There will be enormous potential for discovering the parts of the universe that are invisible by other means, such as black holes, the Big Bang, and other, as yet unknown objects. eLISA will complement our knowledge about the beginning, evolution and structure of our universe.
The Gravitational Universe is a science theme aiming to explore the Astrophysical Universe and the Laws of Nature. Carriers of this information are low frequency gravitational waves in the milli-Hertz frequency range, emitted by coalescing binary black holes and ultra-compact galactic binaries.
Gravitational waves travel undisturbed through spacetime. Thus, the cosmic horizon of the Gravitational Universe is the deepest and widest possible.
A suitable instrument for measuring gravitational waves over a broad band of low frequencies is a laser interferometer with an arm length as large as possible and long integration times, the primary impetus for a space-borne detector. Hence eLISA can be thought of basically as a high precision Michelson interferometer in space with an arm length of 1 million km. The arm length has been carefully chosen to allow observation of most of the interesting sources of gravitational waves.
eLISA will complement traditional astronomical observations based on the electromagnetic spectrum (for example, observations from visible light, infra-red or x-rays). It will open the gravitational wave window in space and measure gravitational radiation over a broad band of frequencies, from about 0.1 mHz to 100 mHz, a band where the Universe is richly populated by strong sources of gravitational waves.