The Hubble tension and the quantum vacuum


  Dror Berechya  ,  Ulf Leonhardt  
Weizmann Institute of Science

The Hubble tension, arguably, the most intriguing open problem/crisis in cosmology, is a discrepancy in the value of the Hubble constant inferred from measurements of early-Universe phenomena, mainly cosmic microwave background fluctuations, to the value measured by late-Universe observations. This tension is significant, reaching up to 6 sigmas (depending on the choice of data combinations). While the late-Universe measurements are direct in nature and model-independent, the measurements done on the early-Universe are indirect and rely heavily on the cosmological model. Hence, the crisis. Lambda-CDM, the current standard model of cosmology, has a remarkable explanatory power; it is partly based on fundamental physics and partly based on observationally-motivated simple assumptions; but this humble, naive-almost model can describe with exquisite precision many complex cosmological phenomena. Yet, Lambda-CDM fails in the "end-to-end" test posed by the Hubble constant.

Lambda-CDM assumes that dark energy, which makes about 70% of the Universe's energy budget at present, is described by a cosmological constant. Recently, a new model was proposed. The "Lifshitz theory in cosmology" takes the starting point of Lambda-CDM but only changes its description of dark energy, wherein this new model is no longer a cosmological constant. The Lifshitz theory explains dark energy as a Casimir force. While the idea of dark energy being a quantum vacuum force has been around since the 1960s, it has never, up to now, proved to be right. The Lifshitz theory in cosmology changes the picture by basing the calculations on a theory of Casimir forces in dielectric media, with new insights coming from the realm of Atomic, Molecular, and Optical Physics; it provides us with dynamical dark energy, which is quantitatively relevant to cosmology and most importantly - able to resolve the Hubble tension.

While there are other attempts to solve the tension, the Lifshitz theory stands out in a few aspects. Lifshitz cosmology does not require any new physics, but rather relies on a well-established physics; as such, it is automatically compatible with the Standard Model of particle physics (SM). The Lifshitz theory, approximately, does not change the early-Universe dynamics; thus, to a good approximation, it preserves the excellent fit of CMB spectra to Lambda-CDM. For many other attempts, which do change the early-Universe dynamics, this issue is a real challenge. Lastly, the Lifshitz theory in cosmology is not devised for resolving the Hubble tension, but instead, it is devised for explaining the mystery that is dark energy; therefore, the fact that it does resolve the Hubble tension gives it a more natural appeal.

In this talk, I will briefly describe the Hubble tension and the Lifshitz cosmology. Then, I will present a preliminary work indicating that this theory can completely resolve the Hubble tension.