Quantum Nature of the Big Bang in Loop Quantum Cosmology

A central feature of general relativity is that gravity is encoded in the very geometry of space-time. Loop quantum gravity is a non-perturbative approach to unifying general relativity with quantum physics while retaining this interplay between geometry and gravity [1, 2, 3]. There is no background space-time; matter as well as geometry are ‘quantum from birth’. Effects of quantum geometry are negligible under ordinary circumstances but they dominate near singularities. There, quantum space-time is dramatically different from the smooth continuum of general relativity. In particular, quantum geometry effects have led to a natural resolution of space-like singularities in a number of mini and midi-superspaces. These encompass both black hole and cosmological contexts. In the cosmological setting, there are several long-standing questions that have been relegated to quantum gravity. Examples are: • How close to the Big Bang does a smooth space-time of general relativity make sense? In particular, can one show from first principles that this approximation is valid at the onset of inflation? • Is the Big-Bang singularity naturally resolved by quantum gravity? Or, is some external input such as a new principle or a boundary condition at the Big Bang essential? • Is the quantum evolution across the ‘singularity’ deterministic? One needs a fully non-perturbative framework to answer this question in the affirmative. (In the Ekpyrotic and Pre-Big-Bang scenarios, for example, the answer is in the negative.)