This simulation shows the formation of the Cosmic Microwave Background (CMB) at a detailed level. Brown spheres are atoms and green balls are photons. Initially the universe is very young and very dense and very hot, and the atoms are ionised and hence opaque to photons. The photons bounce between the atoms, trapped. As space expands, eventually the atoms become neutral (go darker in the simulation), after which the photons just ignore them and fly around undisturbed in all directions. It is these undisturbed photons that form the cosmic microwave background we see today.
<p>This simulation is one of many made during creation of the Australian National University's free online astrophysics courses:
</p><p><a rel="nofollow" class="external free" href="https://www.edx.org/xseries/astrophysics">https://www.edx.org/xseries/astrophysics</a>
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It was created using the VPython library (vpython.org)
<a rel="nofollow" class="external text" href="https://www.youtube.com/channel/UCra3UzQTPZJjhAu1rJfGhtg">Astronomy Simulations</a>
CC BY 3.0
<bdo lang="en" dir="ltr"><a href="//commons.wikimedia.org/wiki/Commons:YouTube_files" title="Commons:YouTube files">YouTube</a>: <a rel="nofollow" class="external text" href="https://www.youtube.com/watch?v=zpBL68gHi8k"><bdi lang="">Simulation of the Cosmic Microwave Background formation</bdi></a> – View/save archived versions on <a rel="nofollow" class="external text" href="https://web.archive.org/web/*/https://www.youtube.com/watch?v=zpBL68gHi8k">archive.org</a> and <a rel="nofollow" class="external text" href="https://archive.today/https://www.youtube.com/watch?v=zpBL68gHi8k">archive.today</a></bdo>