Einstein's theory of General Relativity has been very successful in describing gravitational physics at a wide range of scales, from the astrophysical to the cosmological. At the largest of these cosmological scales, however, in order to do so the theory requires the inclusion of unknown matter and energy components, which have remained otherwise undetected by other means so far. Due to their mysterious nature, they are commonly referred to as Dark Matter and Dark Energy, respectively. The latter is especially intriguing, as it must have exotic properties in order to fill in the role of feeding the observed accelerating expansion of the universe. What is the true nature of Dark Energy remains a fundamental open question in physics.
A tantalizing possibility is that Dark Energy is not just yet another contribution to the energy budget of the universe, even if it seems to be the majority one, but rather a manifestation of gravity itself behaving differently than expected. This would mean a departure from the theory of General Relativity at the largest of cosmological distances. Nonetheless, testing this hypothesis is not an easy task as the only place where the effect of Dark Energy has been observed so far is the accelerated expansion of the universe. This fact, coupled with the sheer amount of possible ways to extend the theory of General Relativity has been a limiting factor in understanding the nature of Dark Energy from cosmological observations alone. The MGCODE project aimed to find new signatures of Dark Energy in a very different place, at the much shorter scales of black holes. The very strong gravity in their vicinity gives rise to a regime that still remains largely untested, and which is only recently becoming accessible through the detection of gravitational waves produced by the collision of pairs of black holes, neutron stars, and/or other compact objects. For this reason, it is an interesting place to look for deviations from the theory of General Relativity, and while this is certainly usually considered by the astrophysics community, the possible connection to deviations at cosmological scales is often overlooked. Establishing this connection between modifications of the theory of gravity at the very different astrophysical and cosmological scales can lead to powerful novel constraints on the properties of gravity.
This project was successful in finding concrete examples among a wide range of candidate modified gravity theories where Dark Energy, the cause for the accelerated expansion of the universe, also produces observable deviations from what the theory of General Relativity predicts about black-hole physics.
