In the 19th Century astronomers had learned that the light from stars and more distant galaxies is not featureless, but has distinct spectral lines characteristic of the atoms and ions in the gases near the surface of the stars. Using large telescopes to gather light too faint for detection by the unaided human eye, astronomers extended their study to the most distant objects, many of which appeared to have complex shapes. In the early 1920s it was known that some spiral nebulae contained individual stars, but there was no consensus as to whether these were relatively small clusters of stars within the nearby collection called the
Milky Way that stretches in a wide band across the sky, or whether these could be separate galaxies, or
island universes, as big as the Milky Way but much further away. In 1924 Edwin Hubble (1889-1953, at right), using the two largest telescopes in the world at that time, the 60 and 100-inch reflector telescopes on Mount Wilson in southern California, discovered that Cepheids variables of the same period in NGC 6822, and similarly in M33 and M31, were intrinsically equally bright. Hubble used the apparent brightness of the Cepheids to measure the distance to the Andromeda nebula, finding it about a hundred thousand times further far away than the nearest stars. Apparently it was a separate galaxy, comparable in size to the Milky Way but much further away. Based on this view of this galaxy in the Andromeda constellation, it seemed reasonable that we are part of a similarly shaped Milky Way galaxy.
In 1929 Hubble with his assistant Humason discovered that other more distant galaxies had spectra with all colors proportionally shifted toward the red
end of the spectrum. Hubble compared the galaxies' spectra with their distances calculated using the relative brightness of Cepheids, and showed that the amount of red shift was proportional to distance. Realizing that light waves could be contracted or expanded by the relative motion of source and observer much as Austrian physicist Christian Doppler (1803-1853) had proposed nearly a century earlier for double stars in 1842 and investigated in 1845 with sound, Hubble realized that the likely explanation for the red shift was that the galaxies are receding from Earth and each other at speeds proportional to their distance. This was radically contrary to the ancient belief that the stars had pertinently fixed locations on the celestial sphere. In a series of papers with Humason between 1931 and 1936, Hubble verified and extended the relation to large (i.e. 60,000 km/s) red shifts.
In 1948, Russian-born physicist George Gamow (1904-1968) realized that if all the galaxies are flying apart at high speed, the entire universe may have been concentrated in a single point at some time in the past. Another scientist, Fred Hoyle, (1915-2001) coined the name Big Bang to disparage Gamow's idea, but the term became popular and continues in use. Based on this prespective of Gamow and Einstein, Hubble's discovery should be rephrased: Instead of viewing the galaxies as moving away from us through space, consider that space itself is expanding, carrying along all contents, causing the observed redshifts proportional to the distances to the light sources.
This expansion of the universe causes an overall decrease in average temperature. (expanding gases cool)
When Gamow, Alpher, and Herman first formulated the theory, they predicted there would be a leftover radiation
signature from the Big Bang, and realized it might be detectable. Calculating the original temperature of the explosion, taking into account the temperature reduction that would be caused by the universe's subsequent expansion, they estimated a current temperature average of about 5 K.
Between 1963 and 1965 Arno Penzias and Robert Wilson worked at Bell Laboratories in New Jersey investigating
potential astronomy using microwaves. (Bell Telephone was likely more interested in potential use of microwaves for communications.) They were plagued by a background noise equivalent to that generated by a black body at extremely cold 3 K. Finding it constant in all directions, they were unable to find any identifiable source. Thinking it might be generated somehow inside their antenna, they cleaned bird droppings and taped all seams seeking ways to eliminate the detected noise. Meanwhile P. J. E. Peebles in R. H. Dicke’s group at Princeton University was constructing a rooftop antenna to search for the expected signature microwave radiation from the extremely hot condensed phase predicted by an oscillating universe. After Bernard Burke of M.I.T. heard of Penzias's difficulties, he hinted that Peebles might have an explanation for the noise. A telephone query to Peebles established that the annoying background noise that Penzias and Wilson could not eliminate nor explain was the 2.73 K microwave relic confirming the Big Bang! The two teams subsequently published side-by-side letters in the Astrophysical Journal: a letter on the theory from Princeton and one on measurement of excess antenna temperature from Bell Laboratories.
The current "best modern value" (mid-2003) of the Hubble constant is 71 km/s per megaparsec (±0.04). This value comes from the use of type Ia supernovae (which give relative distances to about 5%) along with data from Cepheid variables gathered by the Hubble Space Telescope. A number of independent measurements all suggest that the big bang did occur 13.7 billion years ago. (The big bang event is no longer doubted by scientists. The observed evidence and measurements are compelling. What remains is to determine details of events which occurred in the first few seconds following the big bang.)
The measured red shifts are usually stated in terms of a
z parameter. The largest measured z values are associated with a type of very ancient, bright objects called quasars. These are thought to be the emitted radiation from large amounts of matter being devoured by massive black holes (a million times our Sun's mass) at the centers of very young galaxies. The accelerating masses emit more energy than emitted from entire mature galaxies. The most extreme red shifts indicate that some quasars existed less than a billion years after the big bang. The presence in quasar spectral lines of Iron and Magnesium, elements typically manufactured in red giant and supernova star stages, suggests that perhaps stars were formed even earlier. The relatively high abundance of Iron compared to Magnesium indicated by the intensity of their spectral lines suggest synthesis in intermediate-mass stars which only produce these elements after the long main sequence stage which for them lasts hundreds of millions of years. This implies that early stars may have formed only 200 million years after the Big Bang. Evidence from closer galaxies suggests that massive black holes continue to exist but, having already devoured most nearby matter, now emit much less radiation.
When Gustav Kirchhoff and Robert Bunsen observed spectra of light emitted from very hot elements in the 1850s, they had no clue about the mechanism which caused each element to have a distinct spectrum. Eventually in 1885 Johann Kakob Balmer (1825-1898) found the visible spectrum of Hydrogen fit a mathematical progression. In 1908 the German Louis Carl Heinrich Friedrich Paschen (1865-1947) found two hydrogen lines in the infrared. Between 1906 and 1914 the American Theodore Lyman (1874-1954) found another similar series of spectral lines in the ultraviolet due to hydrogen. But not until 1913 did Niels Bohr (1885-1962) developed a theory of transitions between quantized orbits of electrons in atoms to explain the spectral lines. While Bohr hoped to eventually explain all spectra, his theory only successful predicted the correct wavelengths for Hydrogen. Only the addition of the quantum ideas of wave mechanics developed about 1926 by Werner Heisenberg (1901-1976) and Erwin Schrödinger (1887-1961) successfully explained all spectra.In this experiment we shall measure the prominent spectral line of hydrogen emitted by distant quasars and use it to determine the red shift believed due to the rapid motion of the quasars away from us. The red shift