Universe: Big Bang & Cosmic Microwave Background

• Universe grows and cools until 100 seconds after the Big Bang. The temperature is 1 billion degrees, 109 K. Electrons and positrons annihilate to make more photons, while protons and neutrons combine to make deuterons. Almost all of the deuterons combine to make helium. The final result is about 3/4 hydrogen, 1/4 helium by mass; deuteron/proton ratio 30 parts per million. There are about 2 billion photons per proton or neutron.
• One month after the Big Bang the processes that convert the radiation field to a blackbody spectrum become slower than the expansion of the Universe, so the spectrum of the Cosmic Microwave Background (CMB) preserves information back to this time.
• Matter density equals radiation density 56,000 years after the Big Bang. The temperature is 9000 K. Dark matter inhomogeneities can start to collapse.
• Protons and electrons combine to form neutral hydrogen. Universe becomes transparent. Temperature is T=3000 K, time is 380,000 years after the Big Bang. Ordinary matter can now fall into the dark matter clumps. The CMB travels freely from this time until now, so the CMB anisotropy gives a picture of the Universe at this time.
• The first stars form 100-200 million years after the Big Bang, and reionize the Universe.
• The first supernovae explode and spread carbon, nitrogen, oxygen, silicon, magnesium, iron, and so on up through uranium throughout the Universe.
• Galaxies form as many clumps of dark matter, stars and gas merge together.
• Clusters of galaxies form.
• The Solar System and Sun form 4.6 billion years ago.
• Now: The time is 13.7 Gyr after the Big Bang, and the temperature is T=2.725 K. The homogeneous patch is at least 1029 cm across, which is larger than observable Universe.

Even though the universe is now a chilly 2.725 K, it was once much hotter. In fact, from the moment of the initial big bang singularity until nearly 100,000 years later, the universe was hot enough so that electrons and protons had too much energy to come together and form neutral atoms. What existed was a "primordial soup" full of free electrons, protons and neutrons.