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| quote: | Originally posted by MisterOpus1
If by your 90% figure you're referring to that ambiguous dark matter stuff, I higly doubt it will be able to stop the escape velocity of matter expanding outward. Little is known about this substance, but all indications point toward infinite expansion regardless. It's just too strong for gravity to take hold, though I could be wrong (and I'm sure I am). |
Ah yes, well I can answer both your and Renegade's questions together quite convenientely. Yes I was referring to dark matter. And yes current indications are that the rate of expansion was actually increasing. Now from some lectures I attended a while ago, what's interesting is that the amount of dark matter in the universe is increasing as well. Some theorize that as the universe expands, this expansion creates more dark matter. What does that tell us? That dark matter, despite having mass, has attributes of self-repulsion! Actually, it is not so much the dark matter that exhibits attributes of self-repulsion but rather dark energy. Dark energy is a relatively new theory and in the past mass density alone would be the determinant of the fate of our universe. However, since the apparent acceleration of our universe is fueled by the role of dark energy, the fate of our universe has shifted to the role dark energy plays in controlling our acceleration. What is not known, is anything about the nature of dark energy ... whether it is something that decays or its equation of state changes, etc. Basically, could dark energy actually change its state and have properties of attraction in the future? Well, the article I was reading explains it much better:
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After the introduction of inflationary theory or the very early universe by Guth (9), many theoretical cosmologists became convinced that the universe must be flat and that the total energy density must equal the value (termed the critical value) that distinguishes a positively curved, closed universe from a negatively curved, open universe. Furthermore, noting how the evidence for dark matter was growing and extrapolating from the previous decade of study, the theoretical cosmologists became attracted to the beguiling simplicity of a universe in which virtually all of the energy density consists of some form of matter, roughly 4% being the ordinary matter and 96% the dark matter. In fact, observational studies were never compliant to this vision. Although there was a wide dispersion in total mass density estimates, there never developed any convincing evidence that there was sufficient matter to reach the critical value. The discrepancy between observation and the favored theoretical model became increasingly sharp.
Finally, dark energy came to the rescue (10). The only thing dark energy has in common with dark matter is that both components neither emit nor absorb light. In all other respects, they are different. Microphysically, they are composed of different constituents. Most significantly, dark matter, like ordinary matter, is gravitationally self-attractive and clusters with ordinary matter to form galaxies. Dark energy is gravitationally self-repulsive and remains nearly uniformly spread throughout the universe. Hence, a census of the energy contained in all the galaxies would miss almost all of the dark energy. So, by positing the existence of a dark energy component, it became possible to account for the 70-80% discrepancy between the measured mass density and the critical energy density predicted by inflation (11, 12, 13, 14). But the dark energy dominated models make a strong prediction – that the universe is currently accelerating, due to the gravitational self-repulsion of the dominant dark energy component. This ran contrary to the then-current best observational tests based on the brightness of distant supernovae. Then, two independent groups (15, 16) found evidence of the acceleration from observations of supernovae, and the model with a dominant dark energy component became the concordance model of cosmology.
Dark energy has changed our view of the role of dark matter in the universe and our vocabulary for describing the cosmological possibilities. If this paper had been written a decade ago, before any serious consideration of dark energy, the focus would have been on the mass density. According to Einstein’s general theory of relativity, in a universe composed only of matter (particles and radiation), it is the mass density that determines the geometry, the past history and the future evolution of the universe. For example, if the mass density exceeds the critical value, the self-gravity of the matter would cause the current expansion to eventually halt and reverse and, also, space would be positively curved. If the mass density is right at the critical value, space is flat (Euclidean) and the universe expands forever. Hence, the structure and fate of the universe would rest on the value of the ordinary plus dark matter density. With the addition of a new component, the story is totally different. First, what determines the geometry of the universe is whether the total energy density equals the critical value, where now we add to the mass contribution (identifying its energy according to E=mc2) the dark energy contribution. Second, the period of matter domination has given way to dark energy domination. So, the important cosmological role of dark matter is in the past when it was the dominant contribution to the energy density, roughly the first few billion years. Our future is determined by the nature of the dark energy, which is sufficient to cause the current expansion of the universe to accelerate, and the acceleration will continue unless the dark energy should decay or change its equation of state.
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http://feynman.princeton.edu/~steinh/osdark.pdf
Therefore, when I say that we don't even know what 90% or whatever of the mass of the universe even IS, (or more importantly the nature of the component causing its acceleartion, aka dark energy) I'm saying that we cannot come to an absolute conclusion that the big crunch/bang theory is completely out the window.
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Retro ...
Last edited by occrider on Sep-17-2003 at 21:00
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