Quantum gravity, space-time structure, and cosmology

Quantum effects in gravitational interactions and space-time structure  can have different implications, some of them relevant for cosmology. On  dimensional grounds and using the classical understanding of  covariance, one expects an effective action with higher-curvature  corrections. These terms are very small in standard cosmological regimes  and cannot leave significant imprints on observable structures.  However, the notion of covariance may itself receive quantum  corrections, allowing new and potentially larger cosmological effects.  Examples have been found in loop quantum gravity, with implications  especially for the tensor-to-scalar ratio. In high-curvature regimes  where general relativity becomes singular, loop quantum gravity implies  quantum corrections so strong that they turn Lorentzian space-time into  Euclidean 4-space.