Loop quantum cosmology

Loop quantum cosmology applies quantization methods from loop quantum gravity to symmetry-reduced cosmological models. In many models, quantum geometry replaces the classical big-bang singularity with a bounce.

Loop quantum gravity: A proposed quantum theory of gravity that represents spatial geometry using quantized areas and volumes. It supplies the quantization framework adapted to homogeneous cosmological spacetimes.
Canonical quantum gravity: Approaches to quantum gravity that formulate gravitational dynamics using canonical variables and constraints. Loop quantum cosmology inherits this constraint-based route to quantizing gravity.
Bianchi I spacetime: A homogeneous, spatially flat cosmological spacetime that can expand at different rates along three axes. Its loop quantization tests anisotropic dynamics beyond isotropic cosmologies.
Wheeler–DeWitt equation: A quantum constraint equation for the gravitational wave function in canonical quantum cosmology. It provides a continuum quantization whose standard cosmological models retain the big-bang singularity.
Quantization ambiguity: The non-uniqueness that arises when a classical theory is translated into a quantum theory. Different choices in representing curvature and constraints can change cosmological predictions.
Holonomy: The transformation acquired by a field transported around a closed path, encoding curvature. Loop quantum cosmology quantizes connection variables through holonomies rather than using the connection directly.
Friedmann–Lemaître–Robertson–Walker metric: A homogeneous and isotropic spacetime metric used to model the large-scale universe. Its symmetries reduce cosmological dynamics to a small set of gravitational variables.
Closed FLRW universe: A homogeneous and isotropic cosmological model with positively curved spatial sections. Its recollapse and bounce dynamics differ from those of spatially flat models.
Asymptotic safety: A proposed ultraviolet completion of gravity based on a renormalization-group fixed point. It pursues quantum gravity through scale-dependent couplings rather than loop geometry.
Universality class: A set of systems sharing the same large-scale behavior despite differing microscopic details. Whether bounce predictions persist across quantization choices is a question of universality.

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