Physical Society Colloquium

Stable, Accelerating Universes without Dark Energy

Dimitrios Psaltis

University of Arizona

One of the leading candidates in explaining the accelerated expansion of the Universe is via modifications to Einstein's theory at cosmological scales. A large amount of recent work, however, has shown that most of the modifications explored so far face serious problems with instabilities and solar-system tests. In this talk, I will argue that most of these problems are related to the interpretation of the modifications in the theory of gravity and are mathematical artifacts. I will show that properly treating modifications to Einstein's theory leads to Universes with no instabilities on cosmological scales and to predictions that agree with solar system tests. In fact, simple modifications to the theory of gravity produce a spatially flat, accelerating universe, even in the absence of dark energy and when the matter density is too small to close the universe in the general relativistic case.