If this idea were to be true, it must also be true that up to some time early in the history of the universe it was almost completely full of hot plasma with above about 10,000 degrees. It was so hot that electrons and protons rarely combined to form atoms; and when they did the atoms quickly dissociated.
As the universe expanded, it eventually cooled off enough that electrons and protons began to combine into hydrogen atoms and remained stable. At this point, suddenly, the universe went from being opaque to light to being transparent. This is because individual electrons and protons, being charged, interact strongly with light, which is an electromagnetic wave. Atoms, however, are neutral, and thus don't interact with light nearly as strongly; and light is able to proceed unimpeded through the Universe.
Therefore, it was realized that one could detect this last opaque surface if he looked back far enough. George Gamov, in fact, predicted that the light from this "last scattering surface" would have been emitted so long ago that, despite originally being energetic ultraviolet radiation, it would now be detectable as microwaves. This speculation rested on the shelves and was relegated to the "interesting if true" category.
Enter Arno Penzias and Robert Wilson, radio astronomers at Bell Labs. A few years after Gamov's prediction, these astronomers were making routine observations with the radio telescope at Bell Labs in New Jersey when they realized, quite accidentally, that they were getting a low level of noise from every direction in the sky. At first, of course, they blamed their instrumentation and terrestrial effects, but careful testing and calibration showed that the signal was both real and cosmic. In discussing their problems with colleagues, they began to remember Gamov's prediction and, without further ado, announced that they had discovered his predicted Cosmic Microwave Background (CMB). For disovering this, the remnant of the primeval fireball ignited by the Big Bang, they were granted the Nobel Prize in Physics in 1978.
Quickly, astronomers began to wonder if there were structure in the microwave background. These fluctuations are vital to cosmology, because without them it's impossible to see how structures such as galaxies and galaxy clusters could have arisen out of a perfectly smooth fluid. To their chagrin, however, experiment after experiment only put lower and lower upper limits on the amount of angular variation across the sky in the CMB. Experimenters faced a tough assignment, though. The terrestrial atmosphere introduces a large microwave signal, which is difficult to calibrate out at such fine levels; and it wasn't until 1993 that a space mission finally uncovered the most important empirical discovery in cosmology since Penzias and Wilson. The Cosmic Background Explorer (CoBE) detected subtle variations of just one part in 100,000 in the
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