The Universe is dominated by a mysterious component that we call as Dark Energy, which is believed to be the cause of the accelerated expansion of the Universe. The idea of cosmological constant as Dark Energy is unsettling and a large number of exotic theories for the same have been proposed. While the precise nature of Dark Energy is unknown, it is clear that it must affect how the large-scale-structure grows with cosmic time. Gravitational lensing provides a direct way to measure the matter distribution in the Universe. By measuring the statistical properties of the distribution in cosmic time slices, gravitational lensing can enable us to understand the properties of the dark energy. The strange behaviour of the Universe at large scales may also be attributed to our ignorance about the correct theory of gravity on those scales. Large surveys such as the Kilo-Degree Survey (KiDS), Dark Energy Survey (DES), Hyper-Suprime Cam (HSC) survey and future surveys such as the Large Sky Synoptic Telescope (LSST), Wide-Field Infra-Red Space Telescope (WFIRST) and Euclid attempt to make extremely precise measurements of the Dark Energy parameters and look for deviations from the general theory of relativity. In this talk, after a short overview about the theory of gravitational lensing, I will show the results on the Dark Energy equation of state parameters from the current surveys and present the forecasts for future missions. Estimates of the cosmological parameters obtained from lensing are in mild tension with those obtained from CMB experiments (Planck Collaboration) and I will briefly discuss the attempts made to alleviate this tension.
Arun Kannawadi is a postdoctoral researcher at the Leiden Observatory in the Netherlands, working in the field of weak gravitational lensing. His research mainly revolves around making realistic simulations of galaxy images, which are then used to calibrate the weak lensing shears and understanding the systematic biases that exist. He obtained his PhD in Physics from Carnegie Mellon University, under the supervision of Prof. Rachel Mandelbaum in 2016. Prior to that, Arun obtained his Bachelors in Electrical Engineering from IIT Madras in 2011, with a minor in Physics. Arun is currently involved in assisting the preparation for Euclid, an ESA-NASA collaborative telescope to be launched in the next decade and associated with the ongoing Kilo-Degree Surveys (KiDS). During his PhD, he was also working for the LSST and WFIRST surveys.