Of the many open questions that baffle physicists today, there is one - the most relevant for cosmologists - that crystallises into a single number: the Hubble constant H0, which represents the ratio between the distance of a cosmic object from us and the speed at which it moves away from us due to the accelerated expansion of the Universe. The puzzle arises from the statistically significant discrepancy between the values of H0 obtained using different measurement strategies: the traditional method, based on the observation of cosmic objects such as Cepheid variable stars and supernovae, and a more indirect approach that uses the cosmic microwave background as a starting point to infer H0.
If this 'Hubble tension' is real - that is, if the different values of H0 are not caused by systematic uncertainties in measurements that have yet to be identified - it would require a profound rethinking of our understanding of the evolution and composition of the Universe.
In an article published in the journal Astronomy & Astrophysics , the TDCOSMO collaboration documents its latest effort to find an alternative path to accurately measure the Hubble constant. The team's results support the Hubble tension between H0 measurements of the late and early Universe. Frédéric Courbin, ICREA researcher at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC), explains: "To me the lensing time delays is pivotal in the tension resolution as it is fully independent of any other method and as it does not involve complicated calibrations".
The TDCOSMO collaboration uses a technique known as time-delay cosmography to infer the value of the Hubble constant. In the study, the team applies this method to a sample of cosmic probes known as strongly lensed quasars. A quasar is a distant, extremely bright object produced by an accretion disk of gas and dust falling into a supermassive black hole at the centre of a galaxy.
