Observing the universe with gravitational waves is a brand new field in Astronomy, with the first-ever direct observation occurring in 2015 by the ground-based LIGO observatory – a feat that won the founders of LIGO the 2017 Nobel Prize in physics. LISA will observe gravitational waves in the low-frequency part of the gravitational wave spectrum, which is not accessible from the ground.

The low-frequency spectrum is predicted to be especially rich in sources and in particular, LISA will be capable of detecting the merger of pairs of super-massive black-holes over the entire visible universe – a capability which will dramatically improve our understanding of how such massive black holes form and evolve with time, and provide insights into their role in the formation of galaxies.

LISA will also probe the rate of expansion of the universe across the full range of cosmic scales with high precision; explore the fundamental nature of gravity and black holes; and search for possible stochastic gravitational wave backgrounds, which could tell us about the physics of the very early Universe.

LISA consists of three identical spacecraft flying in an equilateral triangle. Small metal cubes – test masses – inside each spacecraft are placed into a near-perfect freefall and laser interferometry is used to measure the change in separation between the test masses on each spacecraft. The ability to place the test masses into the required state of free-fall was recently demonstrated with great success by the LISA Pathfinder technology demonstrator mission.

In order to achieve the extreme precision required to detect the tiny distance fluctuations that a passing gravitational wave will cause, distance between the spacecraft must be several million km, and the accuracy of the laser displacement measurement must be less than the diameter of an atom. This is equivalent to measuring the displacement between Earth and Alpha Centauri with the accuracy of a human hair width. The UKATC, in collaboration with the University of Glasgow, are core-members of the science consortium responsible for developing LISA, and in particular, we are the lead partner developing and building the optical system of the precision laser interferometer which makes the key displacement measurement containing the science signal.

LISA is a mission of the European Space Agency with contributions from the space agencies of the ESA member states and NASA. The UK teams are funded by the UK Space Agency.