What is the highest temperature in the universe?
Atoms like heat: so much that the more there are, the more they move. And the other way around: the colder, the less they move. In fact, the absolute zero (-273ºC) is like this because the atoms stop completely. And you can not go further.
The hottest thing that the Earth has been (that has been registered, at least) has not been by a natural act: it has been thanks to the Large Hadron Collider. When they break lead particles, for a fraction of a second, the temperature reaches 5.5 trillion degrees of temperature. Warmer even than the explosion of a supernova.
But theoretically we can go further. The first contender for maximum heat is the Planck temperature, which equals 100 million million million degrees. It is almost impossible to put so much heat into perspective.
This is as hot as normal physics can be because, once that temperature is exceeded, conventional physics simply does not work. Rare things begin to happen: the gravitational force becomes as strong as the other three natural forces (electromagnetism and strong and weak nuclear forces), and come together in a single unified force. Understanding how this happens is known as "the theory of everything," the summit of modern theoretical physics ... something that we do not currently understand.
The Hagedorn temperature is the highest temperature we think we could reach. This is the point at which the hadronic matter (all the normal ordinary matter of the universe) is no longer stable and completely decomposes. We arrive at this point at approximately 2 × 10 (12) K. Some theoretical physicists postulate that, at this point, hadronic matter does not "evaporate" but, instead, becomes a matter of quarks, which can then warm up even plus. However, the matter of the quark is a theoretical phase, and we are not sure if it really exists.
Another contender for the hottest temperature in the universe is courtesy of string theorists, who say that the highest temperature is 10 (30) K, a little cooler than the top competitor. This is because string theorists believe that the most basic things in the universe are not the normal particles with which we are all familiar, but the vibrating strings, which have a Hagedorn temperature different from that of hadrons.
Unfortunately, it is impossible to test the predictions made by string theorists (as well as many predictions that exist at such extremes), so as a result we do not know exactly what the highest temperature is. But those mentioned above are the best contenders, according to physicists.
Reference:
Futurism
