Session E4: Cosmological Constraints on Theories of Gravity and Fundamental Physics

Dark Matter and Its Alternatives

Overwhelming evidence shows that systems ranging in size from dwarf galaxies to the universe as a whole experience much larger gravitational forces than can be accounted for by Newtonian gravitation from visible matter. Dark matter, which interacts gravitationally but not via the strong or electromagnetic force, explains many of these observations, and natural candidates for dark matter particles appear in common extensions of the standard model of particle physics. However, dark matter has not been directly detected, and some observations remain difficult to explain in the context of dark matter cosmological models. The alternative possible explanation, that we are observing a breakdown of Newton's law of gravitation on cosmological scales, has been far less studied. No successful cosmological model based on a modification of gravity has been proposed, but the hypothesis provides a simple explanation for some puzzling dark matter problems. I will review evidence favoring both hypotheses, and explain the kinds of observations which can provide definitive tests distinguishing between the two. Recent results from gravitational lensing and X-ray observations will be discussed.   

Theories of Cosmic Acceleration

I will discuss a variety of theoretical approaches to explaining the observed acceleration of the universe. After reviewing the main lines of current research on this front, I will then focus briefly on attempts to explain cosmic acceleration through modifications to General Relativity.   

CMB measurements and early universe cosmology

I will summarize the current constraints on inflation from CMB measurements and complementary probes of cosmology. I will discuss the importance of a measurement of primordial tensor modes, which fixes the energy scale of inflation; this is arguably the single most interesting cosmological parameter that is not currently fixed by observations. Finally, I will consider the exciting prospect of potentially observing modifications to the power spectrum of cosmological perturbations that might be introduced by new physics at the shortest length scales, using future ultra-high-precision CMB data.