New data confirm “cosmological tension” in the measurement of Hubble constant

Of the many open questions that puzzle physicists today, there is one, most relevant to cosmologists that crystallises into a single number: the Hubble constant H₀, which stands for the ratio between how far a cosmic object is from us and how fast it moves away from us due to the accelerating expansion of the Universe. 

The TDCOSMO international collaboration, including researchers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB), has presented new results on this parameter measuring the expansion rate of the Universe. Based on the study of quasars, extremely bright and active galactic nuclei, the team has found confirmation of the so-called “Hubble tension”, the statistically significant discrepancy between the values of H₀ depending on the method used to determine them.

The puzzle comes from the different measurement strategies: the traditional method, based on the observation of cosmic objects such as Cepheid variable stars and supernovae, or a more indirect approach that uses the cosmic microwave background as the starting point for inferring H₀. If this Hubble tension is real, that is, if the differing values of H₀ aren’t caused by yet-to-be-found systematic uncertainties in the measurements, this calls for a profound rethinking of what we believe we know about the evolution and composition of the Universe.

The TDCOSMO collaboration, which brings together researchers from about twenty research centres and universities, documents their latest effort in pursuing an alternative path to the precise measurement of the Hubble constant in a paper just published in the journal Astronomy & Astrophysics. 

The team’s results further support the Hubble tension between late- and early-Universe measurements of H₀. “To me, the lensing time delay method is pivotal in the tension resolution as it is fully independent of any other method and as it does not involve complicated calibrations,” says Frédéric Courbin, ICREA researcher from the IEEC at the ICCUB. 

This international collaboration uses a technique known as time-delay cosmography to infer the value of the Hubble constant. In the paper, 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. When a massive object, such as a galaxy, stands between an observer and a quasar, an effect known as gravitational lensing produces multiple images of the quasar. This is because the so-called lens galaxy, also referred to as the deflector, acts a bit like an optical lens placed on the path of a light beam: it warps space and, as a result, bends and magnifies the light coming from a background object. In the case of a quasar, the light’s deflection produces bright, distorted lensed images around the lens galaxy..

The latest findings from the TDCOSMO collaboration underscore the critical role of international research networks and sustained investment in science. Thanks to the support of the Swiss National Science Foundation (SNSF), researchers were able to collect two decades of time-delay measurements of lensed quasars using the Swiss Leonhard Euler Telescope at ESO. This long-term effort was further strengthened by European funding through the ERC Advanced Grant COSMICLENS, a project with Courbin as principal investigator.

Highlighting the significance of this research, the European Commission has recently awarded a €12 million Synergy Grant to tackle the Hubble tension through the RedH0T project, co-led by researchers Licia Verde (ICREA-ICCUB) and Frédéric Courbin.