zeppelin2k Posted July 2, 2009 Report Posted July 2, 2009 Some theories predict a "big crunch" at some point, as the universe continues to expand. But what I have never understood is, why should there be a period of contraction as the universe reaches it's maximum size? To me, a contraction seems the opposite of what should happen; as the matter in the universe is more thinly spread, the gravitational force between this matter weakens, and expansion should continue at a faster rate. So, why do we say that at some point it should contract? What force am i missing? What force would cause this (i assume i am wrong in thinking gravitational force plays a role). Quote
freeztar Posted July 2, 2009 Report Posted July 2, 2009 There are three main possibilities. The universe will expand forever, the universe will contract, or it will reach a point of stability and neither expand nor contract. Determining the correct scenario depends on finding the correct value for lambda, or the cosmological constant. It is currently believed that the universe does not have enough critical mass to cause a contraction and it will continue to expand indefinitely. Proponents of "Big Crunch" theories typically argue that we have not calculated the correct value for lambda. More technical info here: How does the cosmological constant affect the evolution of the Universe? Jay-qu 1 Quote
zeppelin2k Posted July 7, 2009 Author Report Posted July 7, 2009 Thank you for the response freeztar. I checked out the link you provided, a great source for the evolution of the universe. But maybe you can explain something you mentioned, about 'critical mass' of the universe. The concept makes sense, that beyond a certain mass there would be enough attraction to stop and reverse the expansion of the universe. What i don't understand, however, is if our universe was in fact beyond this critical mass, how could expansion be accelerating (in the positive direction) as it currently is? Big crunch is a theory, so someone believes in its validity, but to me it seems that if a 'crunch' were to occur, the expansion would be accelerating in the opposite direction (slowing expansion). Even if we have calculated the wrong value of lambda, observation shows that expansion is speeding up, not slowing down. Any ideas? Quote
freeztar Posted July 7, 2009 Report Posted July 7, 2009 Here's a link that describes critical density well:How was the critical density of the universe calculated? Currently, most scientists believe that the universe is open and it will continue to expand indefinitely. As for the acceleration of the expansion, it is a bit troubling. One possible explanation for this is a mysterious anti-gravitational force named dark energy. Quote
zeppelin2k Posted July 7, 2009 Author Report Posted July 7, 2009 It seems to me that the observation and the theory are incompatible, that the accelerating expansion (whether or not due to dark energy) rules out a big crunch. Correct me if i'm wrong... From your links, the relationship between critical density and shape/evolution of the universe is fairly clear, and with the observation that expansion is accelerating, I can see why most believe the universe to be open. So i suppose the real answer is, a big crunch is NOT a plausible fate of the universe, given the validity of modern observations? Quote
freeztar Posted July 7, 2009 Report Posted July 7, 2009 It does seem that way. An important next step is to identify what exactly dark energy is or at least how it has functioned through time. Some scientists believe that the dark energy will continue to increase, causing the expansion to become a run away event named the big rip. If the opposite is true and dark energy becomes a less abundant force in the future, it is possible for the expansion to slow down. Whether this could lead to an eventual big crunch depends on many factors, but it is still unlikely. The Planck Observatory, which is now in space and operational as of a few days ago, should give us a *much* more definitive answer about the geometry of the universe. It will improve of the results of WMAP as well as look for the first seed galaxies in the universe. It also could possibly provide evidence for a multiverse or a signature from before the big bang. Exciting times! Quote
zeppelin2k Posted July 8, 2009 Author Report Posted July 8, 2009 Definitely exciting times! I will try to follow Planck's progress, and I'm sure we'll hear from in the near future. Thank you for the help! Quote
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