H0 Lenses in COSMOGRAIL's Wellspring.

We measure the current expansion rate of the Universe using strong lensing and the time-delay distance technique.
And it works!

News & Press

The Hubble Space Telescope / European Space Agency joint press release is now available !

"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..."

Latest Results

The H0LiCOW collaboration has revealed its measurement of the Hubble constant H0 from its blind analysis of three multiply-imaged quasar systems through strong gravitational lensing: H0 = 71.9 +2.4−3.0 km/s/Mpc, at 3.8% precision, in the standard flat-ΛCDM model. This measurement is completely independent of, and in agreement with, the local distance ladder measurements of H0, which is in tension with the most recent Cosmic Microwave Background predictions in flat-ΛCDM, potentially pointing towards new physics beyond the predictions of the standard cosmological model.

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 value of H0 = 72.5 ± 1.2 km/s/Mpc is 3.9σ higher than the most recent measurement of Planck, where H0 = 67.8 ± 0.9 km/s/Mpc.

Note that the H0LiCOW probability distribution is not gaussian, but is well represented by a skewed log-normal distribution. This is why its combination with the SH0ES value gives an error bar smaller than the combination of two gaussians.


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. Weak lensing mass reconstruction of HE0435-1223.

The effectiveness of the quasar time-delay method in constraining the Hubble Constant H0 depends on the lens potential at the position of the quasar images and on the contribution of external galaxies along the line of sight. This external contribution, measured as the dimensionless projected mass κext, can be estimated in various ways based, e.g. on N-body simulations or galaxy numbers counts. In this work we present a more direct measurement of κext for the quadruply imaged quasar HE0435-1223, using weak lensing measurement on deep r-band Subaru SuprimeCam images...


    Paper II provides a catalog of all spectroscopically identified targets in the field of view of HE0435-1223, as well as 1-D spectra for targets observed with the VLT, Gemini and Keck telescopes. Those data are available as a single tar.gz file.

  • Tables and Spectra

    Paper III provides a catalogue of all galaxies down to i<24 mag within a 2 arcmin radius around HE0435-1223. It contains positions, magnitudes, photometric redshifts, stellar and halo masses.

  • Complete Tables A1 and A2

    Paper V presents the 13-years optical light curves of HE0435-1223, as well as mock light curves created with PyCS in order to estimate the error on the time-delay measurement. Both can be downloaded here.

  • HE0435-1223 Light Curves
  • Mock light Curves

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.H. Suyu (PI, MPA/TUM/ASIAA), A. Agnello (ESO), S. Hilbert (TUM/ECU), C. Spiniello (MPA/INAF-OAC), M. Tewes (AIfA), A. Yıldırım (MPA)


A. Sonnenfeld (Kavli IPMU), K.C. Wong (NAOJ)

The Netherlands

L.V.E. Koopmans (KAI)


F. Courbin (EPFL), V. Bonvin (EPFL), O. Tihhonova (EPFL), G. Meylan (EPFL)

United Kingdom

M.W. Auger (IoA Cambridge), T. Collett (ICG Portsmouth)


C.D. Fassnacht (UC Davis), P.J. Marshall (KIPAC), T. Treu (UCLA), R.D. Blandford (KIPAC), G.C.-F. Chen (UC Davis), X. Ding (UCLA), K. Liao (UCLA), N. Rumbaugh (UC Davis), C.E. Rusu (UC Davis)

...& Friends!

M. Barnabè, Y. Hezaveh (KIPAC), E. Komatsu (MPA), E. Linder (LBL), J. McKean (ASTRON), D. Paraficz (EPFL), P. Schneider (AIfA), S. Vegetti (MPA)


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.