minimum possible absolute temperature in the universe? Sorry guys i went wrong in my previous question , actually my question is what is the minimum possible absolute temperature in the universe of what ever substance...?
 A: The absolute zero of temperature is, well, absolute zero.  This is the situation in which everything in is its ground state and there are no excitations in the system.  Most places in the universe are hotter than this: the 4 degree background radiation e.g. keeps them hot.  The coldest places we know of are all artificial and the current record keeps changing but at the moment is probably a few nanokelvins in a dilute gas, cooled first by "optical molasses" and then by evaporative cooling.
The current "best theory" of what will happen to our universe is that it will get, eventually, arbitrarily cold.  The universe is thought to be expanding, and faster and faster, because of "dark energy".  In any case, as the universe expands, it cools, although it also stops being in thermal equilibrium.  So, according to our best current understanding, the stars run out of fuel and make black holes, or grey dwarfs or something.  The universe expands and all radiation is red-shifted / made colder and colder.  The 4 degree Cosmological background and everything else gets colder and colder.  Various things may or may not happen to the colder and colder stuff that is left behind: Black holes collide, grey dwarfs radiate and cool, stuff happens.  But, there will always be radiation from the surface of anything, and that radiation into space will be redshifted and cooled.  And, so things will get colder and colder and colder.  Without, so far as we know, any limit.  And, so, in the end, we all freeze (to any arbitrarily low temperature) in the dark.  That is what is presently thought.  We can be wrong, in various ways.
