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 error bars on the data points are so small that they can not be seen under the predicted curve in the figure! There is no alternative theory yet proposed that predicts this energy spectrum. The accurate measurement of its shape was another important test of the Big Bang theory.
Eventually, the universe cooled sufficiently that protons and electrons could combine to form neutral hydrogen. This occured roughly , years after the Big Bang when the universe was about one eleven hundredth its present size. Cosmic microwave background photons interact very weakly with neutral hydrogen, allowing them to travel in a straight lines.
The behavior of CMB photons moving through the early universe is analogous to the propagation of optical light through the Earth's atmosphere. Water droplets in a cloud are very effective at scattering light, while optical light moves freely through clear air. Thus, on a cloudy day, we can look through the air out towards the clouds, but can not see through the opaque clouds. Cosmologists studying the cosmic microwave background radiation can look through much of the universe back to when it was opaque: a view back to , years after the Big Bang.
When we make maps of the temperature of the CMB, we are mapping this surface of last scattering. As shown above, one of the most striking features about the cosmic microwave background is its uniformity. By studying these fluctuations, cosmologists can learn about the origin of galaxies and large scale structures of galaxies and they can measure the basic parameters of the Big Bang theory. Wilkinson Microwave Anisotropy Probe. In particular, once we remove the dipole that arises due to our motion in the Universe, the CMB is incredibly uniform across the sky, varying by no more than one part in ten thousand.
However, this is not possible given standard Big Bang theory, the age of the Universe, and the finite speed of light. A period of inflation is also necessary so that regions of the early Universe are close enough to thermally equalise.
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.
They pointed the antenna at New York City and found it wasn't due to urban interference. Nor was it radiation from our galaxy or extraterrestrial radio sources. Finally, they decided the problem might be due to the droppings from pigeons roosting in the horn-shaped antenna, contrived a pigeon trap to oust the birds, and spent hours removing pigeon dung from the contraption.
So Penzias and Wilson began looking for theoretical explanations. Around the same time, Princeton University physicist Robert Dicke theorized that if the universe was created according to the Big Bang theory, a low-level background radiation at around 3 degrees Kelvin would exist throughout the universe.
Dicke had begun looking for evidence to support his theory when Penzias and Wilson got in touch with his laboratory. He visited Bell Labs and confirmed that the mysterious radio signal was indeed the cosmic background radiation — proof of the Big Bang. Dicke shared his theoretical work with the Bell Labs researchers, even as he resignedly admitted to his Princeton colleagues, "We've been scooped.
The two groups published their results at the same time in Astrophysical Journal Letters. Penzias and Wilson received the Nobel prize in physics in for their serendipitous discovery of the CMB.
The giant radio antenna at Holmdel was designated a National Historic Landmark in Even the lowly pigeon trap has found its way into posterity.
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