Cryosleep

Mechanism
Cryosleep is handled by putting individual people in a cryopod powered by the fusion reactor that powers the ship.

Limitations
There are three primary, and major, limitations concerning this cryogenic technology, which the Numerius Cult was aware of when they created it, but had to be rediscovered by the Pantheon's Roman Empire, which caused the end of the Expansion Era. They are:


 * 1) The processing systems used to convert reagents into the necessary cryogenic chemicals (used during, and in the awakening from cryosleep) are expensive, difficult to construct (low tolerances and a lot of dangerous components), large in size (a very large processing plant), need a lot of maintenance, a huge staff, and require knowledge that is not widely held.
 * 2) This means that you need a very highly developed, rich society on any given planet in order to be capable of initiating this process; in addition, you need to have contact with some of the Navigus Mechanicus to get information about how to build the plants themselves — meaning if none of the NMs came with you, and there's no new immigrants coming to your planet, you're stuck. Moreover, simply because a civilization is capable of doing this does not mean it will want to: the pressure to move on to the stars which makes it possible to rally the necessary resources only happens when a planet is severely depleted or overpopulated.
 * 3) Even more importantly for our purposes, there is a holding-pattern falloff effect precipitated by the decaying of the chemicals produced by these reagents: after awhile the chemical begins to degrade, no matter how it is stored, meaning that after roughly three thousand years of either storage or use the cryogenic chemicals will become useless; this causes both the destruction of the body of the person in cryosleep as the system stops working and regular ice-crystals form, and makes it impossible to pull the person out of cryosleep since the chemicals necessary for doing so have degraded as well.

This combination of technological and sociological factors is what causes the Caelum Nostrum.