The Cosmological Principle
P1: Cosmologists hypothesized that the distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throughout the universe, and should, therefore, produce no observable irregularities on the large scale. This amounts to the strongly philosophical statement that the Passage of the universe which we can see is fair sample, and that the same physical laws apply throughout. In essence, this in a sense says that the universe is knowable and is playing fair with scientists.
P2: In the 1920s, astronomer Edwin Hubble made a groundbreaking discovery that the universe is not static, but rather is expanding. In 1998, the Hubble Space Telescope, named for the astronomer, studied distant supemovae and found that the universe was expanding more slowly a long time ago compared with the pace of its expansion today. But one thing that's certain is that the expansion of the universe was the primary fact of cosmological significance that cosmological theories had to accommodate. In every direction we look, distant galaxies are moving away from each other. The scientific community was once divided between supporters of two different expanding universe theories: the evolutionary (Big Bang) and the Steady State theory.
P3: The essential idea of evolutionary cosmology is that there was a beginning— a moment of creation at which the universe came into existence in a hot, violent explosion—the Big Bang. Scientists believe that the entire vastness of the observable universe was compressed into a hot, dense mass just a few millimeters across. This nearly incomprehensible state is theorized to have existed for just a fraction of the first second of time. Scientists can't be sure exactly how the universe evolved after the big bang. Many believe that as time passed and matter cooled, more diverse kinds of atoms began to form, and they eventually condensed into the stars and galaxies of our present universe some 10 billion to 20 billion years ago.
P4: In the steady-state theory, the universe would have existed for ever and would have looked the same at all times, thus adhering to the perfect cosmological principle, a principle that asserts that the observable universe is basically the same at any time as well as at any place. This view is consistent with philosophical approaches that reject the notion of an absolute beginning of the universe as unacceptable. The steady-state universe would have no beginning and no end.
P5: In an expanding universe, as galaxies moved apart, spreading matter more thinly over space, new galaxies would form from matter—in the form of hydrogen— that was supposed to be continually being created throughout space. In addition, the perfect cosmological principle requires that the universe is always expanding but maintaining a constant average density, matter being continuously created to form new stars and galaxies at the same rate that old ones become unobservable with available instruments as a consequence of their increasing distance and velocity of recession. Thus in the steady-state universe, from any point within it the view on the grand scale—the general character and the overall density—is the same. Galaxies of all possible ages are intermingled. Through continuous creation, the steady-state theory is finally compatible with the expanding universe. In this special sense, the steady-state universe itself does not evolve.
P6: Both of the two theories account for the cosmological principle effectively. However, the discovery in the 1960s of comparatively small star-like objects called quasars tipped the scales in favor of Big Bang cosmology. Many astronomers believe that quasars are the most distant objects yet detected in the universe, emitting radio waves and visible light up to 100 times the luminosity of the entire Milky Way Galaxy.
P7: Despite their brightness, due to their great distance from Earth, no quasars can be seen with an unaided eye. Energy from quasars takes billions of years to reach the Earth's atmosphere. ■ For this reason, the study of quasars can provide astronomers with information about the state of the universe billions of years ago. ■ The fact that almost all quasars are very far away implies that earlier in the history of the universe quasars were developing more frequently than they are now. ■ This evolution is consistent with the Big Bang theory, but it violates the perfect cosmological principle. ■