As a universe grows from young to old, it also moves from being hot to cold, unstable to stable, small to large, ordered to disordered, and dense to sparse, among other conditions. This is all part of the life cycle of a typical space-time continuum, which takes trillions upon trillions of years from start to finish. This covers many of the core concepts in contemporary cosmology, from the birth to the death of a given universe.
HOT → COLD
When the local universe began it was a scorching 2.55×10³² degrees Fahrenheit, which is the hottest that anything could ever be. In the absolute temperature scale, the Planck temperature is equal to 1 while that of complete cold is 0. This is the temperature of a Big Bang and Big Freeze, respectively. Thus, when the universe ends it will achieve a temperature of –459.67°F (0°K). In this way, absolute hot and cold are the highest and lowest attainable temperatures in existence.
UNSTABLE → STABLE
At the temperature of absolute heat all matter is unstable, so a universe will always work toward achieving a stable state. In other words, a universe switches from a dynamic to a static condition over time. At 0°K matter is in its ground state, which is the point of lowest internal energy. This is the moment of rest at the end of a universe’s life. In this way, every universe begins with a Big Bang and ends with a Big Freeze.
SMALL → LARGE
The universe started out roughly a million billion billion times smaller than a single atom and has grown to around 7 trillion light-years in diameter. The universe is also getting larger at an increasingly accelerated rate as time goes on. It will eventually just keep getting bigger and colder and darker until everything finally stops moving altogether. As part of this, in about 150 billion years, it will become impossible for events in the local group to effect other galaxies, and vice versa.
ORDERED → DISORDERED
As a universe ages it progresses from a condition of minimum to maximum entropy, along with size. This results in uniformity in the early near-equilibrium state, which then grows random over time as entropy increases. So, the local universe began in a uniform, high density state at its very early stages, and it will end in a random, low density state at its very late stages. Ultimately, the older the universe gets the longer the process takes as everything slows down and spreads out.
DENSE → SPARSE
Every universe begins as a tiny disturbance of tightly packed primordial matter. After a universe rapidly inflates it starts to expand and everything not bound by gravity within it is pushed further and further apart. In the end everything in a universe becomes so far removed from everything else that nothing exerts any influence on anything else, so everything in a space-time continuum eventually just stops doing anything. Then, after one universe ends another begins.