There are two types of instruments to observe gravitational waves (GW) at low frequency: space-based interferometer in the milliHertz (mHz) band, and Pulsar Timing Array (PTA) in the nanoHertz (nHz) band. They are complementary either by observing two parts of the same sources as for stochastic backgrounds or two parts of the same population of sources as for massive black hole binaries.
LISA is space-based GWs observatory which is planned for launch in 2035. It consists of three satellites in the free fall in the heliocentric orbit forming an equilateral triangle. Satellites exchange laser light forming multiple interferometers allowing to observe a plethora of astrophysical and cosmological sources of GWs. These sources include galactic white dwarf binaries, extreme mass-ratio inspirals, massive black hole binaries, stochastic backgrounds.
PTA is using the timing of millisecond pulsars to observe GWs. Millisecond pulsars emit about hundreds of radio pulses per second with very high regularity. GWs passing between pulsar and Earth, modifies the time of arrival of the pulses. The timing an array of pulsars, enable to make a galactic scale GW detector. Multiple radio-telescopes contribute to PTA, in particular the Nançay Radio-Telescope. In June 2023, 4 PTA collaborations announced the results of 20 years of pulsar timing: strong evidence for a GWs signal. The signal still needs to be characterized and its origin established. It could have been emitted by an ensemble of super-massive black holes or by processes in the primordial Universe. While the two observing systems are different, the data analysis methods are similar. A large parameter space needs to be sampled to extract overlapping sources and disentangle them from the non-stationary noises.
GWs are a new way to learn about fundamental physics. For example, we can test general relativity with the merger of super-massive black holes binary and Extreme Mass ratio Inspiral and test particle physics beyond the standard model, thanks to the detection of stochastic background (SGWB) from phase transitions in the early Universe. The candidate will work at the CEA-IRFU (Institut de Recherche sur les Lois Fondamentales de l'Univers) as part of a cross-disciplinary team conducting research into GWs. This activity ranges from instrumental involvement in the LISA mission to the astrophysical or cosmological consequences of exploiting the signals, via the development of algorithms, simulations and data analysis. IRFU is also involved in PTA-France and International PTA. Developing methods for detecting gravitational wave sources and deducing the associated physical consequences is at the heart of the proposed thesis topic. The candidate will have the opportunity to take an interest in all aspects of the host team's activity and to interact with each of its members. The main objectives of the proposed work are to develop data analysis methods for LISA, taking advantage of developments in PTA and LISA, and to study the synergy between LISA and PTA observations for fundamental physics, in particular with SGWBs and Massive Black Holes (MBHs). The methods developed can also be adapted and applied to real PTA data. The candidate will be a member of the collaborations LISA, PTA-France, EPTA and IPTA. He/she will interact with members of the Groupement de Recherche Ondes Gravitationnelles and collaborate with physicists from the Astroparticles et Cosmologie (APC) laboratory. He will present his results within the LISA and PTA consortiums and at international conferences.