H0 Lenses in COSMOGRAIL's Wellspring.

We measure the current expansion rate of the Universe using time-delay cosmography.
And it works!

Latest Results

The H0LiCOW collaboration has revealed its measurement of the Hubble constant H0 from its blind analysis of six multiply-imaged quasar systems through strong gravitational lensing: H0 = 73.3 +1.7−1.8 km/s/Mpc, at 2.4% precision, in the standard flat ΛCDM model.

Fig 1 : Probability distribution of the 6 lenses of the H0LiCOW sample presented in H0LICOW XIII. Figure credits : Martin Millon/Vivien Bonvin.

This measurement is completely independent of, and in agreement with, the local distance ladder measurements of H0. On the other hand, it is higher than the most recent Cosmic Microwave Background predictions in flat-ΛCDM, as well as galaxy clustering combined to weak lensing, potentially pointing towards new physics beyond the predictions of the standard cosmological model.

Fig 2 : Comparison of H0 constraints for early-Universe and late-Universe probes in a flat ΛCDM cosmology (H0LICOW XIII.) Figure credits : Martin Millon/Vivien Bonvin.

Since the value measured from the SH0ES project using Cepheids and Supernovae is completely independent of the H0LiCOW value, both can be combined into a single measurement of the Hubble Constant in the Local Universe. This new combined value is 5.3σ higher than the most recent CMB prediction from the Planck satellite and 4.1σ than the galaxy clustering and weak lensing measurement from the DES collaboration.

You can reproduce the H0LiCOW 6-lens result in flat-LCDM following the notebook available on our Data Products page here. The posterior chains for our 6 lenses are also available there.

News & Press

Hubble Space Telescope press release (2020)

"People use the phrase "Holy Cow" to express excitement. Playing with that phrase, researchers from an international collaboration developed an acronym—H0LiCOW—for their project's name that expresses the excitement over their Hubble Space Telescope measurements of the universe's expansion rate..."

UCLA press release (2019)

"Seeing double could help resolve dispute about how fast the universe is expanding; the question has been bugging astronomers for almost a century. Different studies keep coming up with different answers — which has some researchers wondering if they’ve overlooked a key mechanism in the machinery that drives the cosmos. At the heart of the dispute is the Hubble constant..."

The Hubble Space Telescope / European Space Agency joint press release (2017)

"By using galaxies as giant gravitational lenses, an international group of astronomers using the NASA/ESA Hubble Space Telescope have made an independent measurement of how fast the Universe is expanding. The newly measured expansion rate for the local Universe is consistent with earlier findings. These are, however, in intriguing disagreement with measurements of the early Universe. This hints at a fundamental problem at the very heart of our understanding of the cosmos..."


H0LiCOW I. H0 Lenses in COSMOGRAIL’s Wellspring: Program Overview.

Strong gravitational lens systems with time delays between the multiple images allow measurements of time-delay distances, which are primarily sensitive to the Hubble constant that is key to probing dark energy, neutrino physics, and the spatial curvature of the Universe, as well as discovering new physics. We present H0LiCOW (H0 Lenses in COSMOGRAIL’s Wellspring), a program that aims to measure H0 with <3.5% uncertainty from five lens systems (B1608+656, RXJ1131−1231, HE0435−1223, WFI2033−4723 and HE 1104−1805)...

H0LiCOW II. Spectroscopic survey and galaxy-group identification of the strong gravitational lens system HE0435−1223.

Galaxies located in the environment or on the line of sight towards gravitational lenses can significantly affect lensing observables, and can lead to systematic errors on the measurement of H0 from the time-delay technique. We present the results of a systematic spectroscopic identification of the galaxies in the field of view of the lensed quasar HE0435−1223 using the W.M. Keck, Gemini and ESO-Very Large telescopes. Our new catalog triples the number of known galaxy redshifts in the vicinity of the lens, expanding to 100 the number of measured redshifts for galaxies separated by less than 3' from the lens...

H0LiCOW III. Quantifying the effect of mass along the line of sight to the gravitational lens HE0435−1223 through weighted galaxy counts.

Based on spectroscopy and multiband wide-field observations of the gravitationally lensed quasar HE0435−1223, we determine the probability distribution function of the external convergence κext for this system. We measure the under/overdensity of the line of sight towards the lens system and compare it to the average line of sight throughout the universe, determined by using the CFHTLenS as a control field. Aiming to constrain κext as tightly as possible, we determine under/overdensities using various combinations of relevant informative weighing schemes for the galaxy counts, such as projected distance to the lens, redshift, and stellar mass...

H0LiCOW IV. Lens mass model of HE0435−1223 and blind measurement of its time-delay distance for cosmology.

Strong gravitational lenses with measured time delays between the multiple images allowa directmeasurement of the time-delay distance to the lens, and thus ameasure of cosmological parameters, particularly the Hubble constant, H0. We present a blind lens model analysis of the quadruply-imaged quasar lens HE0435−1223 using deep Hubble Space Telescope imaging, updated time-delay measurements from the COSmological MOnitoring of GRAvItational Lenses (COSMOGRAIL), ameasurement of the velocity dispersion of the lens galaxy based on Keck data, and a characterization of the mass distribution along the line of sight...

H0LiCOW V. New COSMOGRAIL time delays of HE0435−1223: H0 to 3.8% precision from strong lensing in a flat ΛCDM model.

We present a new measurement of the Hubble Constant H0 and other cosmological parameters based on the joint analysis of three multiply-imaged quasar systems with measured gravitational time delays. First, we measure the time delay of HE0435-1223 from 13-year light curves obtained as part of the COSMOGRAIL project. Companion papers detail the modeling of the main deflectors and line of sight effects, and how these data are combined to determine the time-delay distance of HE 0435-1223. Crucially, the measurements are carried out blindly with respect to cosmological parameters in order to avoid confirmation bias...

H0LiCOW VI. Testing the fidelity of lensed quasar host galaxy reconstruction.

The empirical correlation between the mass of a super-massive black hole (MBH) and its host galaxy properties is widely considered to be evidence of their co-evolution. A powerful way to test the co-evolution scenario and learn about the feedback processes linking galaxies and nuclear activity is to measure these correlations as a function of redshift. Unfortunately, currently MBH can only be estimated in active galaxies at cosmological distances. At these distances, bright active galactic nuclei (AGN) can outshine the host galaxy, making it extremely difficult to measure the host’s luminosity...

H0LiCOW VII. Cosmic evolution of the correlation between black hole mass and host galaxy luminosity.

Strongly lensed active galactic nuclei (AGN) provide a unique opportunity to make progress in the study of the evolution of the correlation between the mass of supermassive black holes and their host galaxy luminosity. We demonstrate the power of lensing by analyzing two systems for which state-of-the-art lens modelling techniques have been applied to deep Hubble Space Telescope imaging data. We use the reconstructed images to infer the total and bulge luminosity of the host and {published broad-line spectroscopy to estimate the black hole mass using the so-called virial method. We then combine our new measurements with new calibration of previously published measurements to study the evolution of the correlation out to z~4.5...

H0LiCOW VIII. A weak lensing measurement of the external convergence in the field of the lensed quasar HE 0435−1223.

We present a weak gravitational lensing measurement of the external convergence along the line of sight to the quadruply lensed quasar HE0435-1223. Using deep r-band images from Subaru-Suprime-Cam we observe galaxies down to a 3 sigmas limiting magnitude of ~26 mags resulting in a source galaxy density of 14 galaxies per square arcmin after redshift-based cuts. Using an inpainting technique and Multi-Scale Entropy filtering algorithm, we find that the region in close proximity to the lens has an estimated external convergence of κext = -0.012 +0.020-0.013 and is hence marginally under-dense. We also rule out the presence of any halo with a mass greater than Mvir=1.6 x 1014 1/h MSol (68% confidence limit). Our results, consistent with previous studies of this lens, confirm that the intervening mass along the line of sight to HE0435-1223 does not affect significantly the cosmological results inferred from the time delay measurements of that specific object.

H0LiCOW IX. Cosmographic analysis of the doubly imaged quasar SDSS 1206+4332 and a new measurement of the Hubble constant.

We present a blind time-delay strong lensing (TDSL) cosmographic analysis of the doubly imaged quasar SDSS 1206+4332. We combine the relative time delay between the quasar images, Hubble Space Telescope imaging, the Keck stellar velocity dispersion of the lensing galaxy, and wide-field photometric and spectroscopic data of the field to constrain two angular diameter distance relations. The combined analysis is performed by forward modelling the individual data sets through a Bayesian hierarchical framework, and it is kept blind until the very end to prevent experimenter bias. After unblinding...

H0LiCOW X. Spectroscopic/imaging survey and galaxy-group identification around the strong gravitational lens system WFI2033-4723.

Galaxies and galaxy groups located along the line of sight towards gravitationally lensed quasars produce high-order perturbations of the gravitational potential at the lens position. When these perturbation are too large, they can induce a systematic error on H0 of a few-percent if the lens system is used for cosmological inference and the perturbers are not explicitly accounted for in the lens model. In this work, we present a detailed characterization of the environment of the lens system WFI 2033−4723 (zsrc = 1.662, zlens =0.6575), one of the core targets of the H0LiCOW project for which we present cosmological inferences in a companion paper (Paper XII).

H0LiCOW XII. Lens mass model of WFI2033−4723 and blind measurement of its time-delay distance and H0.

We present the lens mass model of the quadruply-imaged gravitationally lensed quasar WFI2033−4723, and perform a blind cosmographical analysis based on this system. Our analysis combines (1) time-delay measurements from 14 years of data obtained by the COSmological MOnitoring of GRAvItational Lenses (COSMOGRAIL) collaboration, (2) high-resolution Hubble Space Telescope imaging, (3) a measurement of the velocity dispersion of the lens galaxy based on ESO-MUSE data, and (4) multi-band, wide-field imaging and spectroscopy characterizing the lens environment. We account for all known sources of systematics, including the influence of nearby perturbers ...

H0LiCOW XIII. A 2.4% measurement of H0 from lensed quasars: 5.3σ tension between early and late-Universe probes.

We present a measurement of the Hubble constant (H0) and other cosmological pa- rameters from a joint analysis of six gravitationally lensed quasars with measured time delays. All lenses except the first are analyzed blindly with respect to the cosmologi- cal parameters. In a flat ΛCDM cosmology, we find H0 = 73.3 +1.7−1.8 km/s/Mpc, a 2.4% precision measurement, in agreement with local measurements of H0 from type Ia supernovae calibrated by the distance ladder, but in 3.1σ tension with Planck ob- servations of the cosmic microwave background (CMB). This method is completely independent of both the supernovae and CMB analyses. A combination ...

A SHARP view of H0LiCOW: H0 from three time-delay gravitational lens systems with adaptive optics imaging

We present the measurement of the Hubble Constant, H0, with three strong gravitational lens systems. We describe a blind analysis of both PG1115+080 and HE0435−1223 as well as an extension of our previous analysis of RXJ1131−1231. For each lens, we combine new adaptive optics (AO) imaging from the Keck Telescope, obtained as part of the SHARP AO effort, with Hubble Space Telescope (HST) imaging, velocity dispersion measurements, and a description of the line-of-sight mass distribution to build an accurate and precise lens mass model. This mass model is then combined with the COSMOGRAIL measured time delays in these systems to determine H0. We do both an AO-only and an AO+HST analysis...

Data Products

Below is a link to data products from the H0LiCOW project, including posterior PDFs of the time-delay distances and angular diameter distances, tables and spectra for galaxies in the field of view, light curves, as well as python notebooks to calculate the combined cosmological inference.

Data Products

Who we are

A fine bunch of H0LiCOWers in Leiden for the GravLens 2016 meeting.
From left to right: Alessandro, Tom, Chris, Chih-Fan, Stefan, Sherry, Tommaso, Adri, Ken, Dominique, Olga, Vivien and Fred.

H0LiCOW is an international collaboration that regroups people from all around the world


D. Sluse (STAR Institute), S. Mukherjee (STAR Institute), L. Van de Vyvere (STAR Institute)


A. Agnello (DARK)


S.H. Suyu (PI, MPA/TUM/ASIAA), S. Hilbert (MPA), E. Komatsu (MPA), S. Taubenberger (MPA), A. Yıldırım (MPA)


C. Spiniello (INAF-OAC/ESO)


C.E. Rusu (NAOJ), K.C. Wong (Kavli IPMU)

The Netherlands

L.V.E. Koopmans (KAI), A. Sonnenfeld (Leiden Observatory)


F. Courbin (EPFL), V. Bonvin (EPFL), J.H.H. Chan (EPFL), A. Galan (EPFL), M. Millon (EPFL), C. Lemon (EPFL), O. Tihhonova (EPFL), G. Meylan (EPFL)

United Kingdom

M.W. Auger (IoA Cambridge)


S. Birrer (UCLA), R.D. Blandford (KIPAC), G.C.-F. Chen (UC Davis), X. Ding (UCLA), C.D. Fassnacht (UC Davis), P.J. Marshall (KIPAC), T. Schmidt (UCLA), A. Shajib (UCLA), T. Treu (UCLA)

...& Friends!

M. Barnabè, Y. Hezaveh, I. Jee, K. Liao, E. Linder, J. McKean, P. Schneider, N. Rumbaugh, M. Tewes, S. Vegetti


Is there anything you want to tell us? Do not hesitate to get in touch!
Sherry is the PI of H0LiCOW, and the lead author of Paper I.