  Quantum gravity, as a fundamental theory of space-time, is expected to reveal
how the universe may have started, perhaps during or before an inflationary
epoch. It may then leave a potentially observable (but probably minuscule)
trace in cosmic large-scale structures that seem to match well with predictions
of inflation models. A systematic quest to derive such tiny effects using one
approach, loop quantum gravity, has, however, led to unexpected obstacles. Such
models remain incomplete, and it is not clear whether loop quantum gravity can
be consistent as a full theory. But some surprising effects appear to be
generic and would drastically alter our understanding of space-time at large
density. These new high-curvature phenomena are a consequence of a widening gap
between quantum gravity and ordinary quantum-field theory on a background.
