The construction of a quantum theory of gravity is one of the important challenges in theoretical physics. Such a theory is required to address questions concerning the very early universe, and the properties of black holes.
String theory is a proposal to solve this problem. To show that it is indeed a useful approach towards quantum gravity, it needs to give an answer to the questions mentioned above. For instance, string theory should provide an explanation for the period of inflation, during which the early universe was blown up to enormous size. The expansion of the present universe is also increasing, one would like to understand why. String theory should explain the origin of this constant. It should explain the paradoxes of black hole physics: does quantum physics really allow the disappearance of objects behind the black hole horizon?
String theory should also provide an understanding of the Standard Model of particle physics, and a solution to problems that are still present in this model. Why do the elementary particles have the particular mass hierarchy that has been discovered, does the proposed Higgs mechanism fit into string theory? These questions concern the symmetries of the Standard Model. String theory would like to understand these in terms of supersymmetry, in which bosons and fermions are treated on an equal footing.
All these questions require experimental and observational input. The astronomical  input comes from observations of the universe, such as the WMAP and the Planck satellite experiments. The particle physics side is going to profit in the coming years
from the Large Hadron Collider (LHC) in Geneva, which will certainly result in advances in particle physics, and hopefully also make contact with aspects of string theory.
Other hints for string theory could come from recent applications to the quark-gluon plasma and quantum critical systems in condensed matter physics. The celebrated AdS/CFT correspondence plays a crucial role in these applications.
Our work in Groningen concern several aspects of string theory. We work on cosmological aspects in the context of supergravity models, which provide an approximation at low energy of string theory. We also work on more formal aspects. Some questions of quantum gravity can be discussed in simplified models, for instance,
by assuming two, rather than three, space dimensions. One hopes that such thought experiments lead to new insights in the real thing. In our research the relation with experiments and observations are essential. In Groningen we are in contact with the Kapteyn Astronomical Institute, and with the KVI, the Nuclear Physics Accelerator Institute, and of course with the String Cosmology group of the CTN. In the near future we hope, with out colleagues of Kapteyn and KVI, to set up coherent Master programme that will guide the interested  students to research in different aspects of  the theoretical, experimental and observational side of Quantum Gravity.