Absolute hot

Absolute hot is a theoretical upper limit to the thermodynamic temperature scale, conceived as an opposite to absolute zero.
Planck temperature
Some speculate that the highest possible temperature is the Planck temperature, There is no existing scientific theory for the behavior of matter at these energies; a quantum theory of gravity would be required. The models of the origin of the universe based on the Big Bang theory assume that the universe passed through this temperature about (one Planck time) after the Big Bang as a result of enormous entropy expansion.
Hagedorn temperature
Another theory of absolute hot is based on the Hagedorn temperature, where the thermal energies of the particles exceed the mass-energy of a hadron particle-antiparticle pair. Instead of temperature rising, at the Hagedorn temperature more and heavier particles are produced by pair production, thus preventing effective further heating, given that only hadrons are produced. However, further heating is possible (with pressure) if the matter undergoes a phase change into a quark-gluon plasma. Therefore, this temperature is more akin to a boiling point rather than an insurmountable barrier. For hadrons, the Hagedorn temperature is , which has been reached and exceeded in LHC and RHIC experiments.
In string theory, a separate Hagedorn temperature can be defined, where strings similarly provide the extra degrees of freedom. However, it is so high () that no current or foreseeable experiment can reach it.

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