The idea of absolute hot springs from its better-known cousin, absolute zero, which, as you may recall, is 0 K, -273.15° C or -459.67° F. And although abridged definitions of the lowest temperature will frequently state it is the point at which matter stops moving, this is technically incorrect. Absolute zero is actually the point where molecular motion no longer produces heat (but does have zero-point energy).

Conversely, absolute hot, then, could be defined as the point where molecular motion couldn’t produce any more heat, no matter what the circumstances.

In the Standard Model of the universe, the hottest possible temperature ever reached occurred a fraction of a second (10^-43) after the Big Bang. During that minuscule period of time (called one Planck time), the universe is thought to have only been one tiny Planck length (10^-35 meters) and have reached absolute hot at 10³² K (called Planck temperature). For comparison, our Sun is a measly 1.571 x 10⁷ K at its center and the highest temperature ever created by man is currently 5.5 X 10¹² K.

Beyond Planck temperature being the hottest temperature ever theoretically reached in our universe, physicists hypothesize that at any temperature higher than Planck, the gravitational forces of the affected particles would become equally as strong as the other fundamental forces (electromagnetic and weak and strong nuclear), resulting in all four becoming unified as one force. What happens then?  Nobody knows as currently accepted conventional models of physics break down after that point. Of course, all of this is theoretical, since no one has yet to come up with an accepted quantum theory of gravity. As Nobel laureate Steven Weinberg described it, whatever happens at temperatures above 10³² K remains obscured by a “veil.”

It should be noted that not all physicists follow the Standard Model, and some prefer, for instance, String Theory, which attempts to describe all four fundamental forces as different manifestations of a single basic object (a string). For string theorists, the highest possible temperature is far lower than that postulated by the Standard Model; called the Hagedorn temperature, it is the point at which ordinary matter is no longer stable and either “evaporates” or changes into quark matter. Under this theory, the point at which that happens, or absolute hot, is thought to be just 10³⁰ K, or about 1% Planck temperature.

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Bonus Facts:

• While heating something to anywhere close to Planck temperature is far beyond our technology at present, cooling something down to close to absolute zero is not.  For instance, in 2015 researchers at MIT managed to cool sodium potassium molecules down to just 500 nanokelvins or 500 billionths of 1 K.
• At least one animal can survive cold approaching absolute zero – the tardigrade. Also known as the water bear, this microscopic being has been shown to be able to survive being frozen for several minutes at a mere 1 degree above absolute zero. It can also survive being heated to temperatures well beyond the boiling temperature of water. Not their only astounding survival trick, tardigrades can survive numerous other extremes we humans would die instantly in. You can learn more about these fascinating creatures that may even be currently hanging out in your backyard here: The Amazing Tardigrade
• Just for fun: The energy required to stop the Earth orbiting the Sun is about 2.6478 × 10³³ joules or 7.3551 × 10²⁹ watt hours or 6.3285 x 10¹⁷ megatons of TNT.  For reference, the largest nuclear explosion ever detonated (the Tsar Bomba by the Soviet Union) “only” produced 50 megatons of TNT worth of energy.  So it would take about 12,657,000,000,000,000 of those nuclear bombs detonated at the correct location to stop the Earth in its tracks in its orbit around the Sun.
• Amazingly, if we were actually able to convert matter perfectly to energy with 1 kg of matter being completely annihilated, the energy produced from just that small amount of matter is about 42.95 mega tons of TNT.  So an adult male weighing in at around 200 pounds has somewhere in the vicinity of 4000 megatons of TNT potential in their matter if completely annihilated. This is about 80 times more energy than was produced by the aforementioned Tzar Bomba, which itself produced a blast about 1,400 times more powerful than the combined explosions of the bombs dropped on Hiroshima and Nagasaki. To further illustrate, 1 megaton of TNT, when converted to kilowatt hours, makes enough electricity to power an average American home for about 100,000 years.  It is also enough to power the entire United States for a little over 3 days.  So 1 kg of some matter being completely annihilated would be able to power the entire United States for about four months.  One average adult male then, when completely annihilated, would produce enough energy to power the U.S. for about 30 years if we could harness all that energy.  Energy crisis solved. 😉
• On a completely baffling scale, a typical supernova explosion will give off about 10,000,000,000,000,000,000,000,000,000 megatons of TNT.