Talks

In this talk we assume that Quantum Einstein Gravity (QEG) is the correct theory of gravity on all length scales. We use both analytical results from nonperturbative renormalization group (RG) equations and experimental input in order to describe the special RG trajectory of QEG which is realized in Nature. We identify a regime of scales where gravitational physics is well described by classical General Relativity. Strong renormalization effects occur at both larger and smaller momentum scales. The former are related to the (conjectured) nonperturbative renormalizability of QEG. The latter lead to a growth of Newton's constant at large distances. We argue that this effect becomes visible at the scale of galaxies and could provide a solution to the astrophysical missing mass problem which does not require dark matter. A possible resolution of the cosmological constant problem is proposed by noting that all RG trajectories admitting a long classical regime automatically imply a small cosmological constant.

We propose a grand unified theory (GUT) in which the gauge symmetry is dynamically broken by a strongly coupled gauge interaction, analogous to the chiral symmetry breaking in QCD or technicolor theory. GUT is a beautiful idea and surprisingly consistent with supersymmetry (SUSY). As well as the fact that all the fermions fit to a representation in GUT groups, the three gauge coupling constants meet at a very high energy scale with the particle content of the minimal SUSY standard model. However, the realistic model building of GUT has various difficulties such as the doublet-triplet Higgs mass splitting problem and the too rapid proton decay. Also, since the GUT appears to be a theory at very high energy scale, it is the usual case that there is no definite prediction to the low energy physics. We propose a realistic model without above problems by using the dynamical GUT symmetry breaking. The model provides an interesting predictions to the gaugino mass relation in low energy which should be easily testable with the LHC and a linear collider.

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