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In current cosmological models, the universe was at first both extremely hot and incredibly dense, with temperatures exceeding billions of degrees. In the first second after the big bang, as the universe expanded and cooled, elementary particles such as quarks and electrons formed.

After about one second, the universe had cooled enough that protons had formed out of the quarks. For the next 1,000 seconds—in what is now known as the era of nucleosynthesis—hydrogen, deuterium, helium, and some lithium and beryllium formed. Electrons began to combine with protons to make hydrogen atoms about 300,000 years after the big bang. The process continued until about 1 million years after the big bang, when the universe had cooled to about 3000°C (about 5000°F). Before this era, photons of light could not travel far in the universe without bouncing off electrons. The formation of hydrogen atoms, however, used up many of the free electrons and allowed light to travel quite far. The radiation that was set free at that time has cooled as the universe has expanded. Today the temperature of this background radiation is approximately 3 K (-270°C, or -450°F).

The Cosmic Background Explorer (COBE) spacecraft accurately measured the spectrum of the background radiation from 1989 to 1993. COBE measured radiation from the sky, then subtracted known sources of radiation from its measurements to reveal the background radiation. The measured background radiation fits the radiation predicted by the big bang theory so accurately that scientists consider it conclusive evidence that the big bang theory is the correct explanation for the beginning of the universe.

One of the experiments on the COBE spacecraft found small irregularities, or ripples, in the background radiation that are thought to be the clumps of matter in the early universe—the seeds from which galaxies and clusters of galaxies developed. These ripples were studied in more detail in limited regions of the sky by a variety of ground-based and balloon-based experiments. A more recent spacecraft, NASA's Wilkinson Microwave Anisotropy Probe (WMAP), was designed to make even more accurate observations of these ripples across the entire sky, as COBE did. In 2003 WMAP’s results confirmed and extended the intermediate experiments, providing a full-sky map of the ripples.


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