Hunting for dark matter in the early Universe
It is now well established that a large part of the matter in the Universe is some substance which appears to be oblivious to any force but gravity. The nature of this ``dark matter” remains a mystery. Could it be a new particle, as light as an electron, or might it be made of black holes as massive as many Suns? In this talk, I will first describe how one can hope to tease out some of the properties of dark matter from the Cosmic Microwave Background (CMB), the relic radiation from the very early Universe. I will review the basic physics underlying the CMB, in particular why it has a nearly perfect blackbody spectrum, and what the tiny changes of temperature across the sky tell us about the history and contents of the Universe. I will then illustrate how precise measurements of the CMB properties may inform us about the nature of dark matter. Last but not least, I will explain how gravitational waves, recently directly detected by LIGO, may provide an entirely new window into the mystery of dark matter.