Global InformationLoop quantum cosmology information
Loop quantum cosmology (LQC)[1][2][3][4][5] is a finite, symmetry-reduced model of loop quantum gravity (LQG) that predicts a "quantum bridge" between contracting and expanding cosmological branches.
The distinguishing feature of LQC is the prominent role played by the quantum geometry effects of loop quantum gravity (LQG). In particular, quantum geometry creates a brand new repulsive force which is totally negligible at low space-time curvature but rises very rapidly in the Planck regime, overwhelming the classical gravitational attraction and thereby resolving singularities of general relativity. Once singularities are resolved, the conceptual paradigm of cosmology changes and one has to revisit many of the standard issues—e.g., the "horizon problem"—from a new perspective.
Since LQG is based on a specific quantum theory of Riemannian geometry,[6][7] geometric observables display a fundamental discreteness that play a key role in quantum dynamics: While predictions of LQC are very close to those of quantum geometrodynamics (QGD) away from the Planck regime, there is a dramatic difference once densities and curvatures enter the Planck scale. In LQC the Big Bang is replaced by a quantum bounce.
Study of LQC has led to many successes, including the emergence of a possible mechanism for cosmic inflation, resolution of gravitational singularities, as well as the development of effective semi-classical Hamiltonians.
This subfield originated in 1999 by Martin Bojowald, and further developed in particular by Abhay Ashtekar and Jerzy Lewandowski, as well as Tomasz Pawłowski and Parampreet Singh, et al. In late 2012 LQC represented a very active field in physics, with about three hundred papers on the subject published in the literature. There has also recently been work by Carlo Rovelli, et al. on relating LQC to spinfoam cosmology.
However, the results obtained in LQC are subject to the usual restriction that a truncated classical theory, then quantized, might not display the true behaviour of the full theory due to artificial suppression of degrees of freedom that might have large quantum fluctuations in the full theory. It has been argued that singularity avoidance in LQC are by mechanisms only available in these restrictive models and that singularity avoidance in the full theory can still be obtained but by a more subtle feature of LQG.[8][9]
- ^ Agullo, Ivan; Singh, Parampreet (2016). "[1612.01236] Loop Quantum Cosmology: A brief review". arXiv:1612.01236 [gr-qc].
- ^ Bojowald, Martin (2020). "Critical Evaluation of Common Claims in Loop Quantum Cosmology". Universe. 6 (3): 36. arXiv:2002.05703. Bibcode:2020Univ....6...36B. doi:10.3390/universe6030036.
- ^ Wilson-Ewing, Edward (2017). "Testing loop quantum cosmology". Comptes Rendus Physique. 18 (3–4): 207–225. arXiv:1612.04551. Bibcode:2017CRPhy..18..207W. doi:10.1016/j.crhy.2017.02.004. S2CID 119261924.
- ^ Struyve, Ward (2017). "Loop quantum cosmology and singularities". Scientific Reports. 7 (1): 8161. arXiv:1703.10274. Bibcode:2017NatSR...7.8161S. doi:10.1038/s41598-017-06616-y. PMC 5557943. PMID 28811562. S2CID 3318033.
- ^ Bojowald, Martin (2019). "Effective Field Theory of Loop Quantum Cosmology". Universe. 5 (2): 44. arXiv:1906.01501. Bibcode:2019Univ....5...44B. doi:10.3390/universe5020044.
- ^ Ashtekar, Abhay (2009). "Loop Quantum Cosmology: An Overview". Gen. Rel. Grav. 41 (4): 707–741. arXiv:0812.0177. Bibcode:2009GReGr..41..707A. doi:10.1007/s10714-009-0763-4. S2CID 115155250.
- ^ Bojowald, Martin (2005). "Loop Quantum Cosmology". Living Reviews in Relativity. 8 (1): 2. arXiv:gr-qc/0502091. Bibcode:2005LRR.....8....2A. doi:10.12942/lrr-2005-2. PMC 5253932. PMID 28163646.
- ^ On (Cosmological) Singularity Avoidance in Loop Quantum Gravity, Johannes Brunnemann, Thomas Thiemann, Class. Quantum Grav. 23 (2006) pp. 1395–1428.
- ^ Unboundedness of Triad – Like Operators in Loop Quantum Gravity, Johannes Brunnemann, Thomas Thiemann, Class. Quantum Grav. 23 (2006) 1429–1484.