Grant: Discovery Grants Program - Individual

Of the four known forces of nature, gravity is the most obvious and has sparked dramatic revolutions in physics, from Newtonian theory to General Relativity and cosmology, predicting exotic phenomena such as black holes and spacetime singularities. A revolution in cosmology started with the 1998 discovery that the present expansion of the universe is accelerated. What causes this acceleration? Assuming the standard theory of gravity-Einstein's General Relativity-76% of the energy content of the universe must be dark energy: exotic (it has negative pressure), not clumping, not detected in the laboratory. The cause of the cosmic acceleration is a major puzzle of fundamental physics. It is possible that dark energy doesn't exist, that our picture of gravity fails at the largest scales, and that we need to modify General Relativity instead of introducing exotic ingredients. My research explores a large class of theories that do that, with attention to their cosmology, comparison with local (i.e., Solar System) experiments, stability, and other criteria that a theory must meet in order to be acceptable.There are plenty of theoretical reasons to believe that gravity should be modified: when attempting to merge General Relativity and Quantum Mechanics, the two major creations of 20th century physics, General Relativity proves to be inconsistent and it must be modified at high energies (short scales). It is quite possible that these modifications survive also at the lower energy scales probed by cosmology. A recurrent idea of modern physics is that classical gravity may emerge from more fundamental constituents, ``atoms of spacetime'', in a way similar to classical fluids emerging from the more fundamental (quantum) atoms and molecules. A very influential idea hypothesizes that General Relativity corresponds to an equilibrium state in a ``thermodynamics of theories of gravity'', and that alternative theories of gravity are deviations from this equilibrium state. The research proposes to tie this idea with the foundations of General Relativity and of other theories of gravity and to probe these ``excitations'' and their approach to equilibrium. -theory of gravitational theories which could help understanding emergent or quantum gravity.