There is a crisis unfolding in the field of cosmology.

Most measurements of the current acceleration of the universe (called the Hubble constant, or H₀) based on stars and other objects relatively close to Earth give a rate of 73 km/s/Mpc. These are referred to as “late-time” measurements. On the other hand, early-time measurements, which are based on the cosmic microwave background emitted just 380,000 years after the Big Bang, give a smaller rate of 68 km/s/Mpc.

They can’t both be right. Either something is wrong with the standard cosmological model for our universe’s evolution, upon which the early-time measurements rest, or something is wrong with the way scientists are working with late-time observations.

Climbing the Distance Ladder

Most of the late-time measurements of H₀ use “distance ladders” to measure cosmic distances further and further outward into the universe. One of the most prolific distance-ladder collaborations is SH0ES (Supernovae and H₀ for the Equation of State of dark energy), which Adam Riess (Johns Hopkins University and the Space Telescope Science Institute) has led for nearly two decades.

The first rung in the SH0ES method uses geometric parallax to double-check the distance to Cepheid variable stars in our galaxy, for which astronomers can also measure distance using their brightness variations. The second rung then compares Cepheids against Type Ia supernovae, another “standard candle” like Cepheids that astronomers can see to greater distances. The third rung compares distances based on supernovae and redshift measurements.

In a Zoom webinar on December 9th, Dan Scolnic (Duke University) announced, on behalf of a collaboration between SH0ES and another group, Pantheon+, that the teams had obtained a new late-time H₀ measurement with the smallest uncertainty yet. The result is posted on the arXiv preprint server. After much data collection and analysis, the teams still find the universe’s expansion is accelerating at a high present-day rate between 72 and 74 km/s/Mpc — a much smaller range than obtained from their previous late-time measurements.

The Hubble Tension is Real

The Pantheon team complemented the SH0ES team’s work by performing a meta-analysis of supernovae surveys, correcting for the inconsistencies that can crop up during the use of different instruments, baselines, and calibration methods. The SH0ES team then used this updated information, along with new Cepheid sightings from the Hubble Space Telescope, to take a closer look at their previously established distance ladder methods.

While the additional data helped reduce the range of possible H₀ values from the team’s calculations, the systematic study of the methods involved is what really sets this study apart from previous ones.

“They’ve done a more complete and thorough cross correlation of terms between the different aspects of the distance ladder,” says Suhail Dhawan (Cambridge University, UK), who was not on either team.

The researchers set up about 70 different scenarios in which they changed the way they added things up along the distance ladder in order to measure systematic error. Small uncertainties can add up in big calculations in a way similar to the “butterfly effect.” Many have postulated that distance-ladder measurements are prone to systematic errors but understanding those systematics has been difficult. Using their dozens of scenarios, the SH0ES team determined the effect any particular error or combination of errors might have on the final Hubble constant calculation.

Thanks to the additional data and analysis, the results breach the “five-sigma threshold,” meaning there is only a 1 in 1 million chance that the discrepancy between late-time and early-time measurements arise from systematics.

In short, the so-called “Hubble tension” seems to be real. And it is looking more and more like something missing or wrong in the standard model of cosmology is causing the difference between early- and late-time measurements. The search is on for such new physics: the discovery of some as yet unknown law, particle, or property that’s causing these disparate measurements of the universe’s current acceleration.

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