The Crisis in Cosmology

Cosmology is facing a direct, high-precision standoff over one number that ties the universe’s past to its future: the Hubble constant (H0), the present-day rate of cosmic expansion. Two of the most powerful measurement approaches—one anchored in the nearby universe using Type Ia supernovae, the other inferred from the early universe using the Cosmic Microwave Background (CMB)—produce values that differ more than random errors should allow. The disagreement is small in absolute terms, but large in statistical weight, and it’s forcing scientists to scrutinize both measurement systematics and the underlying physics used to interpret them.

The supernova route refines H0 by building the “cosmic distance ladder.” Distances to host galaxies are calibrated using Cepheid variable stars, whose pulsation periods map to intrinsic brightness via the period–luminosity relation. That calibration depends on nearby Cepheids with distances determined through stellar parallax, and uncertainties compound step by step. Type Ia supernovae then extend the ladder far beyond Cepheids: these explosions have highly predictable luminosities because runaway fusion in white dwarfs produces detonations with consistent brightness. The SHOES project (Supernovae H0 for the Equation of State) uses the Hubble Space Telescope to improve the calibration by matching older supernova observations with newer, more reliable Cepheid measurements. Recent results narrow H0 to about 73.5 ± 1.7 km/s/Mpc, implying a faster expansion rate in the modern universe.

The early-universe route uses the CMB instead of the distance ladder. The CMB is the leftover glow from roughly 400,000 years after the Big Bang, when the universe cooled enough to become transparent to light. The Planck satellite maps tiny temperature variations across the sky, and those “speckles” encode the physics of baryon acoustic oscillations—sound-wave-like oscillations in the coupled matter–radiation plasma. Once light decouples, the oscillation pattern freezes into the CMB’s angular power spectrum. By fitting the observed spectrum with cosmological models, the Planck team infers parameters that include the expansion rate, yielding H0 ≈ 66.9 ± 0.6 km/s/Mpc.

The two values—73.5 ± 1.7 versus 66.9 ± 0.6—are separated by about 3.7 standard deviations, corresponding to a roughly 1 in 7,000 chance of arising from random measurement error alone. The discrepancy first became prominent in 2016 after Riess’s updated supernova calibration sharpened the supernova-based H0 upward relative to earlier Planck-based estimates. Since then, calibrations and cross-checks have improved, and the error bars have generally tightened rather than disappearing.

Two broad explanations remain on the table. One possibility is hidden systematic error: biases in Cepheid behavior, supernova calibration, or how Planck’s CMB speckles are affected by effects like gravitational lensing. The other possibility is new physics not included in the standard CMB interpretation—such as additional fast-moving particles (e.g., sterile neutrinos), altered dark matter properties, or dark energy that evolves rather than behaving like a constant cosmological constant. Future measurements—using independent probes like gravitational lensing and gravitational waves—are expected to decide whether the tension will converge or persist, potentially reshaping ideas about dark matter, dark energy, and even particles beyond the Standard Model.