The universe has expanded rapidly since the Big Bang launched the whole creation 13.8 billion years ago, but astronomers have never agreed on how fast the expansion rate is. First, it was the Belgian physicist Georges Lemaître (1894 - 1966), who proposed the theory that the universe is expanding in all directions and could be traced back to a single point; a theory that was later demonstrated for the first time by Edwin Hubble (1889 - 1953). Hubble discovered in 1929 that each galaxy moves away from Earth and that more distant galaxies move faster. Meanwhile, there was a time in the past when all galaxies were located in the same place, something that can only be associated with the Big Bang. His research resulted in the law of Hubble-Lemaître, including the Hubble constant (H0), a unit that describes how fast the universe expands to different distances from a particular point in space.
Now, a team of physicists from the University of Geneva (UNIGE) claim to have resolved this complex cosmic debate.
No need for "new physics"
The calculation techniques used so far had given different values with respect to the rate of expansion of the universe, with a considerable deviation. Both methods are opposite, since the first is based on supernovae that appear sporadically in distant galaxies and the other in the cosmic microwave background. The first gave an estimate of 67.4 H0 and the second 74 H0 (km/s/parsec), or the universe is expanding at an average of 70 kilometres per second faster per 3.26 million light years.
"These two values remained more precise for many years without ceasing to be different from each other," explains Lucas Lombriser, assistant professor in the Department of Theoretical Physics at the Faculty of Sciences in Geneva in the magazine Physics Letters B that published the study.
Lombriser argues that the universe is not as uniform as we thought and that there is a large part of the cosmos in a kind of cosmic "bubble" that has skewed the calculations of the past. This bubble extends over 250 million light-years and contains the Milky Way and a few thousand nearby galaxies. Experts believe that the matter inside this bubble is at least half the size of the rest of the universe.
"If we were in a kind of gigantic bubble where the density of matter was significantly lower than the known density of the universe, it would have consequences in the distances of supernovae and ultimately in the determination of H0," explains Lombriser.
All that would be required would be for this "Hubble bubble" to be large enough to include the galaxy that serves as a resource for estimating distances. By establishing a diameter of 250 million light-years for this bubble, the physicist calculated that if the density of this matter were 50% lower than for the rest of the universe, a new value would be obtained for the Hubble constant, which is finally, according to that obtained thanks to the diffuse cosmic background.
"The probability of such a fluctuation on this scale is one in 20, or even one in five, specified Lucas Lombriser. So it’s not the fantasy of a theoretician. There are many regions like ours in the vast universe,” he explained.
Reference: Lucas Lombriser. Consistency of the local Hubble constant with the cosmic microwave background, Physics Letters B (2020). DOI: 10.1016/j.physletb.2020.135303