In one minute

Ziv Landau and Ulf Leonhardt model electromagnetic quantum noise in spatially uniform media with equal electric and magnetic responses that vary over time. Light and its fluctuations propagate using conformal time, while their local, causal renormalization predicts nonzero vacuum energy in the changing medium. They apply this model to spatially flat cosmic expansion, assuming negligible dispersion until near the Planck scale. Their proposed quantum buoyancy reduces the effective gravitational contribution of radiation and matter and can match the Hubble discrepancy for a chosen cutoff. The authors explicitly state that this does not definitively resolve the Hubble tension: the coupling depends on unknown microscopic physics and must be fitted to astronomical data.

Quantum noise in time-dependent media and cosmic expansion