Astrophysics (Index)About

relic

(primordial)
(particles left over from the events of the early universe)

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:

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.


(cosmology,particles,photons)
Further reading:
https://en.wikipedia.org/wiki/Relic_abundance
https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_156_-_A_Cosmology_Workbook/Workbook/22%3A_Hot_and_Cold_Relics_of_the_Big_Bang
https://indico.fnal.gov/event/2341/contributions/72179/attachments/45198/54408/Formaggio_Relics_2009.pdf

Referenced by pages:
Big Bang nucleosynthesis (BBN)
cosmic background radiation (CBR)
cosmic neutrino background (CNB)
free streaming
freeze-out
helium (He)
hydrogen (H)
lithium (Li)
mass fraction
primordial black hole (PBH)
WIMP

Index