Cosmology from gravitational lens time delays and Planck data

TL;DR

This paper improves gravitational lens modeling to better measure the Hubble constant and explores cosmological implications, addressing tensions between Planck data and local measurements within flat Lambda-CDM and alternative models.

Contribution

It introduces more flexible lens models to reduce systematic errors in time-delay measurements and combines these with Planck data to constrain cosmological parameters.

Findings

Flexible lens models constrain the Hubble constant with 6.6% uncertainty.

Combined lens and Planck data favor a flat universe with Omega_k ≈ 0.

Results suggest a dark energy equation of state w ≈ -1.52.

Abstract

Under the assumption of a flat Lambda-CDM 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 Lambda-CDM 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 Lambda-CDM model, the data for RXJ1131-1231 in combination with Planck favor a flat universe with Omega_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).