Sabbatical Thomas Van Riet: A new perspective on the nature of dark energy

The operation of the universe remains a mystery on the very largest scale (cosmology) and the smallest scale (elementary particle physics). This is not a stumbling block to the development of most science and technology, because it focuses on the length scales between the largest and the smallest. However, to get an answer to the fundamental questions it is crucial to understand the extremes. What are the fundamental building blocks (particles) of the universe and their interactions (forces)? What is the Big Bang and what is a black hole? One of the most challenging problems in fundamental physics today is a paradox that connects the smallest with the greatest: the dark energy. Dark energy has been observed since 1998 and is the substance that accounts for nearly 70% of the total energy in the visible universe and is responsible for the repulsive nature of gravity on cosmological longitude scales. Gravity pulls objects together on everyday and astronomical length scales, but appears to be repulsive at cosmological distances. This leads to the universe expanding at an increasingly rapid rate. Quantum mechanics, ie the physics of the smallest, has an explanation for this at first sight: the quantum fluctuations of matter and energy at very short lengths leads to a constant 'vacuum' energy in the universe that can make gravitation repulsive at a certain length. The problem is that the length is extremely small instead of extremely large. So quantum mechanics is not consistent with cosmology. But to do this calculation correctly, we need a theory of quantum gravity. String theory is the most elaborate candidate. My research during the sabbatical is about whether string theory does indeed describe dark energy. My previous research seems to imply that this energy should decrease over time and future observations may confirm this.