The main goal of STRIDES is to discover and follow-up ~100 previously unknown gravitationally lensed quasars in the Dark Energy Survey.

Strongly lensed quasars can be used to measure cosmological parameters. The time delays between the multiple images can be measured via dedicated monitoring campaigns, while the gravitational potential of the lensing galaxy and of structures along the line of sight can be modeled and measured using deep high resolution imaging and spectroscopy. The combination of these observables enables a distance, known as the time-delay distance (a combination of angular diameter distances) to be measured, which in turn can be converted into a measurement of cosmological parameters including those describing the Dark Energy equation of state (e.g. Courbin, Saha & Schechter 2002, LNP, 608, 1; Treu 2010, ARA&A, 87, 48). The constraints obtained from time delays are comparable in precision to those obtained by baryonic acoustic oscillation experiments and provide a powerful complement to other probes of dark energy (Suyu et al. 2014, ApJ, 788, 35; Suyu et al. 2013, ApJ, 766, 70).

At the moment, the main limitation of this technique is the small sample size of lenses that have accurate time delays as well as all the other required ancillary datasets. This limitation is mostly driven by the fact that lensed quasars are rare in the sky (0.2/degree at DES depths), and not enough suitable lenses are known at this time. According to Oguri & Marshall (2010, MNRAS, 405, 2579) there should be approximately 120 gravitationally lensed quasars brighter than i=21 (third faintest image), including 20 high information-content quads, to be discovered in DES.

We recently confirmed two new lensed quasars in the DES Y1A1 release, shown below (Agnello et al. 2015, MNRAS, 454, 1260).