and evolving that model forward in time. Both local and early-universe determinations have been checked with multiple methods, making a single simple systematic error less convincing. The late universe seems harder to blame, because multiple independent late-time probes agree with one another and with an equation-of-state close to -1. A possible resolution may lie in the early universe, where additional components or interactions could alter the sound horizon and inferred Hubble constant. Another anomaly, sigma-eight clumpiness, may or may not be connected to the Hubble tension, and a single theory may have to explain both if the issue is fundamental.

Data Points: Chandrasekhar limit: about 1.4 times the mass of the Sun - Threshold at which a white dwarf can undergo runaway thermonuclear explosion in a Type Ia supernova Hubble constant from early-universe inference: about 67 ± 0.5 km/s/Mpc - Value predicted by the standard cosmological model fit to the CMB Hubble constant from local measurements: about 70–75 km/s/Mpc; average around 73 ± 1.5 km/s/Mpc - Direct late-time measurements using Cepheids and Type Ia supernovae Tension significance: about 4–6 sigma - Current discrepancy between early- and late-universe Hubble constant determinations Type Ia supernova distance precision: about 6% per object - Precision for individual calibrated supernovae in the local distance ladder Sample size for supernova calibration: about 38 objects - Complete sample of Type Ia supernovae calibratable with Cepheids Target precision for Hubble constant: about 1% - Holy grail level for local Hubble constant measurement Earlier Hubble constant uncertainty: 50 vs 100 - Historical uncertainty when Sean Carroll and Adam Rees were graduate students Dark matter / dark energy composition: about 30% matter and 70% dark energy - Approximate energy budget from the accelerating-universe era and cosmological model Sigma-eight tension: about 3 sigma - Clumpiness discrepancy between lensing/velocity measurements and early-universe predictions Age of supernova timing stretch: 20 days to maximum; observed stretched to up to double or triple - Evidence for expansion via time dilation in distant Type Ia supernovae Type Ia supernova rate in a Milky Way-like galaxy: about one per 100 years - Explains why nearby calibration objects are rare Calibration volume for local supernova ladder: 40–50 megaparsecs - Reach of Hubble-based Cepheid calibration for local supernova hosts CMB-era content of the universe: mostly matter, neutrinos, and radiation - Early-universe composition before dark energy dominates later times Late-universe content of the universe: mostly dark energy and a fair bit of matter - Present-day cosmic composition Neutrino mass description: very little mass - Neutrinos are relativistic-like in the early universe and become less important later

Pivotal Quotes: "if the supernova doesn't know how old it is in its galaxy, how does it know how old it is in the universe?" — Adam Rees: Explaining why Type Ia supernovae in different host galaxies appear to be reliable standardizable candles "we made the mistake of continuing to improve the precision of all the measurements" — Adam Rees: A humorous framing of how better data exposed the Hubble tension "We are interested in why we don't get a match between these two ways of looking at things." — Sean Carroll: Clarifying that the central scientific goal is not the numerical value itself but the mismatch

Implications: The Hubble tension may signal missing early-universe physics, hidden systematics, or a deeper flaw in cosmology. Upcoming surveys, CMB data, and gravitational-wave standard sirens should sharpen the test and may force a revision of the standard model.