This is the hypothesis put forward by a theoretical physicist from the University of Geneva (UNIGE) to solve a conundrum that has been splitting the scientific community for a decade: at what speed is the universe expanding? Until now, at least two independent calculation methods have arrived at two values that are different by about 10% with a deviation that is statistically irreconcilable.
This new approach, which is set out in the journal Physics Letters B ("Consistency of the local Hubble constant with the cosmic microwave background"), erases this divergence without making use of any “new physics”.
The universe has been expanding since the Big Bang occurred 13.8 billion years ago – a proposition first made by the Belgian canon and physicist Georges Lemaître (1894-1966), and first demonstrated by Edwin Hubble (1889-1953). The American astronomer discovered in 1929 that every galaxy is pulling away from us, and that the most distant galaxies are moving the most quickly. This suggests that there was a time in the past when all the galaxies were located at the same spot, a time that can only correspond to the Big Bang.
This research gave rise to the Hubble-Lemaître law, including the Hubble constant (H0), which denotes the universe’s rate of expansion. The best H0 estimates currently lie around 70 (km/s)/Mpc (meaning that the universe is expanding 70 kilometres a second more quickly every 3.26 million light years). The problem is that there are two conflicting methods of calculation.
Sporadic supernovae
The first is based on the cosmic microwave background: this is the microwave radiation that comes at us from everywhere, emitted at the time the universe became cold enough for light finally to be able to circulate freely (about 370,000 years after the Big Bang).
Using the precise data supplied by the Planck space mission, and given the fact that the universe is homogeneous and isotropic, a value of 67.4 is obtained for H0 using Einstein’s theory of general relativity to run through the scenario.
The second calculation method is based on the supernovae which appear sporadically in distant galaxies. These very bright events provide the observer with highly precise distances, an approach that has made it possible to determine a value for H0 of 74.
Lucas Lombriser, a professor in the Theoretical Physics Department in UNIGE’s Faculty of Sciences, explains: “These two values carried on becoming more precise for many years while remaining different from each other. It didn’t take much to spark a scientific controversy and even to arouse the exciting hope that we were perhaps dealing with a ‘new physics’.”