Strong Lensing and $H_0$

TL;DR

This paper reviews how time delays in strong gravitational lensing can independently measure the Hubble constant, highlighting recent breakthroughs with specific case studies and discussing future prospects in the field.

Contribution

It provides a comprehensive review of time-delay cosmography, emphasizing recent case studies that have advanced the measurement of H_0 using strong lensing.

Findings

Successful measurement of H_0 from the quasar RXJ1131$-$1231

First time-delay measurement from the supernova Refsdal

Discussion of future potential in the field

Abstract

Time delays from strong gravitational lensing provide a one-step absolute distance measurement. Thus, they measure $H_0$ independently of all other probes. We first review the foundations and history of time-delay cosmography. Then, we illustrate the current state of the art by means of two recent case studies that have been real breakthroughs: i) the quadruply imaged quasar lensed by a galaxy-scale deflector RXJ1131$-$1231, for which spatially resolved stellar kinematics is available; ii) the multiply imaged supernova "Refsdal", the first with measured time delays, lensed by cluster MACS1149.5$+$2223. We conclude by discussing the exciting future prospects of time-delay cosmography in the coming decade.