Cosmological singularity resolution from quantum gravity: The emergent-bouncing universe

Abstract

Alternative scenarios to the Big Bang singularity have been subject of intense research for several decades by now. Most popular in this sense have been frameworks were such singularity is replaced by a bounce around some minimal cosmological volume or by some early quantum phase. This latter scenario was devised a long time ago and referred as an "emergent universe" (in the sense that our universe emerged from a constant volume quantum phase). We show here that within an improved framework of canonical quantum gravity (the so called Quantum Reduced Loop Gravity) the Friedmann equations for cosmology are modified in such a way to replace the big bang singularity with a short bounce preceded by a metastable quantum phase in which the volume of the universe oscillates between a series of local maxima and minima. We call this hybrid scenario an "emergentbouncing universe" since after a pure oscillating quantum phase the classical Friedmann spacetime emerges. Perspective developments and possible tests of this scenario are discussed in the end.

Equilibrium properties of blackbody radiation with an ultraviolet energy cut-off

“…We can also see its application in case of "bouncing" loop quantum cosmology theories (see [63] [64] [65] and the references therein), where normally we consider specific modifications to the spacetime geometry which effectively puts a bound on the curvature and in this way the Big Bang singularity can be avoided. But for such bouncing models, we cannot use the perturbation technique at the curvature saturation, as the energy density of the cosmic fluid diverges.…”

Bouncing universe of entropy-corrected Friedmann equations

“…Therefore, it can generally give us a full picture of the dynamics combined with phase plane analysis, especially in cosmology. Since such a study can be used to circumvent the need for initial conditions which is as one of the shortcomings of the standard cosmological model (SCM) [25][26][27][28][29][30][31][32][33][34][35][36][37][38]94]. In this right, the phase plane analysis is an invaluable tool in studying and visualizing the behavior of dynamical systems.…”

“…(5), (6)). Considering that a nonlinear autonomous system may be approximated by a linear system through its coefficient matrix (Jacobian), one can obtain the eigenvalues of Jacobian to standard classification of the different types of the fixed points [25][26][27][28][29][30][31][32][33][34][35][36][37][38]. In fact, the critical points of a system can be almost completely classified based on their eigenvalues, e.g.…”

“…Many attempts have been done to resolve this singularity problem through the modified general relativity theory [14][15][16][17][18], although it may be resolved via string theory and/or loop quantum cosmology [19,20] as some candidates for a quantum theory of gravity [21][22][23][24]. In this right, the scenario of an oscillating universe is to avoid the big bang singularity and replace it with a cyclical evolution [25][26][27][28][29][30][31][32][33][34][35][36][37][38].…”

Quantum gravity predictions for black hole interior geometry

“…In the case of LQC techniques applied to the Schwarzschild interior, this was clearly manifest in the use of point holonomies for the construction of the reduced kinematical Hilbert space associated with the 2-spheres that foliate the spacelike surfaces, which completely eliminated all information about the 3-D structure of the graph. Secondly, the reintroduction, by hand, of some 'elementary plaquette' at the end of the quantization procedure, in order to fix the quantum parameters entering the construction of the states, was an ambiguous prescription, with different choices yielding different physical scenarios, as well as undesirable features.In order to construct a more robust and reliable model of quantum black hole geometry that addresses the aforementioned issues, a new framework, the so-called 'quantum reduced loop gravity' approach [15][16][17][18][19][20], has recently been developed in which one starts from the full LQG theory and then performs the symmetry reduction at the quantum level by means of coherent states encoding the information about a semi-classical spherically symmetric black hole geometry. More precisely, computing the action of the quantum Hamiltonian operator on a partially gauge fixed kinematical Hilbert space results in a semiclassical, effective Hamiltonian constraint H eff that now replaces the Hamiltonian constraint of general relativity.…”