• Gravitational Waves
• LIGO
• Ringdown
• Tests of general relativity
• Virgo

The several observations of gravitational waves, generated by binary black holes coalescences as
well as by binary neutron stars mergers, officially ushered the observational era of observational
gravitational waves physics. Access to the most violent stages of the coalescence provides unprecedented
insights in the general relativistic strong-field dynamics.
The information contained inside such signals has already been used by the LIGO and Virgo
collaborations to estimate the parameters of such binaries and to produce some of the most
stringent tests of general relativity [1].
However, current methods are not suited to fully extract information about the black hole born
at the end of the coalescence process.
In fact, the remnant black hole mass and spin are inferred from measurements of the initial stages
of the coalescence in conjunction with predictions from numerical relativity simulations.
In this thesis, we show that, using recent models of the latest stage of the coalescence, known as
the “ringdown”, the final parameters of the coalescence can be directly extracted from the signal.
Combining numerical simulations and parameter estimation methods, we provide a procedure
to measure the parameters of the remnant black hole. For the first time, we demonstrate that
ground-based gravitational waves observatories will allow the direct measurement of the mass
and the spin of the remnant black hole, exclusively from the analysis of the ringdown part of the
signal.
We are also able to determine the “effective start time” of the ringdown, as well as the mass ratio
of the initial black holes, thanks to the imprint it leaves on the ringdown regime.
Independent methods from numerical relativity confirm our results (see [2]).
The start of the ringdown indicates the transition from the non-linear to the linear regime of the
theory of gravity, where perturbation methods can be applied. Its knowledge allows to test for
the presence of violations of general relativity that can be searched for only in the quasi-linear
regime. We quantify the constraints on parametric violations of general relativity with single
ringdown observations and show their improvements as the number of observed events increases.