relic

In cosmology, the term relic (either as a noun or an adjective; the adjective primordial means basically the same thing) refers to some type of particle left over from an event in the early universe, e.g., the phrases "relics from recombination", "relic neutrinos", and "primordial neutrinos". For example much hydrogen and helium is presumed to have been synthesized very soon after the Big Bang (Big Bang nucleosynthesis), in part because the reactions that would take place given the heat and density that would be present at the time would (according to established physics) produce abundances close to today's, i.e., the puzzle fits together. The terms relic and primordial are also used for such characteristics of relic particles, particularly the phrases relic abundances aka primordial abundances.

The CMB is relic photons from recombination, left over from photons that were previously constantly scattered. This was an example of a decoupling (of photons from electrons and protons) and a freeze-out; the Hubble expansion had enlarged space sufficiently that the vast majority of photons were then free streaming, i.e., a significant number have been traveling ever since. Relics include all or a significant fraction of each of these:

• baryons (protons and neutrons).
• hydrogen.
• helium.
• lithium.
• neutrinos.
• photons (CMB).
• WIMPs (or presumably whatever particle constitutes dark matter).
• primordial black holes (if they exist).

Those that are not relics have been transformed by subsequent processes and events, such as radioactive decay, stellar fusion, supernovae, and I'm guessing maybe active galactic nuclei.

The term relic is also used within astrophysics for other objects (either particles or astronomical objects) left over from some event or activity, such as radio relics.