In his new paper in Nature Astronomy, Taylor Hoyt – a researcher in Berkeley Lab’s Physics Division – presents a new approach to calibrating the measurement of the universe’s present day expansion rate.

Hoyt, a physicist in the Supernova Cosmology Project who studies the extragalactic distance scale, has taken a deep dive into various datasets and examined systematic errors that had previously not been taken into consideration when measuring the brightness of red giant stars. These stars are used in a distance measurement technique called the ‘tip of the red giant branch method’ (TRGB), or ‘red giant method,’ a primary astronomical distance indicator that uses the luminosity of the brightest red giant branch stars in a galaxy to gauge the distance to that galaxy.

In 2019, a group at the University of Chicago (Freedman et al.) released a new measurement of the universe’s present day expansion rate – the Hubble constant – using the TRGB, yielding results that were broadly consistent with the standard cosmological model. Their calibration of the TRGB was based on measurements made in the Small and Large Magellanic Clouds (SMC and LMC), a pair of galaxies nearest our own Milky Way. That calibration has since been challenged because identifying the TRGB in the LMC is not a straightforward task. This is due to its recent gravitational interaction with the SMC, which stirred up gas and dust, and initiated the formation of new stars – all factors that can introduce systematic uncertainties into TRGB measurements, and were thus driving the dispute over the method’s calibration.

After a close reinspection of all the data, Hoyt found consistency with the 2019 calibration, and that the alternative calibrations that challenged the 2019 result contained biases. According to Hoyt, “Circumventing these biases led to a number of striking new results, such as a sub-per-cent measurement of the red giant stars’ distinctive signal (a factor of two improvement in precision over our 2019 measurement of the LMC’s TRGB), a verification that the brightness of these red giant stars is a deterministic function of their observed colors; as well as a first-ever measurement of the LMC’s three-dimensional tilt on the sky using only the TRGB.” This newest calibration of the TRGB method from Hoyt suggests a value of the Hubble Constant that is in closer alignment with the standard model of physics than other determinations, and strengthens the 2019 Freedman et al. findings.

Hoyt plans to continue this work using a library of high-quality datasets from the Nearby Supernova Factory. These new measurements will enable the collaboration to develop a new, more accurate way of measuring the Hubble constant.

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Sub-per-cent determination of the brightness at the tip of the red giant branch in the Magellanic Clouds
March 16, 2023 / Taylor Hoyt / Nature Astronomy