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13 years ago
Briefly describe the two key pieces of evidence that support the Big Bang theory.
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13 years ago
The two key pieces of evidence that support the Big Bang theory are the cosmic background radiation and the observed helium content of the universe. The cosmic background radiation consists of photons arriving at Earth directly from the end of the era of nuclei. The radiation came from the heat of the universe and should have a thermal radiation spectrum with a peak wavelength corresponding to 3,000 K, the temperature the universe was at the end of that era. Since the universe has expanded by a factor of about 1,000 since that time, we observe the radiation with a peak wavelength of about a millimeter, corresponding to a temperature of 2.73 K. The Big Bang theory predicts that the universe should have had a composition of 75 percent hydrogen and 25 percent helium by mass at the end of the era of nucleosynthesis. The Milky Way's helium fraction is about 28 percent, and no galaxy has a helium fraction lower than 25 percent.
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