Quantum Gravity

We introduce a framework that allows to calculate cosmological perturbations in a gauge invariant manner to any order. The two main features of this framework are to take physical observables as basic objects and to treat the variables describing the background geometry as fully dynamical. Backreaction effects can therefore naturally adressed. At the end I will mention applications to Loop Quantum Cosmology.

With a cosmic flight simulator, we'll take a scenic journey through space and time. After exploring our local Galactic neighborhood, we'll travel back 13.7 billion years to explore the Big Bang itself and how state-of-the-art measurements are transforming our understanding of our cosmic origin and ultimate fate. We then turn to the question of whether this can all be described purely mathematically, and discuss implications ranging from standard physics topics like symmetries, irreducible representations, units, free parameters and initial conditions to broader issues like parallel universes, simulations and and Goedel incompleteness.

Hints from quantum gravity suggest that a preferred frame may actually exist. One way to accommodate such a frame in general relativity without sacrificing diffeomorphism invariance is to couple the metric to a dynamical, time like, unit-norm vector field--the "aether". I will discuss properties and observational tests of a class of such theories, including post-Newtonian effects and radiation from binary pulsar systems.

A classical Hamiltonian system can be reduced to a subsystem of "relevant observables" using the pull-back under a Poisson embedding of the "relevant phase space" into the "full phase space". Since a quantum theory can be thought of a noncommutative phase space, one encounters the problem of the embedding of noncommutative spaces, when one tries to extend the reduction via a pull-back to a quantum theory. This problem can be solved for a class of physically interesting quantum systems and embeddings using an analogy to finding the base space of an embedded fibre bundle via the projection in the full fibre bundle. The resulting construction is then applied to Loop Quantum Gravity to extract a cosmological sector. This sector turns out to be similar but not equivalent to Loop Quantum Cosmology.

The description of noncommutative space will be given. I will show the relation between field theory on kappa-Minkowski space and the one in Minkowski. This construction leads to deformed energy momentum conservation law for energies close to the Planck scale.

We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological ``constant\'\' and may provide a mechanism for naturally small ``dark energy\'\'.

We consider the hypothesis that quantum mechanics is an approximation to another, cosmological theory, accurate only for the description of subsystems of the universe. Quantum theory is then to be derived from the cosmological theory by averaging over variables which are not internal to the subsystem, which may be considered non-local hidden variables. I will explain the motivation for this view, give some examples of theories of this kind and investigate general conditions for such an approach to succeed.