Emergent Spacetime in Stochastically Evolving Dimensions

TL;DR

This paper proposes a model where the number of spacetime dimensions evolves with temperature, starting from a high-dimensional state at the Big Bang and opening up to our familiar 3+1 dimensions as the universe cools, with implications for quantum gravity and cosmology.

Contribution

It introduces a string theory-based model of evolving dimensions with temperature-dependent mass terms, providing a mechanism for dimensional emergence without compactification.

Findings

Dimensions open sequentially as the universe cools.

Emergence of (3+1)-dimensional spacetime from a causal set.

Potential observational signatures in cosmic rays and high-multiplicity collisions.

Abstract

Changing the dimensionality of the space-time at the smallest and largest distances has manifold theoretical advantages. If the space is lower dimensional in the high energy regime, then there are no ultraviolet divergencies in field theories, it is possible to quantize gravity, and the theory of matter plus gravity is free of divergencies or renormalizable. If the space is higher dimensional at cosmological scales, then some cosmological problems (including the cosmological constant problem) can be attacked from a completely new perspective. In this paper, we construct an explicit model of "evolving dimensions" in which the dimensions open up as the temperature of the universe drops. We adopt the string theory framework in which the dimensions are fields that live on the string worldsheet, and add temperature dependent mass terms for them. At the Big Bang, all the dimensions are very heavy and are not excited. As the universe cools down, dimensions open up one by one. Thus, the dimensionality of the space we live in depends on the energy or temperature that we are probing. In particular, we provide a kinematic Brandenberger-Vafa argument for how a discrete {\it causal set}, and eventually a continuum $(3+1)$-dim spacetime along with Einstein gravity emerge in the Infrared from the worldsheet action. The $(3+1)$-dim Planck mass and the string scale become directly related, {\it without any} compactification. Amongst other predictions, we argue that LHC might be blind to new physics even if it comes at the TeV scale. In contrast, cosmic ray experiments, especially those that can register the very beginning of the shower, and collisions with high multiplicity and density of particles, might be sensitive to the dimensional cross-over.