  General relativity describes the gravitational field geometrically and in a
self-interacting way because it couples to all forms of energy, including its
own. Both features make finding a quantum theory difficult, yet it is important
in the high-energy regime of the very early universe. This review article
introduces some of the results for the quantum nature of space-time which
indicate that there is a discrete, atomic picture not just for matter but also
for space and time. At high energy scales, such deviations from the continuum
affect the propagation of matter, the expansion of the universe, and perhaps
even the form of symmetries such as Lorentz or CP transformations. All these
effects may leave traces detectable by sensitive measurements, as pointed out
here by examples.
