I know there is such a thing as absolute zero. Why isn't there anything such as absolute 'hot'? What is the hottest temperature (from an object and not from the initial big bang) in the universe? I know the CMB is supposed to be one of the coldest places, but where is the hottest?
It is interesting that infinite temperatures are not ruled out by the theory of thermodynamics, but it is also interesting that it is not possible to heat a body to an infinite temperature. In order to heat a body you need a hotter body - which is not possible when you reach infinite temperatures.
Temperature actually is pretty hard to define. You probably think of it as something very simple - you can feel it after all. But can you imagine explaining what it is to an alien who cannot feel it? Scientists have many definitions of temperature, but it still is hard sometimes.
Some really hot things that I can think of are:
- The centre of the Sun: 15,700,000 K
- The corona (outer atmosphere) of the Sun: over 1,000,000 K
- The tenuous electron gas in clusters of galaxies: 100,000,000 K
- Accretion disks around stellar mass black holes: over 1,000,000 K
(K stands for Kelvin - the preferred unit of temperature for scientists. 0 K is defined as absolute zero (-273 Celsius). Actually at such high temperatures as above the difference between units of temperature is pretty unimportant - even the difference between Kelvin and Fahrenheit.)