Sackler Lecture/Physics Colloquium

In 1998 two teams presented evidence of an accelerating Universe by tracing the cosmic expansion to more than 1/2 the age of the Universe using a class of exploding stars known as type Ia supernovae. In the ensuing decade the weight of experimental evidence has accumulated such that the standard model of the Universe now includes a component of matter with negative pressure (commonly referred to as Dark Energy) driving the acceleration. But the physical nature of what is causing the acceleration is still a mystery, with no compelling theoretical explanation yet available. To help differentiate the broad classes of models for understanding the acceleration, huge observational projects have been put into motion to more precisely measure the cosmic expansion rate back in time. A decade after its discovery, supernovae still provide the most constraining measurements of the cosmic acceleration phenomenon, but other measurement techniques are rapidly being developed and refined. I will discuss the evidence and constraints on cosmic acceleration, focusing on type Ia supernovae, but including a wide variety of techniques. In addition, I will critically look at what the next 5-10 years of experiments are likely to achieve at clarifying the mystery of an accelerating Universe.