Frontiers of physics - an underappreciated point

In what branch of physics are the most extreme conditions reached?  If asked, I'm sure the vast majority of people would guess particle physics. Enormous machines (and they want bigger ones all the time) are used to accelerate particles up to a hairsbreadth below the speed of light and smash the particles into each other or into targets.  The energy densities in those collisions are enormous and by intent are meant to rival conditions in the earliest moments of the universe or in extreme astrophysical conditions.  Still, while the details are special (nature doesn't collide directed bunches of ultrarelativistic protons head on), the fact is that there are, or at least have been, naturally arising processes that approach those conditions.  

The fact is, condensed matter physics (CMP) and atomic/molecular/optical (AMO) physics are actually more extreme, reaching conditions that do not ever happen spontaneously, anywhere.  Now-common laboratory techniques in CMP and AMO can produce experimental conditions that, as far as we know, simply do not occur in nature without the direct intervention of intelligent beings.  

The particular condition I'm talking about is temperature.  As I discussed a little here, temperature is a parameter that tells us the direction that energy tends to flow when two systems (say a coffee cup and a coaster) are allowed to exchange energy via microscopic degrees of freedom that we don't track, like the kinetic jiggling of vibrating atoms in a solid.  When the cup and coaster are at the same temperature, there is no net flow of energy between them, even though some amount of energy is fluctuating back and forth all the time.  

The cosmic microwave background, the relic electromagnetic radiation left over from the early universe, is described by an intensity vs. frequency distribution that we would expect from radiation in thermal equilibrium with a system at a temperature of 2.726 K.  What this means is, if you had some lump of matter floating in interstellar space, and you waited a very long time, the temperature of that lump would eventually settle down to 2.726 K, absent other effects.  It would never be colder.

In CMP labs around the world, however, macroscopic lumps of matter are routinely cooled to temperatures far colder than this.  With a conventional dilution refrigerator (see here) it is possible to cool kgs of material down to milliKelvin temperatures.  Through demagnetization cooling, particularly of materials with nuclear magnetic moments, microKelvin temperatures may be reached.  In AMO labs, laser cooling techniques can get clouds of atoms down to nanoKelvin temperatures, though typically the number of atoms involved is far smaller.  Pretty amazing, when you think about it!