Introduction

Here in our expanding Universe, ultra-distant objects aren't just speeding away from us, the rate at which they're speeding away is increasing: teaching us that the Universe is accelerating. When we examine how the Universe is accelerating, we find that it's behaving as though the Universe is filled with some sort of energy inherent to space: dark energy, or a cosmological constant. But theoretically, we have no idea how to calculate what the value of dark energy ought to be. Its extremely small but non-zero value remains a tremendous puzzle in fundamental physics.