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Matter dominates over radiation

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TIME: 10,000 years

. As usual, the Universe is still cooling and expanding. But as this happens, more and more matter is being created by the high energy radiation. And as the Universe expands, the matter loses less energy than does the radiation.
. Eventually, the energy density in the matter -- mostly in the newly-formed nuclei -- becomes larger than the energy density in radiation, in massless or nearly massless particles, mainly photons. This means that in the equations of relativity that described cosmic expansion, the number representing the energy density of matter becomes much bigger than the number that represents the energy density of radiation, and we can forget about the radiation in those equations and only concentrate on what happens to the matter. The matter then dominates in determining us how the Universe expands from this era on.
. At the end of this process, photons scatter much more with each other than they do with matter. As a result, the energy exchange between matter and radiation becomes less efficient. The photons thermalize and start behaving as thermal black body radiation. We can measure this cosmic background radiation today.
. After having cooled off for many billions of years, the temperature of this radiation is just a few degrees above absolute zero. But we can measure this temperature, and we can also measure how this temperature of the cosmic background radiation varies with direction in the Universe. This tells us important details about the Big Bang and about particle physics as well.

How old is the Universe? // Structure of the Universe // The Big Bang // Before the Big Bang? // What about string theory?

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