The field of modern cosmology has undergone something of a revolution since the late 1990s. This comes on the heels of a century that dramatically transformed man’s view of the cosmos. For those of us that learned about cosmology in the 70s, 80s, and early 90s, there were two possible universes. One was an “open” universe, that began with a big bang, and ended with a sea of lifeless particles, separated from each other by infinite distances. The other was a “closed” universe. The universe was finite in space, but with no boundaries. The surface of a globe is an example of a 2-dimentional surface, that is finite, but with no boundaries. It exists in 3-dimentional space. The universe was pictured as a 3-dimentional surface of a 4-dimentional sphere. In this universe, there would eventually be a re-collapse, a “big-crunch”, and the cycle would start over again.

The first universe fit in well with traditional monotheism. The second gave a very materialistic picture. The universe was self-contained and needed no outside support. It was also of finite size and complexity. Given an infinite amount of time, all events would have to repeat. Thus there would be something like the re-incarnation of the Eastern religions. The two choices given by modern science were a universe infinite in space, but finite in time, or a universe infinite in time, but finite in space.

In the late 1990s, observations of supernovas led to the discovery that the expansion of the universe was accelerating. This was about as surprising as finding a ball accelerating uphill. The acceleration has been attributed to “dark-energy”, sort of an anti-gravity force, and it has been connected to the virtual particles found in the vacuum. (See
quantum mechanics). Recently more evidence confirming the existence of dark-energy was discovered.

This has eliminated the possibility of a closed universe, but it has also opened up a range of new possibilities. But first, it is important to understand that the big bang is not an explosion in space. According to modern cosmology, the moment after the big bang, our universe, which is infinite in space was filled everywhere with high density energy. Over time the universe then expanded, or if the idea of something infinitely large expanding causes conceptual problems, we could say instead that the energy became less dense with time. This is the standard picture.

In some of the new models, the big bang may not be the beginning of all time and space. There is very high matter density, but not infinite density. The big bang takes place in a vacuum filled with virtual particles (dark energy). The big bang fills the universe, with normal matter, which then expands. Eventually the expansion force of the dark energy starts to win out over the gravity of the ordinary matter, and the expansion accelerates. Because of the acceleration, the portion of the universe that can be seen from any point starts to decrease. That is, today, the big bang took place some 13 billion years ago, and 13 billion light-years away is our “event-horizon”. Beyond that we would be looking back at a time before that big bang. Also, that horizon is moving away at the speed of light. The acceleration of the universe means that things will actually disappear over that horizon.

Eventually, if the expansion keeps accelerating the event horizon would become smaller and smaller, until one particle might fill its own universe. At this point we might get a new big bang, and start the process all over. A new bunch of matter is ripped out of the vacuum, and temporarily puts a break on the expansion.

There are a number of possible scenarios like this that are being studied, that lead to either periodic or sporadic big-bangs. Here is one current theory:

Thus, modern science now presents us with the possibility of a universe infinite in both space and time. Although, it has not eliminated the possibility of the big-bang as an actual beginning of everything. The real answer, at this point is, "We don't know".

One of the key questions is how the density of the dark energy changes, as the universe expands. Answering this question will be a top goal of cosmologists, for at least the near future, and may help us narrow down the possibilities. This new way of looking at the cosmos will take some time to digest, and new theories will take some time to become established.
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