In one minute

Ziv Landau and Ulf Leonhardt show that in a uniform medium whose equal electric and magnetic response changes in time, light and its quantum fluctuations travel exactly as in empty space, but on a transformed clock called conformal time. The renormalised vacuum energy is exactly zero in empty flat space but not in such media, because renormalisation is local and causal and cannot fully undo the change to conformal time. To the electromagnetic field, the expanding universe is exactly such a medium. During cosmic expansion this vacuum energy effectively lowers the gravitational weight of radiation and matter by characteristic factors. This quantum buoyancy naturally resolves the Hubble tension between measured and inferred values of the Hubble constant.

Quantum noise in time-dependent media and cosmic expansion