Re: What has the highest temperature in the universe?

This is actually not an easy question! It depends on what you mean by temperature. Here on Earth, in our cozy blanket of air at one atmospheric pressure, we think of temperature as how hot something is. But on a microscopic level, that really means how vigorously molecules and atoms are bumping into each other. The faster they move, the harder they bump, and the higher the temperature.

However, have you ever wondered why water at 70 degrees Centigrade might scald you, but air at that temperature won't? Air is less dense than water, so it carries less energy total even if the temperature is the same. The atoms are jiggling just as fast, but there are fewer of them.

I bring this up because there can be ridiculously high temperatures in the Universe, but they don't mean much! For example, the Voyager probe measured a temperature of over one billion degrees in the magnetosphere of Uranus. Imagine! But really what it was measuring were particles moving extremely quickly. If you stood (well, floated) in Uranus' magnetosphere, you wouldn't suddenly vaporize. On the contrary, you'd freeze! The particles may be very very hot, but there simply aren't enough of them for them to heat you up.

So to answer your question, probably the hottest temperatures reached in the Universe today are in the cores of supernovae (stars that explode), or possibly in the mysterious gamma ray bursters, extremely energetic explosions the cause of which no one knows. But the hottest temperature of all time would have been the Big Bang: since it had all the matter and energy squeezed into one point, it must have had nearly infinite temperature!