Physics has been full of astonishing discoveries over the past century. But they open up even bigger mysteries that scientists are working feverishly to explain. What is dark energy? And why is the expansion of the universe accelerating? 

In public talks at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, two prominent physicists – Sarah Shandera of Penn State University and Stanford University’s Savas Dimopoulos – discuss the breakthroughs of recent decades and what it will take to solve the most nettlesome mysteries that have deepened in their wake.