Loop quantum cosmology of the $k=1$ FRW: A tale of two bounces

We consider the $k=1$ Friedman-Robertson-Walker (FRW) model within loop quantum cosmology, paying special attention to the existence of an ambiguity in the quantization process. In spatially nonflat anisotropic models such as Bianchi II and IX, the standard method of defining the curvature through closed holonomies is not admissible. Instead, one has to implement the quantum constraints by approximating the connection via open holonomies. In the case of flat $k=0$ FRW and Bianchi I models, these two quantization methods coincide, but in the case of the closed $k=1$ FRW model they might yield different quantum theories. In this manuscript we explore these two quantizations and the different effective descriptions they provide of the bouncing cyclic universe. In particular, as we show in detail, the most dramatic difference is that in the theory defined by the new quantization method, there is not one, but two different bounces through which the cyclic universe alternates. We show that for a “large” universe, these two bounces are very similar and, therefore, practically indistinguishable, approaching the dynamics of the “curvature-based” quantum theory.

Figure 1

For three values of the volume at the bounce $V_b$, we plot the time evolution of the volume $V$ (left) and the density $\rho$ (right). These correspond to the values $V_b=500{\ell }_{\mathrm{Pl}}$ (solid line), $V_b=1000{\ell }_{\mathrm{Pl}}$ (dashed line), and $V_b=4000{\ell }_{\mathrm{Pl}}$ (dashed and dotted line).

Figure 2

For three values of the volume at the bounce $V_b$, we plot the time evolution of the volume $V$ (left) and the density $\rho$ (right). These correspond to the values $V_b=500{\ell }_{\mathrm{Pl}}$ (solid line), $V_b=1000{\ell }_{\mathrm{Pl}}$ (dashed line), and $V_b=4000{\ell }_{\mathrm{Pl}}$ (dashed and dotted line).

Figure 3

We plot the time evolution of the volume $V$ (left) and the density $\rho$ (right), for the two quantization methods, for $p_{\varphi }={10}^5$.