Towards easing the Hubble constant tension?

Astronomical observations suggest that we are living in a ΛCDM universe, which is made of three major ingredients: the cosmological constant (Λ) associated with the dark energy (70%), the cold dark matter (CDM; 26%) and ordinary matter (4%). Most recent measurements seem to confirm this model with an interesting exception: the value of the Hubble constant, which quantifies the current expansion rate of the universe.

The most recent observations of the cosmic microwave background radiation, the primordial radiation residual of the Big Bang, carried out by the Planck satellite, predict that the universe should be currently expanding at the rate of H0=67.4 km/s/Mpc. On the other hand, a team led by Adam Riess, Nobel laureate in physics in 2011, has recently found H0=73.2 km/s/Mpc using direct measurements of distances derived from observations of Cepheids and type Ia supernovae in the nearby universe. Taking both results at face value, the measurement of the expansion rate of the universe based on observations related to the “young” universe is a full 10% smaller than the rate derived from observing distance indicators in the nearby (i.e. “old”) universe. Both measurements are so precise that there is only a 1 in 100 000 chance that the discrepancy between the two values of the Hubble constant happens simply as a statistical fluke. Why does the "old" universe seem to expand faster than the "young" universe ? What lies behind this “tension”: a fascinating new exotic physics or more “trivial” hidden systematic uncertainties? These unresolved questions continue to stimulate the work of cosmologists. 

An international team of scientists including researchers from DARK (Niels Bohr Institute), funded by grants from the VILLUM FOUNDATION to Christa Gall, Jens Hjorth, and Adriano Agnello, and led by Nandita Khetan, PhD student at the Gran Sasso Science Institute, has proposed to change the methodology for calibrating distances to type Ia supernovae by using the surface brightness fluctuations (SBF) method rather than Cepheids. “After using a calibration sample of 24 supernovae which exploded in galaxies with distances estimated using SBF, the team found a Hubble constant of H0 = 70.5 km/s/Mpc lying between the value obtained with the supernovae calibrated with Cepheids and the value obtained from the cosmic microwave background“ - says Nandita Khetan - “and it is consistent within the uncertainties with both the expansion rate measured by Planck and by type Ia supernovae calibrated with Cepheids”. 

Although the new determination of the Hubble constant does not resolve the Hubble constant tension, it demonstrates the importance of employing different techniques and observations to verify the robustness of the expansion rate measurement in the nearby universe. The new proposed calibration method does not only set an independent absolute distance scale for type Ia supernovae, but it also probes different supernova environments than those from the Cepheid calibration (while Cepheids are found in spiral galaxies with young stellar populations, the SBF measurements are primarily evaluated for elliptical galaxies with old stellar populations). “With this result in mind, the tension in the Hubble constant measurements could still hide some systematic uncertainties of astrophysical interest such as differences in the properties of the progenitors of type Ia supernovae belonging to different parent stellar populations or a different dust extinction at the location where supernovae have been observed” according to Luca Izzo, researcher at DARK.