We present a new measurement of the Hubble Constant H 0 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 HE 0435−1223 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. We then combine the timedelay distance of HE 0435−1223 with previous measurements from systems B1608+656 and RXJ1131−1231 to create a Time Delay Strong Lensing probe (TDSL). In flat ΛCDM with free matter and energy density, we find H 0 = 71.9 +2.4 −3.0 km s −1 Mpc −1 and Ω Λ = 0.62 +0.24 −0.35 . This measurement is completely independent of, and in agreement with, the local distance ladder measurements of H 0 . We explore more general cosmological models combining TDSL with other probes, illustrating its power to break degeneracies inherent to other methods. The joint constraints from TDSL and Planck are H 0 = 69.2 +1.4 −2.2 km s −1 Mpc −1 , Ω Λ = 0.70 +0.01 −0.01 and Ω k = 0.003 +0.004 −0.006 in open ΛCDM and H 0 = 79.0 +4.4 −4.2 km s −1 Mpc −1 , Ω de = 0.77 +0.02 −0.03 and w = −1.38 +0.14 −0.16 in flat w CDM. In combination with Planck and Baryon Acoustic Oscillation data, when relaxing the constraints on the numbers of relativistic species we find N eff = 3.34 +0.21 −0.21 in N eff ΛCDM and when relaxing the total mass of neutrinos we find Σm ν ≤ 0.182 eV in m ν ΛCDM. Finally, in an open w CDM in combination with Planck and CMB lensing we find H 0 = 77.9 +5.0 −4.2 km s −1 Mpc −1 , Ω de = 0.77 +0.03 −0.03 , Ω k = −0.003 +0.004 −0.004 and w = −1.37 +0.18 −0.23 .

Under the assumption of a flat ΛCDM cosmology, recent data from the Planck satellite point toward a Hubble constant that is in tension with that measured by gravitational lens time delays and by the local distance ladder. Prosaically, this difference could arise from unknown systematic uncertainties in some of the measurements. More interestingly -if systematics were ruled out -resolving the tension would require a departure from the flat ΛCDM cosmology, introducing for example a modest amount of spatial curvature, or a non-trivial dark energy equation of state. To begin to address these issues, we present here an analysis of the gravitational lens RXJ1131−1231 that is improved in one particular regard: we examine the issue of systematic error introduced by an assumed lens model density profile. We use more flexible gravitational lens models with baryonic and dark matter components, and find that the exquisite Hubble Space Telescope image with thousands of intensity pixels in the Einstein ring and the stellar velocity dispersion of the lens contain sufficient information to constrain these more flexible models. The total uncertainty on the time-delay distance is 6.6% for a single system. We proceed to combine our improved time-delay distance measurements with the WMAP9 and Planck posteriors. In an open ΛCDM model, the data for RXJ1131−1231 in combination with Planck favor a flat universe with Ω k = 0.00 +0.01 −0.02 (68% CI). In a flat wCDM model, the combination of RXJ1131−1231 and Planck yields w = −1.52 +0.19 −0.20 (68% CI).

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, HE 0435−1223, WFI2033−4723 and HE 1104−1805). 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 ΛCDM, we expect to measure H 0 to < 3.5% in most models, spatial curvature Ω k to 0.004, w to 0.14, and the effective number of neutrino species to 0.2 (1σ uncertainties) when combined with current CMB experiments. These are, respectively, a factor of ∼ 15, ∼ 2, and ∼ 1.5 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.

We investigate the optical spectral region of spectra of ∼ 1000 stars searching for IMFsensitive features to constrain the low-mass end of the initial mass function (IMF) slope in elliptical galaxies. The use of indicators bluer than NIR features (NaI, CaT, Wing-Ford FeH) is crucial if we want to compare our observations to optical simple stellar population (SSP) models. We use the MILES stellar library (Sánchez-Blázquez et al. 2006) in the wavelength range 3500-7500Å to select indices that are sensitive to cool dwarf stars and that do not or only weakly depend on age and metallicity. We find several promising indices of molecular TiO and CaH lines. In this wavelength range, the response of a change in the effective temperature of the cool red giant (RGB) population is similar to the response of a change in the number of dwarf stars in the galaxy. We therefore investigate the degeneracy between IMF variation and ∆T eff,RGB and show that it is possible to break this degeneracy with the new IMF indicators defined here. In particular, we define a CaH1 index around λ6380Å that arises purely from cool dwarfs, does not strongly depend on age and is anti-correlated with [α/Fe]. This index allows the determination of the low-mass end of the IMF slope from integrated-light measurements when combined with different TiO lines and age-and metallicity-dependent features such as Hβ, Mgb, Fe5270 and Fe5335. The use of several indicators is crucial to break degeneracies between IMF variations, age, abundance pattern and effective temperature of the cool red giant (RGB) population. We measure line-index strengths of our new optical IMF indicators in the Conroy & van Dokkum (2012a) SSP models and compare these with index strengths of the same spectral features in a sample of stacked Sloan Digital Sky Survey (SDSS) early-type galaxy (ETG) spectra with varying velocity dispersions. Using different indicators, we find a clear trend of a steepening IMF with increasing velocity dispersion from 150 to 310 km s −1 described by the linear equation x = (2.3 ± 0.1) log σ 200 + (2.13 ± 0.15), where x is the IMF slope and σ 200 is the central stellar velocity dispersion measured in units of 200 km s −1 . We test the robustness of this relation by repeating the analysis with ten different sets of indicators. We found that the NaD feature has the largest impact on the IMF slope, if we assume solar [Na/Fe] abundance. By including NaD the slope of the linear relation increases by 0.3 (2.6 ± 0.2). We compute the "IMF mismatch" parameter as the ratio of stellar mass-to-light ratio predicted from the x − σ 200 relation to that inferred from SSP models assuming a Salpeter IMF and find good agreement with independent published results.