earthist Posted July 10, 2011 Report Posted July 10, 2011 When we see galaxies X billion light years away, we're seeing them as they were then. When we say that the further away they are, the faster they are receding from us, it seems to me we're saying that things were moving faster near the (the Big Bang) beginning of the universe than they are now. In other words, it appears that the universe was expanding faster then than it is now. That makes sense to me: things should be slowing down now; the energy from the original expansion should be dissipating over time. But astro-physicists talk about it as if the distant galaxies were receding faster now. They say that the further away things are, the faster they are receding from us. This looks rather paradoxical to me. So, what am I missing here? Quote
JMJones0424 Posted July 11, 2011 Report Posted July 11, 2011 The title of this thread is somewhat misleading, but I understand what you meant. A lightyear is a measure of distance, not time. However, since light travels at a finite speed, the farther back in space we look, the events we see are farther back in time. When we say that the further away they are, the faster they are receding from us, it seems to me we're saying that things were moving faster near the (the Big Bang) beginning of the universe than they are now. In other words, it appears that the universe was expanding faster then than it is now.Your conclusion does not follow. Imagine three objects, a,b, and c are located at x=1, 2, and 3 respectively at time = 0. The x axis expands at a fixed rate of 3 per unit of time, so that at t=1, a is at 2, b at 4, and c at 6. At t=2, a is at 3, b at 6, and c at 9. At t=10, a is at 11, b is at 22, and c is at 33. We can see that even though expansion rate was was fixed, the greater the distance between two objects, the faster the separation increases. The support for the accelerating expansion of the universe comes from observing the redshift and brightness of many different type 1a supernovae. Because these supernovae are so similar, we can determine their distance from their brightness, and we determine their relative velocity from the redshift of the spectral lines in their light. When we plot these, we see that nearby supernova are moving more quickly then they would be if expansion was fixed or decelerating, and distant supernova aren't moving fast enough.(Image is from the very useful hyperphysics site.) Quote
earthist Posted July 11, 2011 Author Report Posted July 11, 2011 Thanks, Sir, for your time. Your reply is not immediately obvious to me (x1,2,3 apparently become a,b,c; t2 of a becomes not 3 but 2 -- I need to contemplate a bit, and I will do so). I will consider it for a while and pursue the links you provided. I expect I'll be back for more help, though, and meanwhile, if anyone else has something to add, I'd welcome it. Quote
JMJones0424 Posted July 11, 2011 Report Posted July 11, 2011 (edited) No worries, I don't have a scanner at home, and obviously my attempt to verbally describe a table failed :) The table on the left is what I was trying to describe earlier, it is a counter-example to your notion that "things were moving faster near the (the Big Bang) beginning of the universe than they are now" because we observe far galaxies receding more quickly than near galaxies. My table shows that even with a fixed expansion rate of 3/unit of time, c moves away twice as fast from a than b does. The graph on the right is an exaggerated version of the Type 1a Supernovae plot in my first post. Line A is what we would expect to see if the expansion of the universe was constant, (like the table I provided.) Line B is what astronomers expected to see when they first started going over the data. Line B is the result of a gravity dominated universe, and in such a case, the only question would be if the initial expansion was great enough to forever overcome gravity, or if gravity would eventually cause a "Big Crunch". Line C is what they actually found, distant galaxies were receding slower than they would be if expansion were fixed or slowing, leading to the concept of dark energy, as it appears that some force is causing expansion to accelerate and overcome gravity's influence. Edited July 12, 2011 by JMJones0424 Quote
earthist Posted July 13, 2011 Author Report Posted July 13, 2011 Wow!! I preesheeayts it! I now get what you're trying to tell me (I think). I still need time to process it past my pre-conceived (probably ill-conceived) notions; ie: get it through my thick skull :lol: Can't thank you enough, Sir. Quote
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