Stringy Models of Modified Gravity: Space-time defects and Structure Formation

TL;DR

This paper develops a string-inspired modified gravity model involving space-time defects, showing that D-particle recoil effects can seed cosmic structure growth during the Universe's evolution.

Contribution

It introduces a new effective action from microscopic space-time foam models with D-particles, linking defect recoil to cosmic structure formation.

Findings

Recoil vector fields provide seeds for structure growth.

A critical variance of D-particle velocities is necessary for sustained growth.

The model connects string theory concepts with cosmological evolution.

Abstract

Starting from microscopic models of space-time foam, based on brane universes propagating in bulk space-times populated by D0-brane defects ("D-particles"), we arrive at effective actions used by a low-energy observer on the brane world to describe his/her observations of the Universe. These actions include, apart from the metric tensor field, also scalar (dilaton) and vector fields, the latter describing the interactions of low-energy matter on the brane world with the recoiling point-like space-time defect (D-particle). The vector field is proportional to the recoil velocity of the D-particle and as such it satisfies a certain constraint. The vector breaks locally Lorentz invariance, which however is assumed to be conserved on average in a space-time foam situation, involving the interaction of matter with populations of D-particle defects. In this paper we demonstrate that, already at the end of the radiation era, the (constrained) vector field associated with the recoil of the defects provides the seeds for a growing mode in the evolution of the Universe. Such a growing mode survives during the matter dominated era, provided the variance of the D-particle recoil velocities on the brane is larger than a critical value.