Big Bang Theory

In the early twentieth century, there was a debate among astronomers over the nature of the spiral nebulae, the diffuse spiral-shaped structures visible (through telescopes) in most parts of the sky. Some believed these were nearby objects that were part of our Milky Way Galaxy, while others thought that they were much further away, and in fact were "island universes," or separate galaxies. If the distances to these objects could be measured, then the debate could be settled, and important knowledge gained about the structure of the Universe.

In 1914, U.S. astronomer Vesto M. Slipher (1875–1969) presented figures for the velocities of 14 spiral nebulae, obtained by measuring the Doppler shifts of their spectral lines as described above. Slipher found that most of the nebulae were, surprisingly, moving away from earth. If the nebulae's motions were random, just as many of them would be expected to move toward earth as were moving away. Why should the sun just happen to be at the center of an organized pattern of motion? Another puzzling finding was the large velocities at which these objects were receding. The nearby Andromeda nebula, for example, was speeding toward earth at 180 miles per second (300 km per second). Many astronomers interpreted this to mean that the nebulae must be outside our galaxy.

In 1923, U.S. astronomer Edwin Hubble (1889–1953), using the 60-in and 100-in (152-cm and 254-cm) telescopes at Mount Wilson Observatory, succeeded in identifying Cepheid variables in the outer regions of two nebulae, M31 and M33. By measuring the periods of these Cepheids and using the formula developed by Leavitt several years earlier, he calculated that they are about 930,000 light years distant. From these distances and the observed sizes of the nebulae, their actual sizes could be calculated. These turned out to be similar to that of Earth's galaxy, strongly supporting the idea that the nebulae are galaxies in their own right.

In 1929, Hubble plotted data on a number of galaxies on a graph. He plotted the distance to the galaxy along the horizontal axis and the velocity of the galaxy's recession along the vertical axis. From this limited data, it was clear that a simple, linear relationship between the two quantities existed; on average, the velocity of a galaxy's recession was proportional to the distance to the galaxy. (The Andromeda galaxy is a member of our local galactic group and does not obey this general rule.) The constant of proportionality between distance and velocity, now called the Hubble's constant (H), was given by the slope of the line. From those data, Hubble's constant was estimated to be 310 miles per second per megaparsec (500 km/s/Mpc)—that is, a galaxy 1 megaparsec (one million parsecs, where one parsec = 3.26 light-years) away would be moving away from our galaxy at 310 mi/s (500 km/s), while a galaxy ten times further away would be moving ten times as fast. Modern values for H are much smaller than Hubble's estimate of 500 km/s/Mpc. The relationship between speed and distance governed by Hubble's constant is termed Hubble's law.