## Mid-Term Report Summary - EQU (Exploring the Quantum Universe)

The project ``Exploring the Quantum Universe'' investigates a theoretical model which combines quantum mechanics and general relativity. Such a unification has long been major goal in theoretical physics. The model is extremely simple. It tames the wildest quantum fluctuations by constructing the universe from simple small building blocks. While one can easily build a classical universe this way, quantum mechanics is implemented by taking an average of all possible universes one can construct from the building blocks, the weight of each such universe being dictated by its classical so-called action. This is the ``sum over histories'' representation of quantum mechanics. The wild quantum fluctuations are then re-introduced by decreasing the size of the building blocks and the main goal is to investigate if one can define the limit where the size of the building blocks goes to zero.

In that case one has defined a continuum theory of quantum geometries, which hopefully can be promoted to a continuum theory of quantum gravity.

It has been shown that the quantum average of the model, called Causal Dynamical Triangulations, in the infrared, i.e. at large distances, seems to be very similar to our real universe. When one probes the smallest distances, the so-called ultraviolet limit, where quantum fluctuations are expected to be important, the model seems most naturally related to a quantum gravity theory called Horava-Lifshitz gravity. This theory is characterized by time and space scaling differently at the shortest distances. For Causal Dynamical Triangulations these results have been obtained by extensive computer simulations of the model.

Presently these simulations are performed for universes without matter but for universes with a cosmological constant (like seemingly our own universe). It is the intention to include matter in the computer simulations and the hope is in this way to obtain a realistic quantum model of our universe.

In that case one has defined a continuum theory of quantum geometries, which hopefully can be promoted to a continuum theory of quantum gravity.

It has been shown that the quantum average of the model, called Causal Dynamical Triangulations, in the infrared, i.e. at large distances, seems to be very similar to our real universe. When one probes the smallest distances, the so-called ultraviolet limit, where quantum fluctuations are expected to be important, the model seems most naturally related to a quantum gravity theory called Horava-Lifshitz gravity. This theory is characterized by time and space scaling differently at the shortest distances. For Causal Dynamical Triangulations these results have been obtained by extensive computer simulations of the model.

Presently these simulations are performed for universes without matter but for universes with a cosmological constant (like seemingly our own universe). It is the intention to include matter in the computer simulations and the hope is in this way to obtain a realistic quantum model of our universe.