  Signature change at high density has been obtained as a possible consequence
of deformed space-time structures in models of loop quantum gravity. This
article provides a conceptual discussion of implications for cosmological
scenarios, based on an application of mathematical results for mixed-type
partial differential equations (the Tricomi problem). While the effective
equations from which signature change has been derived are shown to be locally
regular and therefore reliable, the underlying theory of loop quantum gravity
may face several global problems in its semiclassical solutions.
