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Chameleon scalar fields are candidates for the dark energy, the mysterious component causing the observed acceleration of the universe. Their defining property is a mass which depends on the local matter density: they are massive on Earth, where the density is high, but essentially massless in the cosmos, where the density is much lower. All current constraints from tests of general relativity are satisfied. Nevertheless, chameleons lead to striking predictions for tests of gravity in the laboratory and in space. For example, near-future satellite experiments could measure an effective Newton's constant in space different by a factor of order unity from that on Earth, as well as violations of the Equivalence Principle stronger than currently allowed by laboratory experiments. Such signatures raise the exciting possibility of detecting dark energy through local tests of gravity.

The existence, and enigmatic nature, of 'Dark Energy' is one of the biggest theoretical upsets of recent times. In this seminar we present ideas on alternative theoretical and phenomenological approaches to the Dark Energy problem, in particular the issue of whether dark energy is a matter or gravity-based phenomenon, and the ways in which such approaches can been constrained and guided by observation. We also focus on some of the exciting future approaches that could provide unprecedented insights into the fundamentals of Dark Energy

The cosmic microwave background (CMB) is our most direct cosmological observable, encoding critical information about the evolution and development of the universe. The Wilkinson Microwave Anisotropy Probe (WMAP) has measured the angular power spectrum of the CMB temperature and temperature-polarization power spectra with unprecedented accuracy from its first year in flight. These recent observations along with developments in supernovae and galaxy surveys are generating critical challenges for theoretical physics, producing fundamental, intriguing questions in particle physics, cosmology and astrophysics that are as yet unresolved. We discuss in this seminar how the theoretical picture of the nature of dark matter and dark energy, and the origins of the universe, is being guided and modified by observations.

Understanding how galaxies form is a major current goal in physical cosmology: although a basic picture is well-accepted, there are outstanding mysteries to be solved. First, what is the origin of the heavy elements seen outside of galaxies? Given that these elements are created only inside galaxies, there must be a process whereby galaxies can expel gas rather than accrete it. Second, galaxy properties are somewhat different from theory predicts, yet extremely regular -- to the extent that it has been seriously argued that modified gravity, rather than dark matter, explains them. I will discuss these mysteries and the possibility that the same culprit -- galactic winds -- may play a key role in solving both.