4 research outputs found
Emergent cosmology from quantum gravity in the Lorentzian Barrett-Crane tensorial group field theory model
We study the cosmological sector of the Lorentzian Barrett-Crane (BC) model coupled to a free massless scalar field in its Group Field Theory (GFT) formulation, corresponding to the mean-field hydrodynamics obtained from coherent condensate states. The relational evolution of the condensate with respect to the scalar field yields effective dynamics of homogeneous and isotropic cosmologies, similar to those previously obtained in SU(2)-based EPRL-like models. Also in this manifestly Lorentzian setting, in which only continuous SL(2,Bbb C)-representations are used, we obtain generalized Friedmann equations that generically exhibit a quantum bounce, and can reproduce all of the features of the cosmological dynamics of EPRL-like models. This lends support to the expectation that the EPRL-like and BC models may lie in the same continuum universality class, and that the quantum gravity mechanism producing effective bouncing scenarios may not depend directly on the discretization of geometric observables
The Complete Barrett-Crane Model and its Causal Structure
The causal structure is a quintessential element of continuum spacetime
physics and needs to be properly encoded in a theory of Lorentzian quantum
gravity. Established spin foam (and tensorial group field theory (TGFT)) models
mostly work with relatively special classes of Lorentzian triangulations (e.g.
built from spacelike tetrahedra only), obscuring the explicit implementation of
the local causal structure at the microscopic level. We overcome this
limitation and construct a full-fledged model for Lorentzian quantum geometry
the building blocks of which include spacelike, lightlike and timelike
tetrahedra. We realize this within the context of the Barrett-Crane TGFT model.
Following an explicit characterization of the amplitudes via methods of
integral geometry, and the ensuing clear identification of local causal
structure, we analyze the model's amplitudes with respect to its
(space)time-orientation properties and provide also a more detailed comparison
with the framework of causal dynamical triangulations (CDT).Comment: 40 + 14 pages, 7 figure
Scalar Cosmological Perturbations from Quantum Gravitational Entanglement
A major challenge at the interface of quantum gravity and cosmology is to
explain how the large-scale structure of the Universe emerges from physics at
the Planck scale. In this letter, we take an important step in this direction
by extracting the dynamics of scalar isotropic cosmological perturbations from
full quantum gravity, as described by the causally complete Barrett-Crane group
field theory model. From the perspective of the underlying quantum gravity
theory, cosmological perturbations are represented as nearest-neighbor two-body
entanglement of group field theory quanta. Their effective dynamics is obtained
via mean-field methods and described relationally with respect to a physical
Lorentz frame causally coupled to the quantum geometry. We quantitatively study
these effective dynamical equations and show that at low energies they are
perfectly consistent with those of General Relativity, while for
trans-Planckian scales quantum effects become important. These results
therefore not only provide crucial insights into the potentially purely quantum
gravitational nature of cosmological perturbations, but also offer rich
phenomenological implications for the physics of the early Universe.Comment: 6+1 pages, 2 figure