Since the universe was about 300,000 years old at recombination, the horizon size then was about 300,000 light years.
Each atom at the surface of last scattering could only be in causal contact (and therefore in thermal equilibrium) with with other atoms within a radius of about 300,000 light years..
The epoch at which atoms form, when the universe was at an age of 300,000 years and a temperature of around 3000 K is somewhat oxymoronically referred to as "recombination", despite the fact that electrons and nuclei had never before "combined" into atoms..
Until recombination, the universe was opaque to electromagnetic radiation due to scattering of the photons by free electrons.
As recombination occurred, the density of free electrons diminished greatly, leading to the decoupling of matter and radiation as the universe became transparent to light..
It is commonly said that after the universe cooled enough for ionized Hydrogen to settle down into neutral Hydrogen, i.e. recombination, the universe became transparent..
It is commonly said that after the universe cooled enough for ionized Hydrogen to settle down into neutral Hydrogen, i.e. recombination, the universe became transparent..
When the temperature of the Universe fell below a certain threshold, protons and electrons could, at last, combine together to form neutral atoms, mostly hydrogen and helium.
This event occurred about 380,000 years after the initial Big Bang, and the Universe then stayed neutral until much later..
The cosmic ionization history is generally described in terms of the free electron fraction xe as a function of redshift
Helium nuclei are produced during Big Bang nucleosynthesis, and make up about 24% of the total mass of baryonic matter
Prior to recombination, photons were not able to freely travel through the universe, as they constantly scattered off the free electrons and protons
• Chronology of the universe• Age of the universe• Big
• Bromm, Volker (17 April 2014). "AST 376 Cosmology—Lecture Notes: Cosmic Microwave Background (CMB)"(PDF). Austin
Topics referred to by the same term
