According to scientists, the Hubble constant, a fundamental cosmological parameter that measures the universe's rate of expansion, is not actually constant. The discovery comes from new research published in The Astrophysical Journal and carried out by an international team comprising researchers from Sapienza University, the National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), the universities of Pisa, Salerno and Michigan, coordinated by the National Astronomical Observatory in Japan. 

According to the study, the value of the constant is not fixed but evolves according to the cosmological distance of the object being studied. The scientists came to this conclusion on the basis of the 'Hubble constant tension', i.e. a discrepancy between independent measurements of the universe's expansion rate. Specifically, the values that are not statistically compatible are the data provided by satellites for the study of 'primordial fossil radiation' (a map of the residual light from the very first photons released into the universe, only 380000 years after the Big Bang) and those received from Type Ia supernovae (SNe Ia) (violent stellar explosions, which in 1998 led to the discovery that the universe is expanding at an accelerated rate).

Knowing the value of the Hubble constant is crucial because, together with other cosmological parameters, it serves to estimate, for example, the age of the universe (about 13.8 billion years), the distance to faraway objects and the ultimate fate of the expansion of the universe.

"To test whether Hubble's constant is indeed constant," says Giovanni Montani of the Department of Physics at Sapienza University of Rome, "we took a sample of SNe la and sorted it according to its distance from us, which we then divided into many small intervals. Then, through intensive data analysis in our laboratories, we obtained estimates of the Hubble constant within each interval. Hence the surprise: the Hubble constant seems to show a slow and unexpected evolution as the distance varies, which would explain the discrepancy between the independent measurements, primordial fossil radiation and SNe Ia."

The strain on the Hubble constant is a topic of considerable interest since it could signal that something is wrong with the current standard cosmological model. If the observed effect is not due to systematic effects of unconsidered Type la supernovae, then the possible decrease in the Hubble constant represents a symptom of a hidden evolutionary effect. That opens the way to multiple interpretive scenarios in which modified gravity models, for example, could be tested.

References:

On the Hubble Constant Tension in the SNe Ia Pantheon Sample - Maria G. Dainotti, Biagio De Simone, Tiziano Schiavone, Giovanni Montani, Enrico Rinaldi, Gaetano Lambiase - The Astrophysical Journal 2021  DOI: 10.3847/1538-4357/abeb73

Further Information

Giovanni Montani 
Department of Physics
giovanni.montani@uniroma1.it