Physical time and other conceptual issues of QG on the example of LQC

Abstract

Several conceptual aspects of quantum gravity are studied on the example of the homogeneous isotropic LQC model. In particular: $(i)$ The proper time of the co-moving observers is showed to be a quantum operator {and} a quantum spacetime metric tensor operator is derived. $(ii)$ Solutions of the quantum scalar constraint for two different choices of the lapse function are compared and contrasted. In particular it is shown that in case of model with masless scalar field and cosmological constant $\Lambda$ the physical Hilbert spaces constructed for two choices of lapse are the same for $\Lambda <0$ while they are significantly different for $\Lambda >0$. $(iii)$ The mechanism of the singularity avoidance is analyzed via detailed studies of an energy density operator, whose essential spectrum was shown to be an interval $[0,\rhoc]$, where $\rhoc\approx 0.41ρ_{\Pl}$. $(iv)$ The relation between the kinematical and the physical quantum geometry is discussed on the level of relation between observables.

Revtex4, 19 pages, 1 figure, revised to match the version published in CQG

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• Loop quantum cosmology of a radiation-dominated flat FLRW universe
• Loop Quantum Cosmology: A brief review
• The Martin-Benito-Mena Marugan-Olmedo prescription for the Dapor-Liegener model of Loop Quantum Cosmology
• The high-density regime of kinetic-dominated loop quantum cosmology
• Two-point functions in (loop) quantum cosmology
• Fluctuation energies in quantum cosmology
• Time evolution in quantum cosmology
• On the evolution of the volume in Loop Quantum Cosmology
• The Effect of a positive cosmological constant on the bounce of Loop Quantum Cosmology