💫 A new method for measuring the expansion of the Universe, and solving the "Hubble tension"

How can the Universe be expanding at two distinct rates? This enigma, named the Hubble tension, has puzzled astronomers for several years.

The origin of this tension lies in the discrepancy between two calculation methods. The first, based on the observation of exploding stars in the relatively nearby Universe, provides a certain value for the expansion rate. The second, relying on the analysis of the ancient cosmos via the cosmic microwave background, gives another, significantly lower one. This persistent difference suggests that an element might be missing from our narrative of cosmic evolution.


A team of researchers is now proposing a new lead: the use of gravitational waves. These oscillations of spacetime, predicted by Einstein and experimentally confirmed in 2015, are born from collisions of very massive objects like black holes. They could enable a new, independent measurement of the expansion rate.

This approach, dubbed a "stochastic siren," focuses on the continuous background of gravitational waves, a cosmic hum resulting from the superposition of all black hole mergers that have occurred in the Universe's history. Examining this signal allows an estimation of the density of these events and, thereby, the derivation of a value for the Hubble constant.

For now, current detectors like LIGO and Virgo still lack the sensitivity to clearly isolate this background. However, preliminary analyses of the data seem to indicate values in agreement with the faster expansion rate. This trend motivates the development of much more powerful third-generation instruments.

Thus, in the coming years, this method could deliver a robust measurement and help decide between the two camps of the Hubble tension. Even if one camp gains the upper hand "by two routes versus one," it will still remain to be understood why the other diverges in its measurements.

This work, published in the journal Physical Review Letters, therefore represents an encouraging advance for the field of cosmology.