Spectral dimension with deformed spacetime signature

TL;DR

This paper investigates how quantum gravity effects can cause a transition in spacetime signature from Lorentzian to Euclidean, affecting the spectral dimension and revealing a scale-dependent dimensional reduction.

Contribution

It applies a deformed Poincaré algebra to model signature change in loop quantum gravity-inspired spacetime and computes the resulting spectral dimension behavior.

Findings

Signature change linked to two invariant energy scales.

Derived invariant measure on momentum space for diffusion analysis.

Spectral dimension reduces to 1 in the UV limit for certain models.

Abstract

Studies of the effective regime of loop quantum gravity (LQG) revealed that, in the limit of Planckian curvature scales, spacetime may undergo a transition from the Lorentzian to Euclidean signature. This effect is a consequence of quantum modifications of the hypersurface deformation algebra, which in the linearized case is equivalent to a deformed version of the Poincar\'e algebra. In this paper the latter relation is applied to the LQG-inspired hypersurface deformation algebra that is characterized by the above mentioned signature change. While the exact form of the deformed Poincar\'e algebra is not uniquely determined, the algebra under consideration is representative enough to capture a number of qualitative features. In particular, the analysis reveals that the signature change can be associated with two symmetric invariant energy scales, which separate three physically disconnected momentum subspaces. Furthermore, the invariant measure on momentum space is derived, which allows to properly define the average return probability, characterizing a fictitious diffusion process on spacetime. The diffusion is subsequently studied in the momentum representation for all possible variants of the model. Finally, the spectral dimension of spacetime is calculated in each case as a function of the scale parameter. In the most interesting case the deformation is of the asymptotically ultralocal type and the spectral dimension undergoes a reduction to $d_S = 1$ in the UV limit.