“paving the way for future spaceborne gravitational-wave observatories”
Gravitational wave observation from space and eLISA
Observation of low frequency gravitational waves from a space observatory such as eLISA — proposed for the ESA “Gravitational Universe” large mission (L3) theme selected for the 2030 timeframe — will revolutionize astrophysics, opening a whole new window of discovery for the physics and astronomy of massive black holes at galactic centers, stellar mass compact objects, and the interactions among and between these disparate astrophysical populations.
The gravitational waves produced by these distant sources produce an measurable time varying differential force that on a constellation of free-falling test particles, a “tidal deformation” that alternately shrinks and expands their effective separation.
Einstein’s Geodesic Explorer: LISA Pathfinder
LISA Pathfinder is an ESA mission dedicated to performing a relative acceleration measurement between two “geodesic reference” test masses in near-perfect free-fall. The two free-falling test masses inside a single spacecraft at 40 cm separation, and an optical interferometer measures their differential displacement. The target precision of 3 x 10-14 m/s2/Hz1/2 – 30 fm/s2/Hz1/2 with 1 fm = 1 femto-meter – at 1 mHz, or a resolution of 1 fm/s2 in one cycle of 1000 s period. This is within an order of magnitude of the eLISA sensitivity goal and would guarantee most of the eLISA science return. This is also a several order of magnitude improvement on the current state of the art for gravitational gradient measurements, provided by the ESA geodesy mission GOCE.
LISA Pathfinder represents an in-flight experimental verification of most aspects of the eLISA relative acceleration measurement, as well as an orbiting laboratory for understanding the limits of the most ambitious measurements in spacetime physics and of small forces. Specifically, LISA Pathfinder will demonstrate:
- Test masses in near perfect free-fall
- First high precision interferometric displacement measurement in space
- Drag-free spacecraft control at the nm level
- Flight verification of the eLISA “Gravitational Reference Sensor”
- Physical model of the limits for differential acceleration measurements for gravitational physics
LISA Pathfinder at the University of Trento
The Laboratory of Experimental Gravitation at the University of Trento has worked towards the development of an orbiting gravitational wave observatory since the joint NASA – ESA studies of LISA – Laser Interferometer Space Antenna – in the 1990s. We have been a leader for a metrology demonstrator mission since its approval – as the LISA Technology Package aboard SMART-2 – as an ESA mission in 2002, and one of us (S. Vitale) serves as the LISA Pathfinder mission principal investigator (PI).
Our primary eLISA / LPF research activities, and main contributions to the start of gravitational wave observation from space, include:
- Designing and analyzing an in-orbit experiment to demonstrate sub-femto-g free-fall
- Designing and prototyping “Gravitational Reference Sensors” for eLISA and LISA Pathfinder
- Probing the limits of free-fall in the lab with femtoNewton torsion pendulums