- 4-8 september 2016 : LISASymposium 11 à Zurich
- 7 june 2016 : publication of the first results of LISAPathfinder : PRL.116.231101
- Gérard AUGER (Emeritus CNRS)
- Pierre BINETRUY (Prof. - LISA Responsible at APC lab- Member of LISA consortium board) : Theory
- Eric Bréelle (IR) : R&D
- Hubert HALLOIN (MCF - "Physicien référent" R&D) : R&D, DPC, LISAPathfinder
- Maude LEJEUNE (IR - DPC project manager) : DPC
- Antoine PETITEAU (MCF - PI France LISAPathfinder - "Physicien référent" DPC) : LISAPathfinder, LISA simulation, LISA data analysis, DPC, R&D
- Eric PLAGNOL (Emerite CNRS - Membre du LISA consortium board) : LISAPathfinder
- Ed PORTER (CR CNRS) : Data analysis, theory
- Pierre PRAT (IR - LISAPathfinder project manager) : LISAPathfinder, R&D
- Jean-Baptiste BAYLE (PhD candidate): LISA simulation, LISAPathfinder, Data analysis
- Yann BOUFFANAIS (PhD candidate): Data analysis
- Cecile CAVET(CDD-IR): DPC
- Valerie DOMCKE (Post-doc): Theory
- Henri INCHAUSPE (Post-doc): LISAPathfinder, LISA simulation
- Matthieu LAPORTE (PhD candidate): R&D
- Etienne MARIN-MATHOLAZ: DPC, R&D
- Joseph MARTINO (CDD-IR): LISAPathfinder - LISA
- Mauro PIERONI (PhD candidate): Theory
LISA (Laser Interferometer Space Antenna) is the answer of the science community to the L3 large mission of ESA (European Space Agency) : "The Gravitational Universe". LISA aims at detecting low frequency garvitational waves from space. LISA is an ESA-led project, with a strong participation of the European member states as well as NASA..
As for ground-based detectors, LISA is based on laser interferometry to measure the tiny change in distance between free-falling mirrors, due to the passing of a gravitational wave. Space has two main advantages over ground-based detectors for the science envisioned by LISA: it offers a very quiet environment in the measurement frequency range of the instrument (0.1 mHz to 1 Hz) and allows longer arm-length (and therefore a better sensitivity). In the LISA project, the free falling mirrors consist of test masses, freely floating within the S/C. The S/C is constantly following the test mass, effectively shielding it from any other, non-gravitational, disturbance (e.g. the solar wind). This is the task of the drag-free attitude control system. Perfect free fall also implies that there is no station keeping, nor formation flying. This is not strictly true for LISA, since the satellites need to be rotated to keep each other in the field of view, and long-term maneuvers are required to avoid the constellation drifting apart. So, the test masses are kept free falling only on the sensitive axis (laser links between S/C) and in the desired frequency range (0.1 mHz to 1 Hz).
LISA consists of 3 S/C defining 3 measurement arms. The 3 S/C are put on Earth-like orbits (though slightly inclined), allowing a near-equilateral constellation, with Mkm (1 to 5, probably around 3 Mkm) armlength. The constellation is trailing the Earth by about 20°. This angle doesn't have to be constant over time (possible drift) but should be within ~10 to ~25° over the duration of the mission (at least 5 years).
Each S/C houses two free-falling test masses that define the endpoints of two arms (Figure 3).
The 3 S/C are identical and laser links are exchanged both ways on all arms.
In addition, each spacecraft accommodates the interferometry equipment needed to measure changes in the arm length. A key feature of the LISA concept is that the test masses are protected from disturbances as much as possible by careful design (e.g. internal gravitational field balance) and “drag-free” operation. To establish drag-free operation, a housing around the test mass senses the relative position of test mass and spacecraft, and a control system commands the spacecraft’s thrusters to follow the free-falling mass. Drag-free operation reduces time-varying disturbances to the test masses caused by force gradients arising in a spacecraft that is moving with respect to the test masses. The amplitude spectral density of the residual acceleration of the test mass characterizes the disturbance reduction, the first basic function of the science instrument. An additional benefit of the LISA orbits is the almost constant sun-angle of 30° of the spacecraft, thereby resulting in an extremely stable thermal environment, minimizing thermal disturbances on the spacecraft (especially on the optical bench).
LISA achieves the requisite approximate 3.10-20/√Hz strain sensitivity (averaged over all sky locations and polarizations), which allows to detect a strain of about 3.7.10-24 in a 2-year measurement with an SNR of 1, in part, through a phase resolution of about 10 μcycle/Hz with 1 μm wavelength light, resulting in a displacement sensitivity of 11.10-12 m/√Hz over a path length of 1.109 m. The achievable reductions of disturbances on test masses and the achievable displacement sensitivities by the laser ranging system yield a useful measurement bandwidth in the frequency range from 3.10-5 Hz to 1 Hz. (The requirement is 10-4 Hz to 1 Hz; the goal is 3.10-5 Hz to 1 Hz.)
The distance measuring system is a continuous interferometric laser ranging scheme. Diffraction widens the laser beam so that for each Watt of laser power sent, about 250pW are received. Lasers at each end of each arm operate in a “transponder” mode. A laser beam is sent out from the S/C 1 to S/C 2. The laser in the S/C 2 is then phase-locked to the incoming beam thus returning a high-power phase replica. The returned beam is received by S/C 1 and its phase in turn compared with the phase of the local laser. Similar locking schemes are used for the other arms, resulting in all the laser sources to be phase locked on another one, except for a single source (master laser). The phases of the two lasers serving two arms are compared in the central spacecraft. Two additional measurements are required for the data analysis. One is the absolute distance between the test masses. The main measurement registers changes in the distance with picometer accuracy. In contrast, the absolute distance is needed only to an accuracy of a few meters, which is easily achieved by imprinting a simple ranging code on the laser light. The other additional measurement concerns the time on each spacecraft. As spacecraft clocks with sufficient stability do not exist (and are not likely to come in existence in the next decade) the relative clock error between the spacecraft has to be recorded. For that purpose, the clock signal is imprinted on the laser light as well (offset bands generated by an EOM), allowing an easy comparison between the clocks on different S/C.
The set of phase measurements together with the additional measurements then allows recording the changes in phase due to the optical path difference, laser frequency noise, clock noise, etc. For practical reasons, this measurement is broken up into three distinct parts: the distance from TM to optical bench on SC1, long distance measurement between optical benches of SC1 and SC2, distance from optical bench to TM on SC2. By combining the three measurements, the measurement of the distance between the test masses is reconstructed and kept insensitive to the noise in the position of the spacecraft with respect to the test masses. Normally, such a partitioning would be avoided as it increases the noise due to the number of detectors involved. However, the detector noise is generally negligible in LISA, the partitioning of the measurement has no significant degrading impact on the overall sensitivity.
A large number of sources emit gravitational waves in the LISA frequency band:
- Supermassive black holes (from 104 to 107 solar masses): the spiral phase and the fusion will be observed with signal to noise ratios of 100 to 1000.
- Galactic binaries formed of white dwarfs and neutron stars whose solar mass is typically that of the Sun.
- Binaries with extreme mass ratio (EMRI): a small compact object of a few tens of solar mass orbiting around a supermassive black hole
- Binary black holes of a few tens of solar mass: this type of source has been detected by LIGO / Virgo (GW150914, GW151226, ...) and will be observable by LISA years before the fusion.
- Stochastic backgrounds emitted during the first moments of the Universe
- Bursts of cosmic cord recombination
LISA opens a new window on the Universe. The most interesting sources are therefore those which are not expected!!
The scientific fields covered by LISA are thus:
- The nature of gravity (testing the bases of general relativity)
- The fundamental nature of black holes: existence of a horizon, ...
- The black holes as a source of energy,
- The formation of structures in the universe (non-linear): first object, hierachic evolution, gas accretion,
- The end of life of massive stars,
- The dynamics of the heart of galaxies,
- The very young universe: the physics of the Higgs at the TeV (CERN), the topological defects, ...
- The cosmological models,
To meet the main technological challenges of LISA, the LISAPathfinder (ESA / NASA) technological demonstration mission was launched on December 3, 2015. It was positioned in an orbit around the Lagrange L1 point of the Terre-Soleil system at (1, 5 million kilometers from Earth), two inertial masses at a distance of 38 cm, the relative distance of which is measured with very high precision by heterodyne laser interferometry. These two small cubes of 4cm of side composed of gold and platinum "float" in cubic enclosures spaced 38 cm. The satellite protects the cubes from external influences by constantly adjusting its position thanks to an ultra-precise system of micro-rockets. The external forces disrupting the satellite, such as the solar wind, are thus countered. The cubes remain centered in the cavities, in "free fall", animated by an orbit determined only by gravity. The distance measurement by laser interferometry between these two cubes makes it possible to evaluate the residual acceleration noise between the two masses, in other words the level of the disturbing forces that could not be countered.
The LTP consists of the test masses, the electrode system around the masses, the gas-cold microfuse, the interferometer and an on-board computer called DFACS
The scientific operations began on 1 March 2016 and have since taken place in an environment of great stability. This allows, for example, measurements lasting more than one week without external intervention. The LISAPathfinder team performs many experiments to characterize the sources of residual noise.
Every day the data are received and formatted by ESA and then analyzed by scientists from different European laboratories: AEI (Germany), IEEC (Spain), APC (France), Trento (Italy) and Cardiff, Birmingham, Imperial College (UK). They use a software called LTPDA, which is made up of a set of tools facilitating the implementation of analysis chains with a complete history tracking of the operations performed. In order to test the validity of the tools put in place, "STOC Exercises" were carried out twice a year before the mission.
During the scientific operations phase of LISAPathfinder, a daily analysis of the data is carried out by ESOC (European Space Operation Center, Darmstadt, Germany) by teams taking turns. This so-called "online" analysis consists in processing the data as soon as they arrive by means of a series of framed procedures which check their quality, the presence of any problem and give the first results. At the same time, offline analyzes are carried out by delocalized centers, such as the FACe in Paris, which in particular is equipped with technical support and calculation capabilities (cluster, cloud) that the LISAPathfinder team makes full use of. These detailed analyzes make it possible to refine the results and to understand the operation of the instrument as well as possible. Thus it is possible to quickly use these results to adjust the design of LISA and maximize its scientific return.
- 3/12/2015: take-off from Kourou by the Vega rocket (flight VV06).
- 22/01/2016: arrived in the final orbit and separation of the propulsion module
- 17/12/2015 → 01/03/2016: commissioning: the manufacturers who have delivered the various subsystems verify their good functioning with ESA and scientists
- 01/03/2016 → 27/06/2016: nominal operations of the LTP The European part of LISAPathfinder consisting of the test masses, the electrode system around the masses, the gas-cold microfuses, the interferometer, An on-board computer called DFACS
- 27/06/2016 → 11/2016: nominal DRS operations The NASA part of LISA Pathfinder consisting of micro-rockets to colloid and a computer
- 12/2016 → 31/05/2017: extension of LTP operations
The first results of LISAPathfinder were published on June 7, 2016 in Physical Review Letter (PRL.116.231101: Sub-Femto-g Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results) Two cubes is 100 times more efficient than what was reached in the laboratory: it allows to measure a distance to 30 femto-meters (one ten-thousandth of the size of an atom). The differential acceleration measured between the two test masses at the frequency of 1 milli-Hertz is 5.5 fm.s-2 (fm.s-2 = femto-meters per square seconds = 10-15 ms-2) or Lower than half a billionth of a millionth of the earth's gravity (9.6 ms-2). These performances far exceed the requirements of LISAPathfinder of 30 fm.s-2. They are very close to the desired performances for LISA, which are 4.2 fm.s-2. The residual disturbing forces on the reference masses are therefore less than 8 femto-Newton (10-15 Newton), ie less than the weight of an E. coli bacterium on Earth (1000 times lower than Weight of a cell) which would have an oscillating movement of a period of 1000 seconds. The difference between the objectives of LISAPathfinder and those of LISA is due to the fact that the LISAPathfinder satellite does not attract both masses strictly in the same way, unlike LISA where the masses are in 2 satellites separated by millions of kilometers.
The 5 members of the LISAPathfinder operational team at the APC are working either in "shift" at the ESOC or in the FACe "operations room". They are specialized in:
- The daily verification of the correct operation of the laser injection unit,
- Characterization of thrusters
- The characterization of control loops
- The transfer of the results of LISA Pathfinder to LISA thanks to the simulation expertise for LISA of the laboratory
- The monitoring of the effects in the long term.
- International website of the LISA project: http://www.elisascience.org/
- nternational website of the LISAPathfinder project: http://www.elisascience.org/articles/lisa-pathfinder/lpf-mission
- LISAFrance website: http://elisadpc.in2p3.fr/lisafrance/
- eLISA white paper: «The Gravitational Universe» (http://elisascience.org/dl/TheGravitationalUniverse.pdf)