Context:
The context of this PhD is the study of different aspects in late-time cosmology and gravity in the context of ground-based detectors such as LIGO and Virgo and in particular future 3rd generation detectors such as Einstein telescope (ET).

Introduction:
After 50 years of experimental effort, the direct detection of gravitational waves by the LIGO/Virgo collaboration has demonstrated the great potential of gravitational wave astronomy for the exploration and understanding of the Universe. Its implications go from the discovery of new astrophysical objects, to powerful tests of fundamental physics and cosmology. The detection of the coalescence of two neutron stars (NS), accompanied by the coincident observation of the same event in various electromagnetic bands, has strongly constrained the gravitational-wave propagation speed. Focusing on cosmology, this has drastic consequences for many modified gravity scenarios, candidates to explain the current acceleration of the Universe. Moreover, this observation has provided an independent measurement of the Hubble rate today.

Aims:
The aim of this PhD is develop new and state-of-the-art tools to probe cosmology with gravitational waves, with a paricular focus on ET. Relative to LIGO-Virgo, ET will probe much much larger redshifts, up to z ~ 100 for massive black hole (BH) binaries, and z ~ 3 for binary NS. Furthermore, the event rates will be very large. With such data, one will be able to address fundamental physics questions such as the equation of state of dark energy, the existence of primordial black holes, as well as testing theories of modified gravity. Because of the larger observable volume and bandwidth, additional effects have to be considered with respect to what is used today for LIGO and Virgo. This opens new questions and new techniques are called for.

For instance given that the signal associated with the merger of two NS will exist in the frequency band of ET for ~ days, it will be important to include the effects of the rotation of the Earth. It will also be crucially important to understand how to extract cosmological information from massive BH binaries (and indeed probably a good fraction of NS binaries as well) for which there are no electromagnetic counterparts. These will make up a vast number of the gravitational-wave events. To do so it is required to break the mass-redshift degeneracy that characterises the general relativity wavefrom from such binaries. One way of doing this will be to use astrophysical information regarding the population of BH and NS masses. Another is to use tidal effects of NS, once their equation of state is known.

Environment:
The work will be carried out in the Theory and Gravitation groups of APC, the latter of which also contains experimentalists working on Virgo and LISA.  At the international level, the student will take advantage of the lively and stimulating environment of the LIGO-Virgo collaboration. The international community working on the 3rd generation of detectors is getting organized. In Europe, the newly-born ET collaboration is ramping up and will offer an other interesting forum to present the results obtained during this PhD. Other than becoming a specialist at the forefront of understanding of early- and late-time cosmology and gravitational waves, during the PhD, the student will develop a number of different skills including techniques of mathematical physics, quantum-field theory, statistics and numerical methods.

Comment:
Applications to this PhD subject are closed as it is reserved for a pre-existing candidate.