Phantom singularities and their quantum fate: general relativity and beyond—a CANTATA COST action topic

Abstract

Cosmological observations allow the possibility that dark energy is caused by phantom fields. These fields typically lead to the occurrence of singularities in the late Universe. We review here the status of phantom singularities and their possible avoidance in a quantum theory of gravity. We first introduce phantom energy and discuss its behavior in cosmology. We then list the various types of singularities that can occur from its presence. We also discuss the possibility that phantom behavior is mimicked by an alternative theory of gravity. We finally address the quantum cosmology of these models and discuss in which sense the phantom singularities can be avoided.

Cosmological constraints of phantom dark energy models

“…Nevertheless, the end of the Universe would not be the same when an interaction between matter and DE is considered. The cosmological singularities are often classified according to the divergence of some of the cosmological magnitudes, as for example, the Hubble parameter and its cosmic time derivatives [81]. In addition, we refer to a singularity when it occurs at a finite cosmic time (as it is the case of BR) while we coin it as an abrupt event when it occurs at an infinite cosmic time (as it is the case of LR and LSBR) [45].…”

Classical and Quantum f(R) Cosmology: The Big Rip, the Little Rip and the Little Sibling of the Big Rip

“…For example, a phantom-dominated universe could also reach a big freeze singularity (BF) [58,59], where the scale factor reaches a maximum size at which the Hubble rate and its cosmic time derivative diverge. (See also references [60][61][62] for other examples of cosmological singularities and references [63][64][65] for a detailed classification of DE singularities in cosmology.) However, it is commonly believed that a consistent quantum description of the universe may prevent the appearance of classical singularities, see references [66,67] (see also [64,65,[68][69][70][71]).…”

Classical and quantum fate of the little sibling of the big rip in f(R) cosmology

“…The Universe could also reach a cosmic singularity characterized by a divergent rate of expansion but a finite size of the observable Universe, freezing its evolution at a big freeze [6,7]. (See also [8][9][10][11], and references therein, for other examples of cosmic singularities.) Whereas the big rip and big freeze would take place at a finite cosmic time, the cosmic catastrophe may also be delayed an infinite cosmic time, in which case the singularity is called an abrupt cosmic event.…”

“…For observational constraints on this type of models see references [15][16][17]. Nonetheless, the common belief is that these singularities will be cured or avoided in the quantum realm, as it is assumed to happen with the big bang (see [11,[18][19][20] for reviews on the topic). In Table I we include a summary of these rip-like curvature singularities.…”

“…As the same background cosmic evolution can be described by GR or f (R) gravity, one could wonder whether cosmic singularities will be avoided in the quantum realm for different underlying fundamental theories of gravity [11]. In the framework of quantum geometrodynamics several works have evaluated different kinds of cosmological singularities [44][45][46].…”