The Gravitational Universe
  The last century has seen enormous progress in our understanding of the
Universe. We know the life cycles of stars, the structure of galaxies, the
remnants of the big bang, and have a general understanding of how the Universe
evolved. We have come remarkably far using electromagnetic radiation as our
tool for observing the Universe. However, gravity is the engine behind many of
the processes in the Universe, and much of its action is dark. Opening a
gravitational window on the Universe will let us go further than any
alternative. Gravity has its own messenger: Gravitational waves, ripples in the
fabric of spacetime. They travel essentially undisturbed and let us peer deep
into the formation of the first seed black holes, exploring redshifts as large
as z ~ 20, prior to the epoch of cosmic re-ionisation. Exquisite and
unprecedented measurements of black hole masses and spins will make it possible
to trace the history of black holes across all stages of galaxy evolution, and
at the same time constrain any deviation from the Kerr metric of General
Relativity. eLISA will be the first ever mission to study the entire Universe
with gravitational waves. eLISA is an all-sky monitor and will offer a wide
view of a dynamic cosmos using gravitational waves as new and unique messengers
to unveil The Gravitational Universe. It provides the closest ever view of the
early processes at TeV energies, has guaranteed sources in the form of
verification binaries in the Milky Way, and can probe the entire Universe, from
its smallest scales around singularities and black holes, all the way to
cosmological dimensions.
