DR. S. RATHNA KUMAR

Measurement of time delays between the multiple images of a quasar which is strongly lensed by a massive galaxy that lies in close proximity to its line of sight offers an independent method to measure $H_0$, the Hubble constant at the present epoch, which sets the extragalactic distance scale. This has assumed renewed significance in the light of recent tension between $H_0$ constraints from measurement of Cosmic Microwave Background anisotropies and observations of Type Ia Supernova explosions. There are two main challenges in implementing this method: 1) the need to obtain long-term monitoring light curves to accurately measure time delays in the presence of uncorrelated brightness variations due to microlensing by stars in the lensing galaxy and 2) the presence of degeneracies in modeling the mass distribution of the lensing galaxy. The speaker will present an overview of the advances during the last decade to overcome these difficulties. In particular, he will discuss about difference-smoothing algorithm to measure time delays, testing it on a large sample of simulated light curves from Strong Lens Time Delay Challenge, and our ongoing effort to integrate the algorithm with Python Curve Shifting toolbox, which represents the current state of the art for measuring time delays. He will also discuss about testing simple analytical modeling of lensed quasar systems in the context of Time Delay Lens Modeling Challenge, which is ongoing.