Research - Dr. Valerio Faraoni
Theoretical Cosmology: Dark Energy and Modified Gravity
Dr. Valerio Faraoni
Department of Physics
Theoretical cosmology studies the dynamics, origin, evolution, and fate of the universe, and the formation of structures (galaxies, galaxy clusters, and superclusters) in it. In 1998 it was discovered, by studying distant supernovae, that the expansion of the universe is accelerated. Many theoretical models have been proposed in order to explain this shocking discovery and they mostly fall into two classes: dark energy and modified gravity. Dark energy models assume that Einstein's theory of gravity (general relativity) is valid and that a mysterious form of dark energy of unknown nature permeates the universe and makes up 70% of its energy content. This dark energy must necessarily have exotic properties, such as a negative pressure. The current observational data seem to require an even more negative pressure and more exotic energy (called phantom energy), which may cause the universe to end at a finite time in the future in a Big Rip singularity (the end of time), in which all bound objects-galaxies, planets, humans, atoms-are ripped apart by increasing gravitational forces. If phantom energy is really fueling the cosmic acceleration, we probably have to abandon Einstein's general relativity in favour of alternative gravity theories such as, for example, scalar-tensor gravity, a generalization of Einstein's theory motivated by string/M-theories. The latter attempt to unify gravity with the other three fundamental forces, a goal known as ``the holy grail of theoretical physics''.
The second class of models, modified gravity, does not require exotic dark energy but instead modifies Einstein's relativity with corrections that only affect large (cosmological) scales.
Dr. Faraoni's research explores both classes of models, trying to fully understand their dynamics, explain the cosmic acceleration, develop models that are theoretically consistent and compatible with available experiments, study their predictions (e.g., will the universe accelerate forever? Will it end in a Big Rip?) and related issues such as the production of gravitational waves, or the accretion of phantom energy onto black holes or wormholes. Long term goals include the development of the correct theory of gravity (it is possible that departures from Einstein's gravity are unobservable at the small Solar System scales but are already observed in the cosmic acceleration), finding out if dark energy actually exists and, if so, determining precisely its strange properties, understanding the early universe and obtaining information, otherwise inaccessible on Earth, on the high energy physics that left an imprint in the cosmic microwave background and in the distribution of galaxies and galaxy clusters.
Other recent interests include the study of black holes embedded in a cosmological background and the foundations and possible violations of the Equivalence Principle (the basis of relativistic gravity) in high energy physics.
Dr. Faraoni collaborates with various researchers worldwide on the subjects above and is involved in establishing an international research network on modified gravity. Details on his research and an up-to-date list of publications can be found at http://www.slac.stanford.edu/
