What is the Nature of Dark Energy and Dark Matter?

Tommaso Treu, UC Santa Barbara

I will describe two astronomical experiments based on strong gravitational lensing that can address this question in a novel way. In the first experiment, I use as cosmic "standard rods" strong gravitational lenses where the background source is variable in time and the foreground deflector is a massive galaxy. I will illustrate recent advances in modelling techniques and data quality that enable a 7% measurement of constraints on distance from a single gravitational lens. I will show that results from just two systems yield constraints on the equation of state of dark energy and flatness comparable to those obtained with the best probes. The second experiment uses the flux ratios between multiple images of gravitationally lensed quasars to detect the presence of dark subhalos independent of their stellar content. This tests a fundamental prediction of the cold dark matter model, i.e. that galaxies should be surrounded by large numbers of dark satellite subhalos. Proof that such satellites do not exist would force a revision of the model in favor of more exotic alternatives like warm dark matter. I will show first results from this experiment based on Keck- Adaptive Optics observations. I will then conclude by discussing the bright prospects of studies of the dark sector using gravitational lensing.