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An international collaboration, with the participation of ICREA-ICCUB-IEEC researcher Frédéric Courbin, presents new results on the expansion rate of the Universe based on the study of extremely luminous active galactic nuclei known as quasars.
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Montage of the eight lensed quasar systems in the TDCOSMO-2025 sample. The image for DES J0408−5354 is adapted from Shajib et al. (2020), HE 0435−1223, B1608+656, and WFI 2033−4723 from Suyu et al. (2017), PG 1115+080 and SDSS J1206+4332 from Wong et al. (2020), RX J1131−1231 from Shajib et al. (2024, credits: NASA/ESA/Chandra), and WGD 2038−4008 from Shajib et al. (2022).
Credits
Shajib et al. (2020, 2022 and 2024), Suyu et al. (2017), Wong et al. (2020), NASA/ESA/Chandra

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 puzzle comes from the statistically significant discrepancy between the values of H0 obtained through 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.

In a paper just published in the journal Astronomy & Astrophysics, the TDCOSMO collaboration documents their latest effort in pursuing an alternative path to the precise measurement of the Hubble constant. The team’s results further support the Hubble tension between late- and early-Universe measurements of H0. “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” says 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). 
 

The TDCOSMO 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. 
 

Artist’s illustration of a quasar. Credits: Canva.
Artist’s illustration of a quasar. Credits: Canva.


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 (see header image in this article).
 

Crucially, the signature of gravitational lensing on an observed quasar is not just spatial, but temporal too. If the intensity of the radiation emitted by a quasar varies over time, the light rays coming from multiple images of one lensed quasar reach us with temporal delays. Time-delay cosmography allows researchers to measure what is known as time-delay distance. Provided the gravitational potential of the lens galaxy is sufficiently well known, scientists can infer H0 from the time-delay distance and the light's redshift caused by the expanding Universe.

To obtain the new value of H0, the TDCOSMO team used previously collected as well as new data on eight strongly lensed quasars and took advantage of improved analysis methods. For six out of eight lens galaxies, the data related to stellar kinematics were made accessible by the NIRSpec spectrograph on the James Webb Space Telescope. Stellar kinematics data refer to the motion of stars in a galaxy: they're especially important to address a major source of error when determining H0 with time-delay cosmography. Other data were collected with the Multi Unit Spectroscopic Explorer (MUSE) spectrograph at the Very Large Telescope of the European Southern Observatory (ESO) in Chile and with the Keck Cosmic Web Imager (KCWI) at the Keck Observatory in Hawaii.

The latest findings from the TDCOSMO collaboration underscore the critical role of international research networks and sustained investment in science. Thanks to the visionary 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 (PI: Frédéric Courbin).

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.

 

Reference:

Birrer, S. et al. TDCOSMO 2025: Cosmological constraints from strong lensing time delays. Astronomy & Astrophysics.

https://doi.org/10.1051/0004-6361/202555801

Available at https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202555801

 

Based on the press release from ETH Zurich.