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© Copyright 1997, Jim Loy
The Big Bang was the explosion that was the beginning of the Universe, 10 to 20 billion (10,000,000,000-20,000,000,000) years ago. Is there any evidence of this explosion?
Cosmic Red Shift - Nearly all galaxies show a red shift in their spectra, which shows that they are all receding from us. And the speed is roughly proportional to their distance from us. In other words, they are all receding from each other. They all seem to have begun very near each other, about 20 billion years ago. The rate of expansion should be decreasing. So, the age of the universe is somewhat less than 20 billion years. Apparently the current best estimate is 13 billion.
Cosmic Background Radiation - The hot early universe emitted radiation, just because it was hot. This thermal radiation is called black body radiation. As the universe expanded, it cooled down. It has not completely cooled down, yet. According to theory, the universe should have a black body temperature of about 3 Kelvin (nearly absolute zero). There is a low frequency radiation coming from all directions; that indicates a temperature of about 3 Kelvin. This radiation was accidentally discovered at about the same time that it was being predicted. The temperature and spectrum of this radiation are almost exactly as predicted.
Quasars - Quasars are very bright objects which only exist at immense distances. There are no nearby quasars. Objects seen at immense distances actually are seen as they existed billions of years ago, because it takes that long for their light to get to us. So, the universe was different (had quasars) in the past. This is inconsistent with an unchanging universe, and is consistent with a universe which had a beginning.
General Relativity - Einstein introduced a fudge factor (the cosmological constant) into General Relativity, to compensate for an unchanging universe. This fudge factor is a kind of repulsion, which keeps an unchanging universe from automatically contracting. If the universe expands or contracts, there is no need for the fudge factor.
Cosmologists (astronomers who study the history and future of the entire universe) have not yet determined if there is enough mass in the universe to make it contract again, or if it will just keep expanding. A one-time explosion is not very satisfying, theoretically. So, a cyclic series of infinitely many big bangs is preferred. But, if it keeps expanding, then it only happened once.
The universe that we see is not massive enough to ever begin contracting again. But, there is likely to be unseen mass (the so-called "missing mass"), for a number of reasons. One reason is that without this missing mass, clusters of galaxies (including our own cluster) would have flown apart, by now. Cosmologists continue to search for the missing mass, with both theory and observation.
See Hubble's Law. Note that billion here is the American billion (one thousand million).
Addendum:
More evidence:
Olbers' Paradox - This is a mathematical proof that an infinite non-expanding universe would produce a very bright night time sky. So the universe is either finite, or expanding, or both.
H-R Diagram - This is a graph showing the brightness of stars along one axis, and their colors along the other axis. The use of this diagram has payed off in many ways, including helping to estimate the distance to stars. In many cases, it gives strong clues to the evolution of stars, and can help estimate their ages. In this way, the oldest stars in our galaxy (those in some globular clusters) have been shown to be about 13 billion years old.
The Age of the Solar System - The solar system is quite a bit younger than the universe. Radiation dating of moon rocks and meteors have shown that the solar system is about 4.6 billion years old. This at least shows that this part of the universe is fairly ancient, and is consistent with an older big bang.
Radio Galaxies - Some galaxies are strong sources of radio waves. Radio astronomy has shown that there are relatively more radio galaxies at great distances than at closer distances. As in the argument about quasars, above, this shows that the universe was different in the past. And it is inconsistent with an unchanging universe.
The Abundance of Helium - The universe is mainly made up of hydrogen and empty space. Other relatively light elements (including helium) are made in the interior of stars. Heavier elements are made from the explosive death of some stars. If all of these elements (except hydrogen) were created only by stars, then their relative abundances should vary tremendously from place to place, depending upon the nature of the stars that lived and died. This is indeed true of all of the elements except hydrogen and helium. Helium is distributed fairly evenly throughout the universe. That suggests that its main origin was not in stars, but perhaps in a big bang.
Cosmologists (astronomers who study the history and future of the entire universe) have not yet determined if there is enough mass in the universe to make it contract again, or if it will just keep expanding. A one-time explosion is not very satisfying, theoretically. So, a cyclic series of infinitely many big bangs is preferred. But, if it keeps expanding, then it only happened once.
The universe that we see is not massive enough to ever begin contracting again. But, there is likely to be unseen mass (the so-called "missing mass"), for a number of reasons. One reason is that without this missing mass, clusters of galaxies (including our own cluster) would have flown apart by now. Cosmologists continue to search for the missing mass, with both theory and observation.