For thousands of years, astronomers wrestled with basic questions about the size and age of the universe. Does the universe go on forever, or does it have an edge somewhere? Has it always existed, or did it come to being some time in the past? In 1929, Edwin Hubble, an astronomer at Caltech, made a critical discovery that soon led to scientific answers for these questions: he discovered that the universe is expanding. How to account for the apparent expansion of the universe remained a great debate among cosmologists through the 1950s. Essentially, two mutually exclusive views divided the allegiance of theorists. One, the "steady state" model, held that new matter appeared as space expanded, with the density of the universe remaining constant. The alternative theory, first articulated by the Belgian abbot Georges Lemaitre in 1931 and developed by the Russian émigré physicist George Gamow and colleagues in the 1940s and early 1950s, proposed that the universe originated in an incredibly hot and dense "Big Bang" and has continued to expand since. Gamow and others did realize, though, that if the universe were hotter and denser in the past, radiation should still be left over from the early universe. This radiation would have a well-defined spectrum that depends on its temperature. As the universe expanded, the spectrum of this light would have been redshifted to longer wavelengths, and the temperature associated with the spectrum would have decreased by a factor of over one thousand as the universe cooled. With this theoretical prediction of Gamow, the Big Bang theory became more acceptable after: O The launch of the Hubble Space Telescope and its detailed photography of the remote galaxies O The discovery of cosmic background radiation O The discovery of the expansion of the universe O Mapping the Milky Way Galaxy

For thousands of years, astronomers wrestled with basic questions about the size and age of the universe. Does the universe go on forever, or does it have an edge somewhere? Has it always existed, or did it come to being some time in the past? In 1929, Edwin Hubble, an astronomer at Caltech, made a critical discovery that soon led to scientific answers for these questions: he discovered that the universe is expanding. How to account for the apparent expansion of the universe remained a great debate among cosmologists through the 1950s. Essentially, two mutually exclusive views divided the allegiance of theorists. One, the "steady state" model, held that new matter appeared as space expanded, with the density of the universe remaining constant. The alternative theory, first articulated by the Belgian abbot Georges Lemaitre in 1931 and developed by the Russian émigré physicist George Gamow and colleagues in the 1940s and early 1950s, proposed that the universe originated in an incredibly hot and dense "Big Bang" and has continued to expand since. Gamow and others did realize, though, that if the universe were hotter and denser in the past, radiation should still be left over from the early universe. This radiation would have a well-defined spectrum that depends on its temperature. As the universe expanded, the spectrum of this light would have been redshifted to longer wavelengths, and the temperature associated with the spectrum would have decreased by a factor of over one thousand as the universe cooled. With this theoretical prediction of Gamow, the Big Bang theory became more acceptable after: O The launch of the Hubble Space Telescope and its detailed photography of the remote galaxies O The discovery of cosmic background radiation O The discovery of the expansion of the universe O Mapping the Milky Way Galaxy