Mar. 8, 2016

The probe has entered the core of its mission, with the goal of proving the technological feasibility of gravitational waves detection in space

The LISA Pathfinder probe has started its scientific mission.Launched into orbit on December 3rd last year, LISA Pathfinder reached its operative position on January 22nd, and after a long series of tests it can now begin the scientific and technological experiment for which it was designed. The probe, made by ESA with the crucial contribution of the Italian Space Agency in cooperation with the National Institute for Nuclear Physics and with the University of Trento, aims to demonstrate the technological feasibility of gravitational waves detection in space. Predicted by Albert Einstein a century ago, gravitational waves are fluctuations in space-time fabric generated by extreme astronomic events, such as the explosion of supernovas or the fusion of black holes.

The purpose of the mission is to verify whether it is possible to extend research for gravitational waves into space. In particular, the probe was designed to achieve a ‘free fall’ that is absolutely devoid of external disturbances. This condition, extremely difficult to obtain, is necessary for the construction of a space gravitational observatory. To achieve this, researchers released two test masses in the probe - a pair of identical cubes made of gold and platinum, weighing 2 kilos and with sides measuring 46 millimetres - and are now checking whether they are effectively moving under the sole effect of gravity.

The purpose of LISA should not be taken for granted: in space there are forces such as radiation wind from the Sun which may disturb the motion of the cubes; these must be completely isolated from any and all non-gravitational influence. For this reason, the position of the cubes inside the probe is constantly monitored, and any necessary manoeuvring is made using micro propellers, to avoid all contact with the cubes.

“In order to fall freely in space, the two test masses must be perfectly still, so that no other force can disturb their gravitational motion. This means that nothing but a gravitational wave can make them oscillate”, explains Stefano Vitale of the University of Trento and INFN, science coordinator of LISA Technology Package, the technological core of this mission. In any case, the probe itself cannot identify gravitational waves. For the purpose of the technological test, the two cubes are placed only 38 centimetres apart, too small a distance to register the minuscule oscillations of space-time. Distance variations induced by the passage of a gravitational wave are so small that a gravitational observatory in space must keep test masses one million kilometres apart from one another, therefore measuring variations in the order of one millionth of a millionth of a metre.

“The levels of accuracy necessary to make future observations of gravitational waves in space are so high”, commented Paul McNamara, the mission's project scientist, “that they require an unprecedented understanding of the forces that act on the test masses.”
In short, after releasing the cubes from the mechanism that held them inside and making sure they are in the most accurate position of free fall ever achieved, the researchers of the LISA Pathfinder project will be running a series of experiments in the next six months, “pushing” the masses to verify whether they are actually still in relation to the probe, and free from non-gravitational waves.

The research team will attempt to interfere with the motion of the cubes, studying their reaction to the application of various forces. For instance, one of the experiments will increase the temperature in the high vacuum environment that contains the cubes by heating the few remaining gas molecules, in order to measure whether this produces any effect on the masses. In another experiment, the cubes will be subjected to increasing magnetic and electric forces to estimate the point at which they deviate from free fall.
LISA will carry out six months of scientific activities, and its results will pave the road for mission L3, ESA's next project, dedicated to the exploration of the gravitational universe.

Observation in space will widen the window of observation on gravitational universe that we have just cracked open, as it is sensitive to waves generated by supermassive black holes with masses equal to a million billion times that of the sun, located in the centre of the largest galaxies. When galaxies clash and fuse, the cosmic monsters they had held slowly blend, producing gravitational waves.
These data will provide us with unique clues on the formation of large cosmic structures and, first and foremost, of the evolution of the primordial universe, when the first stars and galaxies were formed.