• Introduction
• Temperature Maps
  • Earth
  • Monopole
  • Dipole
  • COBE
  • Power
  • Precision
  • Features
• Thermal History
  • Expansion
  • Plasma
  • Glue
  • Pressure
  • Thermal
• Acoustic Oscillations
  • Gravity
  • Plane Waves
  • Harmonics
  • Extrema
• Angular Peaks
  • Spatial/Angular
  • Oscillation
  • Freeze Frame
  • Streaming
  • Soundscape
• First Peak
  • Data
  • Curvature
  • Flatness
  • Dark Energy
  • Summary
• Second Peak
  • Data
  • Inertia
  • Baryonmeter
  • Power
  • Dark Baryons
  • Summary
• Higher Peaks
  • Driving Force
  • Power
  • Summary
• Damping Tail
  • Diffusion
  • Ruler
  • Consistency
  • Summary
• Parameter Estimation
  • Parameters
  • Degeneracy
• Polarization
  • What and Why
  • Scattering
  • Projection
  • Power
  • Quadrupoles
  • B-modes
  • Gravity Waves
• Summary

Recombination

Key Concepts

• Photons get hotter as one goes backwards in time.
• At 3000K, CMB photons ionize hydrogen
• Above this temperature, the universe was a photon-baryon (proton) plasma

Going backwards in time, the expansion reverses.  The redshifting of photons by the stretching of space becomes a blueshifting.  When the universe was one thousandths of its current size, the CMB was a thousand times hotter and had a temperature of ~3000K.  At this point, photons in the tails of the distribution had enough energy to break the hydrogen atom into a proton and an electron:

[Note: This process is called recombination since going forwards in time protons and electrons combine to form neutral atomic hydrogen.]

Before this time, the universe consisted of a plasma of mainly electrons, protons, and CMB photons. Because there was also a small amount of helium and heavier elements which contain neutrons, we usually refer to the mix as a photon-baryon plasma.

This process occurs when the universe was a mere ~300,000 years old.  Since the universe is at least 10 billion years old today, CMB photons are nearly as old as the universe itself.  Images of the CMB  represent our earliest direct picture of the universe.

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