Cosmology Theory
Uploaded by Gotskillz on Jul 04, 2004
COSMOLOGY, THEORIES
The twentieth century has seen cosmology transformed from metaphysics into a branch of physics, and the laws governing fundamental forces and elementary particles have been wedded to astronomical observations to produce a description of the past and present states of the visible universe.
Prior to the creation of the general theory of relativity in 1915 by Albert Einstein, no attempts had been made to produce a mathematical description of the entire universe. Einstein's new relativistic theory of gravitation allowed consistent mathematical models of entire universes-even those with infinite size-to be formulated. Einstein's equations describe how these universes will change in time and from place to place. The simplest possible universe that can arise is one that is unchanging in time and uniform from place to place. This static universe was first proposed by Einstein in 1917 as a manifestation of the centuries-old prejudice that the universe as a whole be unchanging. In order to achieve this static state Einstein had to modify his original equations by the addition of a small constant term (dubbed the "cosmological constant") that was allowed, but not required, by the internal consistency of the theory. Subsequently, in the 1920s, it was shown by Willem de Sitter, Alexander Friedmann, and Georges Lemaitre that such static solutions are of a very special sort that would not arise in practice; the slightest deviation from perfect uniformity would cause the universe either to expand or contract as a whole. Following this discovery attention focused upon universes that expand in time.
In the late 1920s Edwin Hubble discovered that the light from distant galaxies is shifted in the direction of the red end of the spectrum of visible light by an amount that is directly proportional to their distance away from us. This redshifting of the spectrum is characteristic of the Doppler shift produced by a receding source of radiation. These observations established the expanding-universe theory as the basic paradigm of twentieth-century cosmology.
The standard theory of the expanding universe is a reconstruction of its past history and is usually called the Hot Big Bang theory (a term invented by Fred Hoyle), because the expansion implies that the universe was hotter and denser in the past. The expansion and the attractive
The expansion and the attractive nature of gravity imply that the expansion must have begun at some finite past time (about 15 billion years ago) if the laws...