Remarks on cosmological issues in some string theoretic brane worlds

TL;DR

This paper investigates how string theory-based five-dimensional brane worlds can self-tune the cosmological constant and recover standard cosmological evolution, highlighting the role of supersymmetry and additional integration constants.

Contribution

It demonstrates that self-tuning of the cosmological constant in stringy brane worlds depends on unbroken supersymmetry and introduces the significance of integration constants from compactification.

Findings

Self tuning occurs only with unbroken supersymmetry.

Adjustment to zero cosmological constant involves integration constants.

Stringy brane worlds allow tuning without fine-tuning due to extra constants.

Abstract

We examine, in the context of certain string compactifications resulting in five dimensional brane worlds the mechanisms of (self) tuning of the cosmological constant and the recovery of standard cosmological evolution. We show that self tuning can occur only as long as supersymmetry is unbroken (unless additional assumptions are made) and that the adjustment of the cosmological constant to zero after supersymmetry breaking and the recovery of standard evolution are the same problem verifying previously made statements in the context of general i.e. not necessarily string theoretic brane worlds. We emphasize, however, that contrary to general brane worlds where the above adjustment requires a fine tuning, stringy brane worlds contain an additional integration constant due to the presence of the compact space thus allowing the adjustment to be done only with integration constants.