In general relativity, Einstein's theory of spacetime and gravity, the geometry of spacetime is not a passive setting for the dynamics of matter and energy, but an equally dynamic player. Matter and energy cause spacetime curvature, which in its turn guides the free fall of matter and energy. Remarkably, spacetime can support curvature without any matter: black holes, the densest masses in the Universe, are objects of pure spacetime wrapped around itself; gravitational waves are self-sustaining, undulatory excitations of spacetime, carrying energy and traveling at the speed of light.

LISA is a space mission designed to measure gravitational waves over a richly populated band of signal frequencies, from 0.03 milliHertz to 0.1 Hertz (i.e., oscillation periods between 10 hours and 10 seconds). LISA will measure signals from several different gravitational-wave sources:

• massive black holes merging at the center of galaxies, or consuming smaller compact objects;
• binaries of compact stars and stellar remnants in our Galaxy;
• possibly, other sources of cosmological origin, including the relic radiation from the very early phase of the Big Bang, and speculative astrophysical objects such as cosmic strings and domain boundaries.

Measuring all these signals will give us insight into a broad range of unanswered science questions: the birth and history of galaxies and massive black holes; the behavior of general relativity and spacetime in their most extreme regime; the expansion history of the Universe; the physics of dense matter and stellar remnants; and possibly new physics characteristic of the early Universe or of string theory.

LISA will feel the beating heart of Einstein's Cosmos: its discoveries will transform astronomy and physics, and reshape the science questions of the new millennium.