Planck temperature

At 1.4 × 10³² kelvin the typical thermal photon has the Planck energy, its wavelength is a Planck length, and the notion of temperature itself becomes suspect because you cannot describe the radiation without a theory of quantum gravity. For comparison, the Sun's core runs at 1.6 × 10⁷ K, a supernova shock at 10¹¹ K, and the quark-gluon plasma made at the LHC at about 5 × 10¹² K — twenty orders of magnitude short.

The universe is believed to have passed through this temperature during the Planck epoch. It is sometimes called an absolute hot, a supposed ceiling matching absolute zero's floor, but that framing is wrong twice over: nothing forbids higher temperatures, and negative absolute temperatures already exist in population-inverted spin systems, which are hotter than any positive temperature. T_P is where our physics stops, not where the thermometer does.