The Science Programme Committee of the European Space Agency (ESA) announced on November 28th that the science theme for the L3 mission opportunity would be "The Gravitational Universe", a proposal to make astrophysical observations in a completely new way, through the detection of gravitational waves.
The Gravitational Universe proposes to detect gravitational waves using laser interferometry in space. The associated mission concept, eLISA, envisages a constellation of three satellites, in a triangular configuration a million kilometres on each side, in an orbit around the Sun that trails behind the Earth. Such an instrument would detect gravitational waves in the millihertz regime, which are generated by the mergers of massive black holes with mass ten thousand to ten million times the mass of the Sun, by binaries of compact objects in our own galaxy, the Milky Way, by the inspirals of compact stellar remnants into massive black holes and perhaps by cosmological processes occurring on the TeV scale in the early Universe.
Most of our current information about astrophysical processes in the Universe has been inferred from observations in the electromagnetic spectrum. However, gravity is the engine behind many of the processes in the Universe, and much of its action is dark – it emits no electromagnetic radiation at all. Opening a gravitational window on the Universe therefore has the potential to transform our current understanding. eLISA will make exquisite and unprecedented measurements of black hole masses and spins from the very earliest epochs in cosmic history (redshift z > 20), making it possible to trace the history of black holes across all stages of galaxy evolution and determine the masses of the seeds from which galactic black holes have grown. These measurements cannot be made by any electromagnetic instrument. eLISA will also provide powerful insights into the fundamentals of gravity, and into Einstein's theory of general relativity, since the precision of the observations will reveal deviations smaller than one part in ten thousand from the predictions of general relativity in a regime where the objects are moving with velocities tens of percent of the speed of light. This regime has never been probed.
A space-based detector will complement other efforts to detect gravitational waves that are currently underway. Ground-based interferometers that are operating or planned in the US (LIGO), Italy (Virgo), Germany (GEO) and Japan (KAGRA) are sensitive to gravitational waves in the 10Hz to 1 kHz range, which are generated by compact objects with mass comparable to or a few tens of times the mass of the Sun. There is also an ongoing effort to detect gravitational waves in the nanohertz band using pulsar timing, the accurate timing of pulsars. Nanohertz gravitational waves are generated by binaries of supermassive black holes (one hundred million times the mass of the Sun or more). It is expected that both of these efforts will yield detections within the next five to ten years, but this prediction is subject to significant uncertainties in the rates of astrophysical events. eLISA will observe gravitational waves in the millihertz band, which cannot be detected by any other instrument, and is unique among existing efforts in that it has guaranteed sources - compact binaries in the Milky Way that have been observed using optical telescopes and must be generating gravitational waves that eLISA can detect if general relativity is correct.
The next step towards revealing the Gravitational Universe will be the launch of LISA Pathfinder in 2015, a mission which will test the key eLISA technologies in space. Between 2014 and 2020, eLISA technology will be optimized, followed by a final mission selection expected around 2020. In 2024 the industrial implementation will begin, with the payload supplied by a European consortium which has also provided the flight hardware for LISA Pathfinder. The eLISA launch is currently expected around 2034.
“The Gravitational Universe” White Paper can be downloaded here: