Gravity may be the weakest of the fundamental forces, but it is the one that rules the evolution and fate of the universe.
Our standard gravity theory is General Relativity (GR), a beautiful theory originally proposed by Einstein describing gravity as the geometry of spacetime. Before this, gravity was best described by the Newtonian universal law of gravitation. This law was able to both describe gravity in our daily life (apples falling) and also the motion of planets around the Sun. However, its validity is limited. For instance, the orbit of Mercury (the planet with strongest gravity) cannot be described in full detail with Newtonian gravity.
One of the early successes of GR was to solve the problem of Mercury’s orbit, and so extend its domain of validity as a gravity theory (see diagram on the right).
Today, we are in a position of needing an alternative theory with a larger domain of validity. As happened with Mercury, current problems on the nature of dark energy and dark matter seem to suggest that something else is needed.
In particular, the strong gravity regime (where GR has not been fully tested yet), and the cosmological regime, are two scenarios where gravity could be modified. To look for alternatives, my research explores scalar-tensor theories, which add a new ingredient (a scalar field) to the gravity theory.
One of the most awesome predictions of General Relativity is that spacetime itself can behave as a wave.
The last decade has seen the rapid rise of gravitational wave (GW) science, upgrading GWs from theoretical predictions to routinely observed phenomena. The field is set to keep growing with the planned construction of advanced land and space detectors.
GWs emitted by merging compact objects (like neutron stars or black holes) schematically look like the diagram on the left, displaying three main stages. Of special interest is the final 'ringdown' stage: the short-lived waves emitted just after the merger. These waves, characterised by their quasinormal modes, contain exceptionally clean information about the black hole's properties and the underlying gravitational theory.
We currently have a large family of modified gravity theories predicting different black hole and GW properties, alongside an increasing number of GW observations sensitive to these modifications. Combining the two is one of the most powerful ways to test gravity today. The aim of my research is to select interesting theories, calculate their predicted ringdown signals, and confront them with observations.
More concretely, I study various scalar-tensor theories and their black hole solutions. Using black hole perturbation theory, I evaluate their stability and extract their quasinormal modes. This allows us to identify the parameters controlling deviations from GR, and therefore use actual data to put constraints on them.
Modifying gravity on cosmological scales creates a challenge: GR already works perfectly within our own Solar System. Therefore, any new theory must somehow "hide" its modifications locally. This is achieved through screening mechanisms: mathematical features that effectively shut off the extra gravitational effects in high-density environments, keeping the theory perfectly consistent with local precision tests.
My work focuses on figuring out exactly how and when these different screening mechanisms activate. While we know several types of screening exist, determining which one is actually at play for any given modified gravity theory is a notoriously tricky problem.