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In 1917 American scientist Harlow Shapley measured the distance to several groups of stars known as globular clusters. He measured these distances by using a method developed in 1912 by American astronomer Henrietta Leavitt. Leavitt’s method relates distance to variations in brightness of Cepheid variables, a class of stars that vary periodically in brightness.

Shapley’s distance measurements showed that the clusters were centered around a point far from the Sun. The arrangement of the clusters was presumed to reflect the overall shape of the galaxy, so Shapley realized that the Sun was not in the center of the galaxy. Just as Copernicus’s observations revealed that Earth was not at the center of the universe, Shapley’s observations revealed that the Sun was not at the center of the galaxy. Cosmologists now realize that Earth and the Sun do not occupy any special position in the universe.

Starting in about 1913, new large telescopes and advances in photography and spectroscopy, the study of the particular colors making up a beam of light, allowed astronomers to observe and begin measuring a reddening of the light from distant galaxies. These redshifts are similar to those caused by the see Doppler effect. The Doppler effect is observed when an object emitting radiation moves with respect to the observer of that radiation. If the object is moving toward the observer, each wave of radiation originates from a place that is a little bit closer to the observer than the previous wave’s point of origin, so the distance between successive wave peaks, called wavelength, is shorter than usual. If the object is moving away from the observer, the wavelength is longer than usual. The wavelength change is proportional to the speed at which the object is moving relative to the observer. In visible light, a shift to longer wavelengths is equivalent to a shift toward the red end of the visible spectrum. Therefore, cosmologists refer to shifts in the color of light coming from galaxies that are moving away from Earth as redshifts. The faster a galaxy is moving away, the more red its light will appear. By measuring the redshifts of distant galaxies, astronomers began to understand how the universe was evolving.

In 1915 German American physicist Albert Einstein, who was working in Switzerland, advanced a theory of gravitation known as the general theory of relativity. His theory involves a four-dimensional space-time continuum that bends in the presence of massive objects. This bending causes light and other objects that are moving near these massive objects to follow a curved path, just as a golfer's ball curves on a warped putting green. In this way, Einstein explained gravity. His theory showed that Newton’s theory of gravitation was a special case, valid in conditions normal to Earth but not in very strong gravitational fields or in other extreme conditions. Einstein’s theory also made several predictions that were not part of Newton's theory. When these predictions were verified, Einstein's theory was accepted. Einstein's equations were very complicated, though, and it was other scientists who eventually found widely accepted solutions to Einstein’s equations. Most of cosmology today is based on the set of solutions found in the 1920s by Russian mathematician Alexander Friedmann. Dutch astronomer Willem de Sitter and Belgian astronomer Georges Lemaître also developed cosmological models based on solutions to Einstein’s equations.

In the early 1920s, astronomers debated about whether the spiral structures seen in the sky, called spiral nebulae, were galaxies like our own Milky Way Galaxy or smaller objects in the Milky Way. Measuring the distances to these galaxies depended on the Leavitt-Shapley method of observing Cepheid variable stars. In 1924 American astronomer Edwin Hubble was able to detect Cepheid variables in other galaxies and show that the galaxies were beyond our own. These findings indicated that the spiral structures were probably galaxies separate from the Milky Way.

In 1929 Hubble had measured enough spectra of galaxies to realize that the galaxies’ light, except for that of the few nearest galaxies, was all shifted toward the red end of the visible spectrum. This shift increased the more distant the galaxies were. Cosmologists soon interpreted these redshifts as akin to Doppler shifts, which meant that the galaxies were moving away from Earth. The redshift, and therefore the speed of the galaxy, was greater for more distant galaxies. Galaxies in different directions at equivalent distances from Earth, however, had equivalent redshifts. This constant relationship between distance and speed led cosmologists to believe that the universe is expanding uniformly. The uniform relationship between velocity of expansion and distance from Earth is known as Hubble's law. The redshifts are not true Doppler shifts but rather result from the expansion of space, which carries the galaxies along with it.


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