![]() As we shall see, we have indeed detected the radiation emitted at this photon decoupling time, when radiation began to stream freely through the universe without interacting with matter ( Figure 29.15).Ĭosmic Microwave Background and Clouds Compared. Observations of it would be very strong evidence that our theoretical calculations about how the universe evolved are correct. If the model of the universe described in the previous section is correct, then-as we look far outward in the universe and thus far back in time-the first “afterglow” of the hot, early universe should still be detectable. Discovery of the Cosmic Background Radiation Suddenly, electromagnetic radiation could really travel, and it has been traveling through the universe ever since. From this point on, matter and radiation interacted much less frequently we say that they decoupled from each other and evolved separately. With no free electrons to scatter photons, the universe became transparent for the first time in cosmic history. Not until a few hundred thousand years after the Big Bang, when the temperature had dropped to about 3000 K and the density of atomic nuclei to about 1000 per cubic centimeter, did the electrons and nuclei manage to combine to form stable atoms of hydrogen and helium ( Figure 29.14). Only after the crowd clears is there a path for the light from his button to reach you. In a way, the universe was like an enormous crowd right after a popular concert if you get separated from a friend, even if he is wearing a flashing button, it is impossible to see through the dense crowd to spot him. It was still too hot for electrons to “settle down” and become associated with a particular nucleus such free electrons are especially effective at scattering photons, thus ensuring that no radiation ever got very far in the early universe without having its path changed. ![]() ![]() It remained hot and opaque, with radiation being scattered from one particle to another. The universe then continued to resemble the interior of a star in some ways for a few hundred thousand years more. The fusion of helium and lithium was completed when the universe was about 4 minutes old. To find out what this milestone was, let’s look at what theory tells us about what happened during the first few hundred thousand years after the Big Bang. Scientists have directly observed what the universe was like at this early stage, and these observations offer some of the strongest support for the Big Bang theory. What predictions does it make? And do observations show those predictions to be accurate? One success of the theory of the first few minutes of the universe is the correct prediction of the amount of helium in the universe.Īnother prediction is that a significant milestone in the history of the universe occurred about 380,000 years after the Big Bang. It is crucial, however, that a scientific theory should be testable. The description of the first few minutes of the universe is based on theoretical calculations. Summarize our current knowledge of the basic properties of the universe including its age and contents.Describe open, flat, and curved universes and explain which type of universe is supported by observations.Discuss the properties of this afterglow as we see it today, including its average temperature and the size of its temperature fluctuations.Explain why we can observe the afterglow of the hot, early universe.By the end of this section, you will be able to:
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