History of Cosmology
From ancient times until 1928, most people, including Einstein, believed the universe was eternal and unchanging. Hubble challenged this view with his discovery that the universe is expanding. Even following this discovery, some cosmologists continued to believe the universe was unchanging, however. In 1948, Fred Hoyle and others pioneered their steady state theory, in which galaxies are receding but new matter is continually being formed in the intervening space. This view did not hold up, however, with Penzias and Wilson's discovery of CMBR, which was the first definitive evidence for the Big Bang.
Until 1980, there were numerous problems with the Big Bang model. For one thing, according to the model as first proposed, the observable universe should not be isotropic - that is, it should not look more or less the same in all directions. Another problem was the flatness problem. Until recently, astronomers could not determine whether the universe would someday stop expanding and start contracting or if it would continue to expand forever. In order for this to be the case, the density of the early universe would need to be very near critical. The third problem was the absence of magnetic monopoles, isolated north or south magnetic poles. Alan Guth solved all these problems with his theory of cosmic inflation, in which the very early universe underwent a dramatic expansion by a factor of 10^50 or so in a matter of just 10^-32 seconds. There are good physical reasons for believing in inflation, though there is yet no direct evidence for it.
The main cosmological breakthrough of the past 10 years or so has been the discovery of dark energy. This mysterious energy, which accounts for approximately 70% of the mass/energy of the observable universe, is causing the expansion of the universe to accelerate. The universe is also composed of 26% mysterious dark matter and just 4% luminous matter, which makes up the stars we can see. No one knows the composition of the dark matter, which is matter that cannot be seen directly but whose existence is inferred by its gravitation effects on galaxies.
Related Tags: astronomy, inflation, wilson, big bang, cosmology, steady state theory, cmbr, hubble, penzias, guth
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