There have been many articles and news stories about the acceleration of the universe's expansion, but fairly little analysis about the meaning or significance of this finding. A Google search on 10/23/2013 for the exact phrase “significance of cosmic acceleration” produces zero matches, as does a Google search for the exact phrase “significance of an accelerating universe.” Let's have a try at discussing the significance of this finding, and to do that we must look at the concept of a cyclical universe.
A cyclical universe theory is one that says that the universe passes through a series of phases or cycles, with each cycle being repeated over and over again. Depending on the theory, each cycle may last billions or trillions of years.
The main theory of a cyclical universe which prevailed around 1985 was the theory of an oscillating universe. To understand this theory, you must understand the concept of critical density. Scientists have long said that if the density of the universe in mass-energy is less than a particular density called the critical density, the universe will keep expanding. But if the density of the universe is greater than this critical density, the universe's expansion will one day slow down and then reverse. If that were to happen, the expansion of the universe (in which the distance between galaxies increases) would turn into a contraction of the universe (in which the distance between galaxies decreases). At the end of the period of contraction would be a Big Crunch in which all of the universe ends up crunched together in a very dense state.
The theory of an oscillating universe held that the universe's density was less than this critical density, and that the current expansion of the universe would one day slow down because of a gravitational attraction of the universe's mass-energy. The theory maintained that at some time billions of years in the future, the universe's expansion would turn into contraction, at which point galaxies would then start moving closer towards each other. The theory maintained that this contraction would eventually result in a Big Crunch that would consist of all the matter in the universe collapsing into a hot dense state (kind of a Big Bang in reverse).
But this theory of an oscillating universe did not maintain that this cycle of a Big Bang followed by Big Crunch would happen just once. The theory maintained that this cycle had been occurring forever, and would continue to occur forever.
If you were to express this theory of an oscillating universe as a piece of computer programming code, it would look like the loop below. The loop is what they call an endless loop, one that repeats endlessly (because 8 is always greater than 7).
While 8 > 7
The theory of an oscillating universe is shown in the diagram below. Each of the semicircles is one of the imagined cycles of expansion and contraction. Each cycle begins with a Big Bang and ends with a Big Crunch. The red line shows the actual observed expansion.
The theory of an oscillating universe was very attractive to many people, because if it were true, it would eliminate the difficulty of explaining the Big Bang. Rather than being a strange one-of-a-kind event (with possible theological implications), the Big Bang would become just a “routine” event that had happened an infinite number of times before.
But even before the acceleration of the universe's expansion was discovered, there were big problems with this theory of an oscillating universe. One huge problem was that there was no good theoretical basis for predicting that a Big Crunch would produce a Big Bang – instead the more likely result would just be that a Big Crunch would cause everything in the universe to end up in a black hole or black holes. Another problem was that entropy would increase in each cycle of an oscillating universe. So instead of there being a possibility of an infinite number of cycles, there could actually only have been a few cycles of an oscillating universe. A cyclical theory that only allows for a few previous cycles is defeating its own rationale of avoiding problems of a beginning.
Despite these problems, many scientists clung to the theory of an oscillating universe, hoping that it would be confirmed. But the discovery of the acceleration of the universe's expansion has put the final nail in the coffin of the theory of an oscillating universe. Because the universe's expansion is accelerating, there will be no Big Crunch. The distance between galaxies will just continue to grow more and more.
Hence the discovery of the acceleration of the universe's expansion has wiped out the most popular theory of a cyclical universe. There has not emerged any widely held theory of a cyclical universe to replace the theory of an oscillating universe. The only remaining theories of a cyclical universe are basically ornate speculative edifices advanced by a single theorist, or a single theorist and a handful of colleagues. None of these theories has gained much traction, partially because none of them have the simplicity of the theory of an oscillating universe.
Anyone trying to come up with a theory of a cyclical universe has the deck stacked against him. You have the Big Bang, the unexplained one-of-a-kind event beginning the universe. You have the Second Law of Thermodynamics, which tells us entropy always increases over long time periods, and the fact that the current entropy level in the universe is vastly lower than it would be if the universe were many times older than 13 billion years. You have the acceleration of the universe's expansion, which tells us that there will be no further opportunities for an event like the Big Bang, when everything was densely packed. These are not facts that lend credence to any cyclical theory of a universe. Nature seems to be screaming at us that the universe is not eternal and cyclical, and it requires a huge amount of speculative imagination to come up with any theory that drowns out that message.
This, then, is the real significance of an accelerating universe. Its significance is that it indicates that we are living not in a cyclical cosmos, but instead the first and last edition of our universe.