Stages of cosmic evolution are illustrated in the diagram above. A sequence of the most significant events in the evolution of the Universe is annotated at the left of the diagram. The sequence begins at the top at a cosmic age of 100 years after the Big Bang, when (following a more exotic period) the Universe settled into a mixture of protons, helium nuclei, and electrons bathed in a sea of photons. The temperature of the whole was about 300,000° Kelvin at this stage. The Universe expanded and cooled until, at about a cosmic age of 300,000 years, electrons were able to combine with protons and helium nuclei to form hydrogen and helium atoms. The radiation emitted during that process, initially with a temperature of 3,000° Kelvin, cooled as the Universe continued to expand to the 3° Kelvin microwave background that is observable today. The atomic material then coalesced gravitationally to form the first protogalaxies, the galaxies like our own and quasars (the most distant observable objects). Viewed upwards from the bottom, the above diagram shows how we see the Universe as we peer into distant space. Because light travels at a large but finite speed, it takes time for the light to cover large distances. When we observe the light of distant objects in the universe, we are actually seeing the light that was emitted from them a long time ago: we see them literally as they were in the remote past. Within a few thousand light years distance are the stars in our own Galaxy. There then lies a gap to the nearest external galaxies at a distance of several million light years; the light which we are now receiving from those distant galaxies began its journey as the first manlike apes walked the Earth. Evolutionary events corresponding to the age of light from distant sources are annotated at the right in the above diagram. |