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Home page > PhD fellowships > Proposed PhD subjects 2016-2019 > Virgo : Revealing gravitational waves out of terrestrial noise: identifying the main sensitivity limitations

Virgo : Revealing gravitational waves out of terrestrial noise: identifying the main sensitivity limitations

by Luc Frappat - 17 December 2015

Topics:  Gravitational waves, Virgo collaboration

Proponents:  Michal Was, Benoît Mours

Address:  LAPP - 9, chemin de Bellevue - BP 110 - 74941 ANNECY-LE-VIEUX CEDEX

Phone:  + 33 (0)450 09 55 20 (M. Was), + 33 (0)450 09 55 21 (B. Mours)

Contact Email:,

Gravitational waves are an early prediction of Einstein’s theory of General Relativity. An indirect proof of their existence was obtained in 1974 through the observation of a pulsar in a tight binary system. The Virgo detector in Italy aims at observing directly these perturbations of space-time. Beyond the confirmation of General Relativity in a new dynamic regime, this discovery will also open anew field of astronomy, where we observe directly the movement of compact objects heavily deforming space-time, such as neutron stars and black holes. The Virgo detector is undergoing the Advanced Virgo upgrade to improve its sensitivity. In 2016 it will join the Advanced LIGO detectors in the USA to form a network that should ultimately detect dozens of coalescing neutron star binaries per year.

The challenge is to measure deformations of space-time that are of the order of 10-22 or lower. The observation relies on a kilometer scale Michelson interferometer, which measures the relative distance between 40 kg mirrors suspended on complex isolation chains with a precision of 10-20 Hz. This measurement is limited by quantum fluctuations of the laser light, the thermal vibration of the mirrors, but also by a large number of environmental and technical noise sources. These other noise sources will initially limit the sensitivity of the upgraded detector, as various parameters are fine-tuned over several years, and additional auxiliary systems are introduced. A task of primordial importance is to measure the different noise contributions, identify the dominant sources and guide the priorities of the detector commissioning. Some examples are: scattered light, ambient electromagnetic noise, photo-diode quantum noise, control noise (sensor and feed-back), etc.

Light scattered by different optics is one of the noise contributors. It couples the vibrations of the scattering optic back into the measurement, short-circuiting the mirrors seismic isolation. Depending on the environmental conditions, this noise has been a limiting factor for all gravitational wave detectors constructed to date. For this reason most auxiliary optics in Advanced Virgo will be placed on suspended benches that hold all the photo-diodes, and the digital acquisition and control electronics. The LAPP group is in charge of these benches. The group is also involved in searching for gravitational waves from coalescing compact object binaries in real time and sending out alerts to radio/optical/X-ray telescopes in order to observe possible transient electromagnetic counterparts.

As a first step the PhD will study the control in position of suspended Advanced Virgo benches and model the impact of light scattered by these benches on the gravitational wave measurement. The goal will be to measure and lower the scattered light coupling with the rest of the interferometer, which will involve optimizing the bench position control and eliminating scattered light sources.
Building on this experience the student will incorporate other noise sources, studied by themselves or Virgo collaborators into a full noise budget that includes all understood noise couplings.This will progressively give a global view of the instrument and the functioning of its many sub-systems. The PhD is expected to take the lead on this project.

The student will also become an expert on some noise sources that will raise their interest. With improved sensitivity, unexpected noise sources may be discovered. This involves not only understanding and characterizing the coupling, but also proposing novel solutions to mitigate them. Depending on the case, this may involve advanced digital control techniques, feedback between sub-systems, or designing additional sensing and isolation equipment. 

The PhD will participate in the Virgo commissioning, and hence directly contribute to reaching the sensitivity required for a gravitational wave discovery. The student will be a member of the Virgo collaboration, which has a data sharing agreement with the LIGO Scientific Collaboration and joint instrument and commissioning meetings.