On Quantum Spacetime and the horizon problem

Abstract

In the special case of a spherically symmetric solution of Einstein equations coupled to a scalar massless field, we examine the consequences on the exact solution imposed by a semiclassical treatment of gravitational interaction when the scalar field is quantized. In agreement with the work of Doplicher, Fredenhagen and Roberts (DFR), imposing the principle of gravitational stability against localization of events, we find that the region where an event is localized, or where initial conditions can be assigned, has a minimal extension, of the order of the Planck length. This conclusion, though limited to the case of spherical symmetry, is more general than that of DFR, since it does not require the use of the notion of energy through the Heisenberg Principle, nor of any approximation as the linearized Einstein equations. We shall then describe the influence of this minimal length scale in a cosmological model, namely a simple universe filled with radiation, which is effectively described by a conformally coupled scalar field in a conformal KMS state. Solving the backreaction, a power law inflation scenario appears close to the initial singularity. Furthermore, the initial singularity becomes light like and thus the standard horizon problem is avoided in this simple model. This indication goes in the same direction as those drawn at a heuristic level from a full use of the principle of gravitational stability against localization of events, which point to a background dependence of the effective Planck length, through which a-causal effects may be transmitted.

26 pages. v3: several discussions and clarifications added, misprints corrected

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• Entropy, energy and temperature-length inequality for Friedmann universes
• Planckian corrections to the Friedmann flat equations from thermodynamics at the apparent horizon
• Length and distance on a quantum space
• Comment on "Can we measure structures to a precision better than the Planck length?"[arXiv:1205.3636], by Sabine Hossenfelder
• Perturbative Algebraic Quantum Field Theory on Quantum Spacetime: Adiabatic and Ultraviolet Convergence
• Pale Glares of Dark Matter in Quantum Spacetime
• The Majid-Ruegg model and the Planck scales
• Physically viable rotating mass solutions surrounding Kerr black hole
• Quantum Spacetime and the Universe at the Big Bang, vanishing interactions and fading degrees of freedom
• High Performance Algorithms for Quantum Gravity and Cosmology
• Renormalization on the DFR Quantum Spacetime