H0LiCOW

H0LiCOW
Name	H0LiCOW
Established	2013
Location	International
Type	Observational cosmology

H0LiCOW

H0LiCOW was an international collaboration that used strong gravitational lensing time delays to measure the Hubble constant. The team combined observations from facilities such as the Hubble Space Telescope, the Keck Observatory, the Very Large Telescope, and radio arrays including the Very Large Array to study lensed quasars and lenses selected from surveys like the Sloan Digital Sky Survey and the Dark Energy Survey. The project aimed to provide an independent local measurement of cosmic expansion to compare with inference from the Cosmic Microwave Background and large-scale structure probes such as Planck and Baryon Acoustic Oscillations.

Overview

H0LiCOW brought together researchers affiliated with institutions including University of Oxford, Max Planck Society, Harvard University, Stanford University, University of California, Berkeley, University of Cambridge, Princeton University, University of Tokyo, ETH Zurich, Australian National University, Kavli Institute for Cosmology, European Southern Observatory, National Radio Astronomy Observatory, and Institut d'Astrophysique de Paris to exploit lensed active galactic nuclei. The collaboration targeted systems like the quadruply imaged quasars found in surveys such as the Cosmic Lens All-Sky Survey and follow-ups from the Canada–France–Hawaii Telescope Legacy Survey to derive time-delay distances, linking to theoretical frameworks developed by researchers at Institute for Advanced Study and groups working on Lambda-CDM and alternative dark energy models. H0LiCOW results were compared against constraints from projects including WMAP, Euclid (spacecraft), Dark Energy Spectroscopic Instrument, Large Synoptic Survey Telescope (now Vera C. Rubin Observatory), and Kilo-Degree Survey.

Methodology

The project measured relative light-travel time delays between multiple images of variable sources, combining high-resolution imaging from Hubble Space Telescope instruments, adaptive optics data from Keck Observatory instruments, and radio monitoring with the Very Large Array. Lens mass modelling used approaches influenced by studies at Max Planck Institute for Astrophysics and techniques described in work by researchers at University of Pennsylvania and Columbia University. The analysis pipeline incorporated stellar kinematics from integral-field spectrographs at European Southern Observatory facilities and population synthesis priors tied to results from Sloan Digital Sky Survey stellar libraries. H0LiCOW used Bayesian inference frameworks developed alongside methods from Carnegie Mellon University and applied cosmological modeling consistent with research from Princeton University Observatory, allowing direct comparison with parameters reported by Planck Collaboration and analyses from Baryon Oscillation Spectroscopic Survey.

Lens Sample and Observations

The lens sample included well-studied systems monitored photometrically and spectroscopically over multi-year campaigns, with targets overlapping catalogs from Sloan Digital Sky Survey, discoveries from Pan-STARRS, and follow-ups from Dark Energy Survey. Imaging and spectroscopy came from teams using Hubble Space Telescope, Keck Observatory, Very Large Telescope, Gemini Observatory, Subaru Telescope, and radio arrays such as the Very Large Array and Atacama Large Millimeter/submillimeter Array. Time-delay measurements were cross-validated using monitoring programs coordinated with groups at University of Chicago, Ohio State University, University of Sydney, and University of Melbourne. Lens environment characterization leveraged redshift surveys by Two Micron All Sky Survey teams and deep imaging from Canada-France-Hawaii Telescope and Blanco Telescope programs.

Results and Cosmological Implications

H0LiCOW published Hubble constant estimates that were compared with local distance-ladder results from teams led by researchers at Carnegie Institution for Science and analyses using Type Ia supernovae compiled by groups at Harvard–Smithsonian Center for Astrophysics. Their measurements contributed to the discussion of the so-called Hubble tension between local probes and early-universe inferences from Planck and WMAP. The collaboration explored implications for cosmological parameters in the context of Lambda-CDM, and alternative scenarios involving additional relativistic species as discussed in work from CERN and neutrino studies at Fermi National Accelerator Laboratory. Results were cited in comparisons with forecasts for missions like Euclid (spacecraft), James Webb Space Telescope, and surveys by Dark Energy Survey teams.

Systematics and Error Analysis

H0LiCOW addressed systematic uncertainties including line-of-sight structure, mass-sheet degeneracy, lens substructure, and anisotropy of stellar orbits, with methods influenced by theoretical work from California Institute of Technology and numerical simulations from groups at Lawrence Berkeley National Laboratory and Max Planck Institute for Astrophysics. The collaboration employed external shear modeling using environment catalogs from Sloan Digital Sky Survey and validation against mock data produced by teams at Flatiron Institute and Simons Foundation. Kinematic modelling incorporated templates and priors informed by observations at European Southern Observatory and stellar population studies from University of Cambridge and Imperial College London.

Related Projects and Legacy

H0LiCOW influenced successor and related initiatives including the STRIDES program supported by Space Telescope Science Institute, the TDCOSMO collaboration linking to groups at Institut de Ciències del Cosmos de Barcelona and Max Planck Institute for Astronomy, and synergies with surveys from Vera C. Rubin Observatory, Euclid (spacecraft), and Nancy Grace Roman Space Telescope. The methodologies informed lensing analyses in projects at University of Pennsylvania, Columbia University, Princeton University, Kavli Institute, and contributed to training of researchers associated with fellowships from Royal Society and grants from European Research Council. The project’s datasets, analysis techniques, and results continue to be cited in studies addressing cosmological parameter estimation and the resolution of the Hubble tension by teams at Harvard University, Stanford University, Max Planck Society, and Carnegie Institution for Science.

Category:Observational cosmology