H0LiCOW – I. H0 Lenses in COSMOGRAIL's Wellspring: program overview

TLDR

Strong gravitational lens systems with time‑delay measurements enable determination of the Hubble constant, probing dark energy, neutrino physics, spatial curvature, and new physics. The H0LiCOW program aims to measure H0 to <3.5 % uncertainty from five lens systems and to establish a foundation for extracting cosmological distances from the hundreds of time‑delay lenses expected in future surveys. The program acquires time delays via COSMOGRAIL and VLA, high‑resolution HST imaging for mass modeling, wide‑field imaging and spectroscopy to characterize the lens environment, and moderate‑resolution spectroscopy to measure stellar velocity dispersions for mass modeling. The program is projected to constrain H0 to <3.5 %, Ωk to 0.004, w to 0.14, and Neff to 0.2 (1σ) when combined with CMB data—improvements of ~15×, ~2×, and ~1.5× over CMB alone—and will also allow studies of the stellar initial mass function and SMBH–host co‑evolution.

Abstract

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 ($H_0$ Lenses in COSMOGRAIL's Wellspring), a program that aims to measure $H_0$ with $<3.5\%$ uncertainty from five lens systems (B1608+656, RXJ1131-1231, HE0435-1223, WFI2033-4723 and HE1104-1805). We have been acquiring (1) time delays through COSMOGRAIL and Very Large Array monitoring, (2) high-resolution Hubble Space Telescope imaging for the lens mass modeling, (3) wide-field imaging and spectroscopy to characterize the lens environment, and (4) moderate-resolution spectroscopy to obtain the stellar velocity dispersion of the lenses for mass modeling. In cosmological models with one-parameter extension to flat $\Lambda$CDM, we expect to measure $H_0$ to $<3.5\%$ in most models, spatial curvature $\Omega_{\rm k}$ to 0.004, $w$ to 0.14, and the effective number of neutrino species to 0.2 (1$\sigma$ uncertainties) when combined with current CMB experiments. These are, respectively, a factor of $\sim15$, $\sim2$, and $\sim1.5$ tighter than CMB alone. Our data set will further enable us to study the stellar initial mass function of the lens galaxies, and the co-evolution of supermassive black holes and their host galaxies. This program will provide a foundation for extracting cosmological distances from the hundreds of time-delay lenses that are expected to be discovered in current and future surveys.