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waynehu

Professor, Department of Astronomy and Astrophysics
University of Chicago

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CMB Introduction '96   Intermediate '01   Polarization Intro '01   Cosmic Symphony '04   Polarization Primer '97   Review '02   Power Animations   Lensing   Power Prehistory   Legacy Material '96   PhD Thesis '95 Baryon Acoustic Oscillations Cosmic Shear Clusters
Transfer Function WMAP Likelihood Reionization PPF for CAMB Halo Mass Conversion Cluster Abundance
Cosmology I [legacy 321] Cosmology II [321] Current Topics [282] Galaxies and Universe [242] Radiative Processes [305] Research Preparation [307] GR Perturbation Theory [408] CMB [448] Cosmic Acceleration [449]

Recombination

Key Concepts

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.