Recombination and Last Scattering

About 380,000 years after the Big Bang the universe cooled enough for electrons and protons to combine into neutral hydrogen, freeing the radiation that we now see as the cosmic microwave background.

Definition

Recombination is the cosmological epoch at which free electrons and protons combined into neutral hydrogen, and last scattering is the moment, slightly later, when photons ceased to scatter frequently off electrons and began to travel freely, defining the effective surface from which the cosmic microwave background originates.

Scope

This topic covers the epoch of recombination, when the primordial plasma became neutral, the resulting decoupling of photons from matter, the surface of last scattering from which the cosmic microwave background streams freely, and the thickness and physics of this transition that shape the observed background.

Core questions

• Why did the universe become transparent at recombination?
• What is the surface of last scattering?
• How does the physics of decoupling shape the cosmic microwave background?

Key concepts

• Recombination
• Decoupling
• Surface of last scattering
• Neutral hydrogen
• Free-electron fraction
• Optical depth

Key theories

Cosmological recombination

As the universe expanded and cooled below a few thousand kelvin, electrons and protons bound into neutral hydrogen, sharply reducing the free-electron density and the scattering of photons.

Photon decoupling

Once recombination removed most free electrons, the mean free path of photons grew larger than the horizon, so they decoupled from matter and have propagated freely ever since from the surface of last scattering.

Mechanisms

Falling temperature shifted the ionization balance so that electrons captured by protons formed neutral hydrogen faster than radiation could re-ionize it; with free electrons depleted, Thomson scattering became rare, the optical depth dropped below one, and photons last scattered to begin their free journey to us.

Clinical relevance

Recombination sets the origin of the cosmic microwave background and fixes the redshift and physical scale of its features: the sound horizon at last scattering acts as a standard ruler, and the details of decoupling determine the damping and visibility of the temperature anisotropies used to measure cosmological parameters.

History

The recombination history was first computed in detail by Peebles and independently by Zeldovich and collaborators in 1968, establishing the redshift of last scattering near 1100; later refinements improved the treatment of hydrogen and helium recombination needed for precision cosmology.

Debates

Recombination accuracy for precision cosmology

Extracting cosmological parameters from the cosmic microwave background at percent precision requires recombination to be modeled to comparable accuracy, motivating ongoing refinement of the atomic physics of the transition.

Key figures

• James Peebles
• Yakov Zeldovich
• Rashid Sunyaev

Seminal works

• peebles1968

Frequently asked questions

Why is it called recombination if electrons and protons were never combined before?

The term is historical and slightly misleading: in the early universe electrons and protons had never previously been bound, so this was actually their first combination, but the name recombination has remained standard in cosmology.

Is the surface of last scattering a real surface?

It is not a physical surface but the set of points from which the cosmic microwave background photons reaching us last scattered; because decoupling took some time, it is actually a shell of finite thickness rather than an infinitely thin surface.