![]() ![]() Spiral nebulae are actually galaxies-vast wheeling clouds of stars, dust, and gas-but this was not known at the time, because the spiral nebulae were visible only as pinwheel-shaped smears of light in even the best telescopes. Historical Background and Scientific FoundationsĪstronomers observed in the 1910s that the spiral nebulae are receding from us. It is the organizing or central concept of most modern cosmology. The big bang began the process of synthesizing (putting together) the nuclei of all the elements of which Earth and we ourselves are made. These include the movements of distant galaxies and the nature of the cosmic microwave background, a sky-filling bath of radio waves that can be explained as the afterglow of the big bang. Although many questions about the big bang remain, and alternative explanations of the universe's history have been proposed, most cosmologists agree that the big bang theory is strongly supported by many observations. The rapid beginning of this expansion is termed the big bang. ![]() Astronomy and Cosmology: Big Bang Theory and Modern Cosmology IntroductionĬosmologists, who are scientists who study the nature and history of the universe as a whole, state that the universe began in a state of infinite or near-infinite density about 13.7 billion years ago and has been expanding and cooling ever since. ![]()
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