First indications of causal set cosmology.

摘要

A variety of observations indicate that the universe is dominated by a so-called "dark energy" with an effective negative pressure, one possibility for which is a cosmological constant. If the dark energy is a cosmological constant, a fundamental question is: Why has it become relevant at so late an epoch, making today the only time in the history of the universe at which the cosmological constant is of order the ambient density. We explore an answer to this question based on the conjugacy - and resulting uncertainty relationship - expected between spacetime volume and the cosmological term in any theory of quantum gravity (and clearly visible in, say, unimodular gravity), and on spacetime discreteness at the fundamental level. The first of these predicts fluctuations in the cosmological constant, and then a fundamentally discrete theory like causal set theory predicts the magnitude of these fluctuations. The resulting ansatz yields a fluctuating cosmological "constant" which is always of order the ambient energy density. The model is not only structurally stable but also does extremely well on the observational side.