Quantum gravity effects on space-time

TL;DR

This paper reviews how quantum gravity, especially loop quantum gravity, influences the structure of space-time at the smallest scales, focusing on effective geometries near the big bang and the role of dynamical states.

Contribution

It provides a comprehensive review of quantum gravity effects on space-time, emphasizing the development of effective equations and the importance of dynamical states in loop quantum cosmology.

Findings

Quantum gravity introduces counter-intuitive effects on space-time structure.

Effective equations help describe the regime near the big bang.

Dynamical states are crucial for understanding quantum space-time geometry.

Abstract

General relativity promotes space-time to a physical, dynamical object subject to equations of motion. Quantum gravity, accordingly, must provide a quantum framework for space-time, applicable on the smallest distance scales. Just like generic states in quantum mechanics, quantum space-time structures may be highly counter-intuitive. But if low-energy effects can be extracted, they shed considerable light on the implications to be expected for a dynamical quantum space-time. Loop quantum gravity has provided several such effects, but even in the symmetry-reduced setting of loop quantum cosmology no complete picture of effective space-time geometries describing especially the regime near the big bang has been obtained. The overall situation regarding space-time structures and cosmology is reviewed here, with an emphasis on the role of dynamical states, effective equations, and general covariance.