The Gravitational Universe will open a new window in astronomy using powerful sources of gravitational waves to probe a universe that cannot be probed by other means.
In the early years of this millennium, our view of the universe has been comfortably consolidated in some aspects, but also profoundly changed in others: General relativity underwent tough tests and ESA´s Planck mission delivered important cosmological findings, a new concept suggests that black holes and galaxies evolve jointly and numerous ultra-compact binary systems in the Milky way were discovered – exquisite laboratories for exploring the extremes of stellar evolution in binary systems.
All of these advances were possible using only our first sense for observing the universe, electromagnetic radiation, tracing electromagnetic interactions of baryonic matter in the universe. However, almost all of the universe remains electromagnetically dark. On astronomical scales gravitation is the real engine of the universe.
Listening to gravity will let us go further than any alternative.
This is possible through the direct observation of gravitational waves. Gravitational waves only weakly interact with matter and travel largely undisturbed over cosmological distances. Their signature is a fractional squeezing of spacetime perpendicuar to the direction of propagation.
Electromagnetic observations of the universe, plus theoretical modeling, suggest that the richest part of the gravitational wave spectrum falls into the freuqency range accessible to a space interferometer, from about 10-4 Hz to 10-1 Hz. In this band, important first-hand information can be gathered to test the history of the universe out to redshifts of order 20, gravity in the dynamical strong field regime and the TeV scale energy of the early universe.
LISA will be the first ever mission surveying the entire universe with Gravitational Waves to address the Science Theme The Gravitational Universe.