Peering through his telescope, astronomer Edwin Hubble observed that the Universe was destined to end in ice. Quantum theories say that it might end in fire. Thermodynamics, an older and even more inviolable pillar of physics, says that it may end in a different way, with all things dim, tepid, and sluggish: in equilibrium.
The eventual fate of the universe, from this view, is something called heat death. Thermodynamics dictates that large systems evolve toward equilibrium over time. This is a balanced, calm state where no more reactions are favorable; nothing has energy to gain or lose compared to anything else.
The universe currently varies tremendously in composition from place to place. We live among glowing filaments of matter and energy, clustered together in a background of vast nothingness. Stars, planetary systems, galaxies, nebulae, black holes -- all are incredibly concentrated specks in the colossal void of space.
If the entire universe can be understood as a thermodynamic system, this theory spells out the certain consequence. In the far distant future, all these hot heavy specks will all be spread out into the enormous cold void, mixing until everything is a thin uniform mist. Like boiling water added to a bowl of cold soup, the two extremes will balance out and leave lukewarm broth.
Heat death originated from the work of several prodigious physicists who began the study of understanding how machines transform heat into mechanical work. Lord Kelvin, Sadi Carnot, and others formed an empirical understanding of how steam engines and other suppliers of motive force do this. They discovered that the machines were harnessing the tendency of energy to flow from hot areas to cold ones. Eventually, the entire system settles down to an intermediate temperature and no more net energy transfer occurs. (This is the maximization of entropy.)
In space, molecules, atoms and subatomic particles will collide with one another, spreading their momentum and energy from the fast to the slow. The motion of all the particles of the universe will gradually thermalize (become random); they will collide and interact with no change of energy. Eventually they will careen out into empty space after some unlucky bounce to spend eternity traveling alone.
Can this dull fate be avoided? Not if the universe is a completely thermodynamic system. However, it is unclear whether the gravitational forces within and between enormous astrophysical objects can be accurately described this way. Brilliant physicist Freeman Dyson believes gravity will prevent heat death from ever occurring.
In any case, don’t lose sleep over this (or any other) far distant cosmic outcome. If it occurs, this equilibrium process will take what seems like an infinitely long time. (Can you imagine the 10^100 years -- a.k.a., "googol" -- required for a large black hole to evaporate completely? I cannot.) We essentially have an eternity of time to enjoy our universe before it fades gently into that all-encompassing night.