Predicting the Cosmological Constant from the Causal Entropic Principle

TL;DR

The paper addresses why the observed cosmological constant is so small by proposing the Causal Entropic Principle (CEP), which weights vacua by the total entropy production, $\Delta S$, within their causal diamonds. By modeling the causal diamond in a flat FRW universe and identifying the dominant entropy source as dust heated by starlight, the authors compute $\Delta S(\rho_\Lambda)$ and derive a probability distribution for $\rho_\Lambda$ that aligns with observations, with the peak corresponding to when the rate of entropy production coincides with the maximum accessible volume. The result contrasts with anthropic weighting by observers per baryon, which suffers from regularization issues and tends to overestimate the preferred $\rho_\Lambda$. The analysis also reveals that the entropy production is largely insensitive to $\rho_\Lambda$ except through geometric effects, and that the CEP can potentially enable predictions across the broader string landscape beyond our vacuum. Overall, the CEP provides a robust, thermodynamically grounded framework for predicting environmental parameters in a multiverse context, with practical implications for how we interpret the smallness of $\Lambda$ and future extensions to multiple varying parameters.

Abstract

We compute the expected value of the cosmological constant in our universe from the Causal Entropic Principle. Since observers must obey the laws of thermodynamics and causality, the principle asserts that physical parameters are most likely to be found in the range of values for which the total entropy production within a causally connected region is maximized. Despite the absence of more explicit anthropic criteria, the resulting probability distribution turns out to be in excellent agreement with observation. In particular, we find that dust heated by stars dominates the entropy production, demonstrating the remarkable power of this thermodynamic selection criterion. The alternative approach - weighting by the number of "observers per baryon" - is less well-defined, requires problematic assumptions about the nature of observers, and yet prefers values larger than present experimental bounds.