- #36
procrastin
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cepheid said:It sounds like you're talking about the so-called "big-bounce" scenario. For what level of schooling are you creating this presentation?
the presentation is over now, but it was for my first year university physics program. i was reffering to the big bounce, indeed, just didn't know the name.
The equations that govern the expansion are not those of a simple harmonic oscillator, although that's not a bad guess. I'm not sure what your math background is, but you might consider looking up the Friedmann equations in order to learn more. Very roughly speaking, these equations describe how the 'scale' of the universe evolves with time. One of the implications of the Friedman equations is that this evolution depends upon the different components that contribute to the energy density of the universe, such as matter, radiation, etc. The component that seems to be dominant at this point in time is something whose nature is not understood at all. It has simply come to be called, The Dark Energy. More on that below.
That depends. The force that wants to slow down the expansion is just gravity. Therefore, the ultimate fate of the universe depends (partly) upon how much matter is present within it. This dependence is a direct result of General Relativity (which is where the Friedmann equations come from in the first place). It is simple to explain what the ultimate fates of universes without dark energy would be. There are three possibilities. If the density of matter in such a universe is less than some critical value, then that universe will continue to expand forever, and the expansion rate, although decreasing, will always be positive. If the density of matter happens to be exactly equal to the critical density, then that universe will also continue to expand forever and will once again be decelerating with its rate never quite reaching zero (but approaching it asymptotically). If the density of matter in the universe is greater than the critical value, then the expansion rate will decrease until it becomes zero, and then negative. In other words, the expansion will slow down, stop, and then reverse. The universe will begin to recollapse. This third possibility is essentially the same as the scenario you outlined, with one key difference. Without dark energy, there is no 'bouncing back.' Everything collapses down to a singularity and that's it. This is the so-called 'big crunch.'
I should emphasize that all of these scenarios have been ruled out by observations, which strongly favour the presence of dark energy. The dark energy makes it a bit more complicated to sort out what the ultimate fate of the universe will be (as compared to the neatness of the three cases presented above). However, in all reasonable models that include dark energy, the universe will continue to expand forever, and the rate of that expansion just gets faster and faster (i.e. the expansion is accelerating).
thanks for that, i learned a lot and this improved my research.
i just want to add that in the link i provided, suggests that dark energy is replaced with "single energy field". quote from the link;
"The new model replaces inflation and dark energy with a single energy field that oscillates in such a way as to sometimes cause expansion and sometimes cause stagnation. At the same time, it continues to explain all the currently observed phenomena of the cosmos in the same detail as the big bang theory."
does it not mean that the dark energy problem is solved? I'm finding it difficult to understand the quoted statement as i don't really know what is being talked about, so if u could please elaborate on this point, it would probably improve my understanding by miles.
thanks again xxx