NOTES ON THE HISTORY OF COSMOLOGY

1000 BC

According to the Rig Vedas -- ancient Hindu texts -- the universe undergoes an endless cycle of fiery deaths and rebirths. This cycle of creation and destruction is a manifestation of the dance of Shiva. The time for each cycle is a "day of Brahma", which lasts 4.32 billion years. (This number is curiously close to the age of the Earth, 4.6 billion years, as determined from the age of the elements.)

350 BC

According to Aristotle, the universe is eternal and has never changed. The Earth is at the center, and the stars are located at the outer boundary, where all things (including space itself) fade into nothingness. (This idea foreshadows the modern concept of the "event horizon" of the universe.)

1576 AD

Thomas Digges publishes "A Perfect Description of the Celestial Orbes", a variant of the theory of Copernicus (with the Sun at the center of the universe), in which Digges claims that the universe is infinite in spatial extent and filled uniformly with stars. This is one of the first clear statements of the "Cosmological Principle."

1590

The fiery monk Giordano Bruno goes one step further: he says that the Sun is not at the center, but that the universe contains numberless solar systems and is teeming with life. The Church was not amused. He was imprisoned, tortured, and finally burned at the stake in 1600.

1610

The great astronomer Johannes Kepler first pointed out a fundamental problem with an infinite universe populated with stars: the entire sky would be blazing hot. In 1720, Edmund Halley, famous for his prediction of the return of Halley's comet, again draws attention to this problem. In 1823, the German astronomer Olbers once again discusses this problem, and suggests that the solution might be that the universe is filled with dust, which prevents the light from distant stars from reaching us. But 20 years later the great English astronomer John Herschel shows that Olbers' explanation won't work. This problem, now known as "Olbers' paradox" was not resolved until 1920, when Edwin Hubble proved that the universe was expanding.

1912-1920

American astronomer Vesto Slipher, working at the Lowell Observatory in Flagstaff, Arizona, began to measure the Doppler shifts of spectral lines from spiral galaxies. He found that the vast majority of galaxies were moving away from the Milky Way (except the Andromeda galaxy M31, which was moving toward the Milky Way).

1916

Albert Einstein published his General Theory of Relativity (GTR), which explains how the presence of matter causes space and time itself to be warped, and how the force of gravity can be regarded as the natural trajectories of objects moving in warped space-time.

1917

Einstein realized that the equations of GTR should be able to explain the structure of the entire universe. He assumed that the universe is infinite in extent, has more or less the same average density everywhere (the "Cosmological Principle"), and that the space must be curved because of the matter in the universe. With these assumptions, he found that his equations do not permit a static universe: the universe must be in motion, either expanding or contracting. Einstein couldn't believe his own equations, because they contradicted the "Perfect Cosmological Principle" -- namely, that notion that the universe must be eternal in time as well as uniform in space. To remedy this, Einstein added a new term to his equations of GTR. This term, called the "cosmological constant", acted as a repulsive force of space and time itself; it could counteract the attractive force of gravity and permit an infinite universe that was neither expanding nor contracting.

1917

Dutch astronomer Willem de Sitter solves Einstein's original equations of GTR, without the cosmological constant and with very low density of matter. His solution shows that such a universe must be expanding.

1920

Russian mathematician Alexander Friedmann finds a general set of solutions to Einstein's equations of GTR with no cosmological constant but with any density of matter. His solutions describe a set of model universes, some of which expand forever and some of which collapse again, depending on the mean density of matter in the universe. Nobody paid much attention to Friedmann's solutions.

1924

Working at at the great new 2.4-m telescope at Mt. Wilson, California, Edwin Hubble began a systematic survey to measure the distances and Doppler shifts of spiral galaxies, following Slipher's work.

1927

Belgian priest Georges Lemaitre rediscovered Friedmann's solutions to Einstein's equations. But, unlike Friedmann, Lemaitre was interested in observational astronomy as well as mathematics and he was aware of Hubble's observations at Mt. Wilson showing that the distant galaxies were expanding away from the Milky Way -- the more distant the galaxy, the faster the expansion. Lemaitre recognized that this observed expansion law was exactly what Einstein's equations predicted, and he told Hubble. Moreover, it was clear that the cosmic expansion was the solution of Olbers' paradox.

1929

Hubble published his observations showing that the expansion of the universe obeyed the equation V = H₀D, now known as "Hubble's Law". He underestimated the distances of the galaxies, thus derived a value of the "Hubble Constant", H₀ = 500 km/s/Mpc, that is almost ten times the modern value.

1932

By this time, Hubble and Lemaitre had convinced the world that the universe was really expanding, according to the Friedmann solutions. Einstein came to Mt. Wilson to meet Hubble, and said that the invention of the cosmological constant was "the biggest blunder of my life." Here's a photo of Lemaitre (center) and Einstein (right) in Pasadena, California in 1933.

1941

Canadian astronomer Andrew McKellar, observing absorption lines in stellar spectra due to CH+ and CN molecules in interstellar clouds noticed that the molecules were excited, as if they were bathed in a cosmic radiation field at a temperature of about 3 K (-270 C). It was a puzzling result that nobody could understand. Everybody ignored it.

1948

Russian physicist George Gamow, now working at George Washington University in Washington, DC, published a famous paper with R. Alpher and Hans Bethe. He realized that the Friedmann solutions implied that the universe had infinite density when the expansion began, some 10 billion years ago, and that if so, the primordial matter in the early universe (less than a minute old!) would be incredibly dense. Such primordial matter in the universe would naturally undergo fusion reactions, leaving a universe today filled with mostly heavy elements and virtually no hydrogen. That prediction was grossly in conflict with astronomical observations, which showed that the most of the matter in the universe was hydrogen. Gamow realized that this dilemma could be resolved if the universe was filled with gamma rays, and that this radiation would cool down as the universe expanded. With his students Alpher and Hermann, Gamow estimated that the radiation would have a temperature of about 5 K today, but in a book he published in 1952, Gamow changed his estimate to 50 K. In fact, Gamow made several errors in his assumptions about the conditions and reactions in the early universe, so the details of his predictions were wrong. Few scientists took Gamow's theory seriously.

1950

Japanese astrophysicist Chushiro Hayashi (the same Hayashi who developed the theory for newly-forming stars) recognized a basic flaw in Gamow's theory for the formation of the elements in the early universe. A gas of pure neutrons could not coexist with such a hot radiation field in the early universe. Hayashi corrected Gamow's theory and published the version that we believe today. But he did not carry out detailed calculations of the radiation temperature.

1964

Russian physicists A. G. Doroshkevich and Igor Novikov, working in Moscow, suggested that it might be possible to observe the left- over radiation filling the cosmos and pointed out that the best telescope to use might be the ATT Bell Labs radio telescope at Holmdel, New Jersey.

1964

Robert Dicke, working at Princeton University, knew about the theory of the hot big bang and was building a radio telescope especially designed to observe the radiation.

1964

Arno Penzias and Robert Wilson, working at the Bell Labs radio telescope, were observing "background radiation" that seemed to come from all directions in the sky. It had a temperature of 3.5 +/- 1 K. Penzias and Wilson thought the noise was not real, but due to some flaw in their receiver. Bernard Burke, a radio astronomer from MIT, was visiting Bell Labs immediately after visiting Dicke at Princeton, some 30 miles away. Burke told Penzias and Wilson that Dicke was trying to build a telescope to observe the cosmic background radiation, and that it was predicted to have exactly the temperature of the "noise" that they were observing. They realized that they had discovered the fires of creation!

1989

NASA launches the Cosmic Background Explorer (COBE) satellite, especially designed to measure the spatial distribution and spectrum of the cosmic background radiation (CBR). COBE observations show that the spectrum is exactly as predicted by the Big Bang Theory. Moreover, COBE measures very slight (about 1 part in 100,000) spatial variations in the temperature of the background radiation. These variations show that the distribution of matter and radiation in the early universe is not absolutely smooth. They are a fundamental clue to how matter in the universe developed the structure that we observe today as clustering of galaxies.

1998

Through observations of distant supernovae, astronomers measure the expansion rate of the universe when it was roughly half its present age, and are astonished to discover that the universe was expanding more slowly then than it is now. It seems that the universe is speeding up! The best way to account for this acceleration is to revive the concept of the cosmological constant, which Einstein invented in 1917 and discarded in 1932. See Breakthrough of the Year: Cosmic Motion Revealed

2000

A consortium of astronomers from the US and Italy report new measurements of the angular structure of the CBR obtained with a telescope carried on a balloon launched from Antarctica. The results confirm the idea that the cosmological constant exists. See the Boomerang home page.

Today, we know the Hubble Constant to an accuracy of about +/-15% (H₀ = 65 +/- 8 km/s/Mpc). Only a few years ago, the uncertainty was a factor of two (H₀ = 50 - 100 km s^-1 Mpc^-1).

There are several more experiments underway to measure the fluctuations of the CBR from the ground. Also, NASA launched the Microwave Anisotropy Probe (MAP) satellite in June 2001, and the European Space Agency plans to launch a more powerful instrument, called PLANCK, in 2007. By measuring the fluctuations of the CMB in exquisite detail, these experiments are expected to determine the fundamental properties of the universe that we need to know to understand its origin and fate.