  Regarded as a promising alternative to spatially shaping matter, time-varying
media can be seized to control and manipulate wave phenomena, including thermal
radiation. Here, based upon the framework of macroscopic quantum
electrodynamics, we elaborate a comprehensive quantum theoretical formulation
that lies the basis for investigating thermal emission effects in
time-modulated media. Our theory unveils new physics brought about by
time-varying media: nontrivial correlations between thermal fluctuating
currents at different frequencies and positions, thermal radiation overcoming
the black-body spectrum, and quantum vacuum amplification effects at finite
temperature. We illustrate how these features lead to striking phenomena and
novel thermal emitters, specifically, showing that the time-modulation releases
strong field fluctuations confined within epsilon-near-zero (ENZ) bodies, and
that, in turn, it enables a narrowband (partially coherent) emission spanning
the whole range of wavevectors, from near to far-field regimes.
