LISA Pathfinder exceeds all expectations

In an article that was published today in Physical Review Letters, the LISA Pathfinder (LPF) team demonstrates that the two test blocks scarcely move with respect to each other. The acceleration measured between the two blocks is less than ten millionths of a billionth of the acceleration due to gravity on Earth (g). The successful test opens up the way for the development of eLISA, a large observatory in space for observing gravitational waves that originate from exotic objects and phenomena in the universe. eLISA will be launched in about 2034.

The existence of gravitational waves was already predicted hundred years ago by Albert Einstein. Gravitational waves are ripples in the fabric of space-time that travel with the speed of light and are caused by the acceleration of extremely massive objects. They can be caused by cosmic phenomena such as supernovas, neutron binary stars that revolve around each other, and merging black holes. But even these extremely energy-rich phenomena only cause very slight ripples in space-time. By the time these reach the Earth the waves disrupt the space-time by less than 1 trillionth of a percent.

Scientists need extremely advanced technologies to be able to measure such minimal ripples. It was not until September 2015 that gravitational waves were detected for the first time by the LIGO detector in de US. The LIGO-Virgo partnership detected a signal of two black holes of 29 and 36 solar masses respectively, which merged into a new black hole of 62 solar masses. In the last 0.3 seconds before the merger 3 solar masses were converted into gravitational waves that were registered by LIGO.

"It is fantastic that after the spectacular discovery made by LIGO, there is once again good news for the discipline," says Jo van den Brand, leader of the gravitational waves research at Nikhef. "We are eagerly awaiting the next discoveries."

Low-frequency vibrations
The signals that LIGO captured had a frequency of about 100 Hz. But gravitational waves cover a far broader spectrum. Low-frequency vibrations in particular, are caused by exotic phenomena such as the merging of two supermassive black holes. We find these supermassive black holes, which have a mass millions to billions times the mass of the Sun, in the centre of galaxies. When two galaxies collide, the black holes gradually merge in the centre of the new galaxy. During the merging process they produce a vast amount of energy in the form of gravitational waves, with a peak in the last few minutes before the final merger.

"eLISA provides entirely new opportunities to study fundamental physics and to follow the evolution of structure in the universe," says Gijs Nelemans of Radboud University and leader of the Dutch eLISA consortium.

To observe these phenomena and to make full use of the new window on the universe that gravitational waves offer, it is vital to be able to measure gravitational waves with a low frequency (0.1 mHz-1 Hz). That means: measuring minimal fluctuations between objects that are located millions of kilometres from each other. That can only be done in space with an observatory that does not suffer from seismic and thermic disruptions on Earth or from the gravitational noise of the Earth itself. Detectors on the ground are, however, hindered by these disruptive factors.

Key technology
LISA Pathfinder has been designed to test the key technology for a space observatory (eLISA). A vital part of the test is using lasers to continuously measure the position of two test blocks that move through space in free fall, influenced only by gravity. Even in space that is extremely difficult because there are a whole range of factors, such as the solar wind and the radiation pressure from sunlight, which can have a disruptive effect. LISA Pathfinder therefore has two identical gold-platinum blocks (2 kg, 46 mm) on board, which fly through space at a distance of 38 cm from each other, surrounded by but not in contact with the satellite that protects the blocks. Using lasers, beam splitters and a series of small mirrors, variations in the distance between the blocks were measured very accurately, almost to the level of a single atom.

LISA Pathfinder was launched on 3 December 2015 and reached its intended position at the end of January 2016 about 1.5 million km from Earth in the direction of the Sun. The experiments started on 1 March.

"The measurements have more than exceeded our most optimistic expectations," says Martijn Smit, who coordinates the SRON contribution to LPF. "On the first day, the LPF team had already achieved the precision needed and in the subsequent weeks it even improved this by a factor of five. That really is a stupendous and a superb piece of precision technology."

The Netherlands
Dutch engineers, physicists and astronomers are closely involved in both missions. Space research institute SRON developed the test equipment for LISA Pathfinder in the run up to the launch. TNO tested and developed various systems including one which ensures that the laser beams of eLISA end up at exactly the right place, even over a distance of 1 million kilometres.

For eLISA Nikhef, Radboud University, University of Amsterdam, Leiden University, University of Groningen, University of Twente, VU University Amsterdam and SRON have joined forces and contributed their scientific strengths. Nikhef, TNO, NOVA and SRON are working together on developing the technology for eLISA.

Reference
Sub-femto-g free-fall for space-borne gravitational wave detectors: LISA Pathfinder results, Physical Review Letters, 7 June 2016.

More information
For further information please contact:
Gijs Nelemans, Radboud University, +31 24 365 29 83
Martijn Smit, SRON Netherlands Institute for Space research, +31 88 77 756 80
or with SRON spokesperson Frans Stravers, +31 6 52 67 93 95.

See also the ESA-media kit