Saltatory Relaxation of the Cosmological Constant

TL;DR

The paper tackles the cosmological-constant problem by extending the Brown-Teitelboim saltation mechanism to string/M-theory, where brane nucleation coupled to a 3-form (and its 4-form flux) drives stepwise reductions of the effective cosmological term. The authors identify two key string-theoretic innovations that can qualitatively alter dynamics: (i) compactification-dependent brane tensions and charges that can yield extremely small, quantized jump units for $\Lambda$, and (ii) exponentially large density-of-states factors for coincident branes that can substantially enhance multi-bounce tunneling. An analysis of naturalness and stability reveals a nontrivial relation between the supersymmetry-breaking scale and the present value of $\Lambda$, with a plausible bound $M_{\rm SUSY}^2 \lesssim (10^{-3}\,\mathrm{eV}) M_{\rm Pl}$. The work proposes two concrete scenarios, one with a single dominant step and another with multi-step relaxation, showing how rapid relaxation can in principle drive $\Lambda$ toward the observed small positive value while maintaining stability on cosmological timescales. While not yet embedded in a full FRW cosmology, the approach offers a framework in which a tiny, positive cosmological constant can arise from fundamental stringy dynamics, potentially avoiding fine-tuning and the need for anthropic arguments by tying the outcome to the structure of the compactification and brane sector.

Abstract

We modify and extend an earlier proposal by Brown and Teitelboim to relax the effective cosmological term by nucleation of branes coupled to a three-index gauge potential. Microscopic considerations from string/M theory suggest two major innovations in the framework. First, the dependence of brane properties on the compactification of extra dimensions may generate a very small quantized unit for jumps in the effective cosmological term. Second, internal degrees of freedom for multiply coincident branes may enhance tunneling rates by exponentially large density of states factors. These new features essentially alter the relaxation dynamics. By requiring stability on the scale of the lifetime of the universe, rather than absolute stability, we derive a non-trivial relation between the supersymmetry breaking scale and the value of the cosmological term. It is plausibly, though not certainly, satisfied in Nature.