1Chapter 17 continuedChapter 17 continuedThe Beginning of TimeThe Beginning of TimeWe saw that strong evidence for the big Bang theory is found in the discovery of the Cosmic Microwave Background (CMB), the residual heatof the universe itself. It is a perfect thermal radiation spectrumwith a temperature of 2.73 K. This is just what we would expect if the universe expanded by about 1000 since the Era of Atoms (about 380,000 years old).THE WMAP SATELLITE TOOK THIS ALL-SKY PICTURE OF THE ULTRA-SMOOTH MICROWAVE BACKGROUND. TEMPERATURE VARIES BY NO MORE THAN 0.0001 K FROM PLACE TO PLACE.What have we learned?• How do we observe the radiation left over from the Big Bang?•Telescopes that can detect microwavesallow us to observe the cosmic microwave background, radiation left over from the Big Bang. •Its spectrum matches the characteristics expected of the radiation released at the end of the era of nuclei, spectacularly confirming a key prediction of the Big Bang theory.Peak wavelength of about 1mm gives us the temperatureFrom NASA’s Cosmic Background Explorer (COBE) satellite: the two lead scientists got the Nobel Prize. Protons and neutrons combined to make long-lasting helium nuclei when the universe was ~3 minutes old. Fusion stopped after that because the universe had already cooled.As the hot dense universe expanded and cooled it went through the temperature range the allows hydrogen to fuseto heliumTHE BIG BANG EXPLAINS THE ABUNDANCE OF HELIUMTHE BIG BANG EXPLAINS THE ABUNDANCE OF HELIUMProtons outnumbered neutrons by 7 to 1Big Bang theory prediction: 75% H, 25% He (by mass)Matches observations of nearly primordial gases
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