We observe Andromeda, the nearest big galaxy, as it was about 2. Astronomers observing distant galaxies with the Hubble Space Telescope can see them as they were only a few billion years after the Big Bang. The CMB radiation was emitted Thus, by studying the detailed physical properties of the radiation, we can learn about conditions in the universe on very large scales at very early times, since the radiation we see today has traveled over such a large distance.
One of the profound observations of the 20th century is that the universe is expanding. This expansion implies the universe was smaller, denser and hotter in the distant past.
When the visible universe was half its present size, the density of matter was eight times higher and the cosmic microwave background was twice as hot. When the visible universe was one hundredth of its present size, the cosmic microwave background was a hundred times hotter degrees above absolute zero or 32 degrees Fahrenheit, the temperature at which water freezes to form ice on the Earth's surface.
In addition to this cosmic microwave background radiation, the early universe was filled with hot hydrogen gas with a density of about atoms per cubic centimeter.
When the visible universe was only one hundred millionth its present size, its temperature was million degrees above absolute zero and the density of matter was comparable to the density of air at the Earth's surface. At these high temperatures, the hydrogen was completely ionized into free protons and electrons. Since the universe was so very hot through most of its early history, there were no atoms in the early universe, only free electrons and nuclei. Nuclei are made of neutrons and protons.
The cosmic microwave background photons easily scatter off of electrons. Thus, photons wandered through the early universe, just as optical light wanders through a dense fog. This figure shows the prediction of the Big Bang theory for the energy spectrum of the cosmic microwave background radiation compared to the observed energy spectrum.
The red line in the figure on the left shows that according to Big Bang theory, the Universe had a radius of more than 10 metres at 10 seconds after the Big Bang. Big Bang theory therefore makes it impossible for the whole Universe to have equalised its temperature at these early times, as not all the Universe was in communication.
In everyday life we cannot receive information beyond our horizon, so this is known as the horizon problem. To resolve the horizon problem, astronomers introduced an inflationary period into the Big Bang model blue region in figure. This sudden increase in the rate of expansion of the Universe soon after the Big Bang, resolves not only the horizon problem, but also the flatness problem.
It has therefore been accepted as part of the current concordance model of cosmology. Answer originally posted October 13, Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Erik M. Leitch of the University of Chicago explains. Get smart.
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