Big Bang Origin?

correct No, redshift only reveals velocity of recession, it does not reveal acceleration/deceleration. In a conventional big bang scenario this is true but the observed acceleration contradicts this. No, this conclusion is not compatable with the evidence. The acceleration appears real. Not necessarily. Slow down occurs in all three original big bang scenarios: closed, flat and open. Only the closed re-collapses. Correct. Just to add, Dark Energy is the name given to the driving agent of the acceleration. The mystery is not why is there dark energy, but why is there so little dark energy. We have long predicted its existence, though some of the predicted values have been a little out (~10^100 times out in some cases !!!)

Hi Trékuhrid Motion of the stars is not related to the expansion of the universe. The stars move within the Galaxy, both orbiting the centre and having their own proper motions. Even nearby galaxies do not reveal the expansion. Our big sister galaxy, Andromeda, is actually on a collison course with us! You have to look much further afield at the distant galaxies to see the overall expansion. We can't do this because our errors in measuring the distance are many times greater than the actual distance a galaxy will move in a human lifetime.We measure the redshift in the galaxys' spectra. This reveals their recessional velocity and in turn we can determine their distance. This is cross-checked with other distance measuring methodologies for closer galaxies. This is a common viewpoint but based upon a very large misconception. The expansion of the universe is not what tells us there was a big bang. The mathematics of General Relativity tells us that the universe "began" in a Big Bang, and so we should see that the universe is expanding. We look, and it is, so the expansion is observational evidence for the Big Bang. So the explanation for the Big Bang is very simple: General Relativity tells us that it happened. The Big Bang has exactly the same explanation as why things "fall" towards the Earth, why the Moon orbits the Earth, etc. It is all part of the same theory of gravitation.Check out my other posts in this forum for more details on what the Big Bang is actually all about.

Very big questions for New Year's Eve!GR is Einstein's masterpiece. Initially he developed Special Relativity, which is the theory that explains the interconnections between space and time, and how that affects what we call motion. Here the ideas of 4 dimensional space-time, the constancy of the speed of light, time dilation, length contraction, etc are all introduced. It enfoced the role of (4d) geometry in physics. To say that SR revolutionised the way we think about space and time is the understatement of the year.SR was published in 1905, and Einstein spent the next 10 years developing GR, which was the incorporation of our concept of gravity into SR. He was sparked by the thought that if you were trapped in a lift, it was impossible for you to determine whether the lift was sat motionless on the ground or was accelerating through empty space at 1g. Both gave exactly the same sensation inside the lift. Acceleration had already been reduced to a geometrical concept in SR, so he reasoned that gravity was not some mysterious Newtonian force, but another geometrical concept within 4d space-time. Gravity became gravitation, the curvature of the 4d space-time that had remained uncurved or flat in SR.His final equation (the Einstein Equation) G=8(pi)T takes a mass distribution and spits out the shape of space-time around that mass, which determines all associated gravity and motion.If you stick in a spherical mass, you get something called the Swcharzschild solution - this gives the space-time around the earth or the Sun (approximating them as spherical and non-rotating - we can correct for these but it gets messy). This space-time gives all of the predictions of Newtonian gravity plus it explains the anomalies that Newtonian gravity could not explain. If the mass is sufficiently dense (restricted to a small enough sphere) then this same solution predicts what we now call a black hole (so a black hole, like the big bang, is a result of the mathematics... not just some fancy physics idea!).Now, if we put in an infinte uniform mass distribution (like dust scattered through empty space) we get a different solution. The idea is that this distribution mimics the universe on the largest scale. This solution describes space-time as a finite hyper-sphere that starts with zero size (and hence infinite density), expands to maximum, and then contracts back to zero size.When Einstein saw this he was most perturbed. In the early 1900s, the universe was considered fixed and static. Einstein introduced a fudge factor to stop this expansion and contraction, and made this universe balance at a fixed size. He was later to call this his greatest mistake, because it wasn't long before Hubble discovered that the Universe was actually expanding!And thus the big bang theory was born, the big bang itself simply being this initial infinte density point, but the theory governing the entire universe, past and future. Notice there is no concept of explosion in space or any other such popular nonsense. The universe simply expands from zero size.Why should we believe this? Can we trust GR? This assumption of a uniform distribution of matter doesn't look very much like our universe!Well, GR is remarkably well tested. It predicts many strange and bizarre phenomena, many of which have been observed and shown to agree perfectly with the predictions of GR. We have also examined literally hundreds of other possible theories of gravity and none come close to GR (other than a few which are so similar to GR, they still predict a big bang).The uniform assumption was always a worry, but we do notice that the larger the scale we look at matter in the universe, the more uniform it appears. The final proof of this was the observation of the cosmic microwave background, which is more than uniform enough to convince us.What about evidence for the big bang? Well, I have work to do, so go check out the Wikipedia entry and you will see all of the evidence laid out.I hope this helps a little.This message has been edited by cavediver, 12-31-2005 04:37 AMThis message has been edited by cavediver, 12-31-2005 04:40 AM

No, it most certainly is not an assumption. It is a prediction of General Relativity. That everything appears to be flying apart from each other is evidence for this prediction. No, they are causing the slow down and eventual re-collapse (in the original closed Big Bang model). The initial outward expansion is effectively a negative energy phenomenon to balance the positive energy of the matter (the "dust"). This is what we mean when we say the universe has zero energy. You are most welcome. Keep asking. This stuff is not intuitive and has little in common with everyday experience. It is difficult enough for scientists that don't happen to work in this particular field.

I don't blame you, but it's not surprising. I told you that you were asking deep questions, and you have pushed beyond Wikipedia's ranbge of utility. I would go further and say that Wikipedia's entry is BS, but I'm not that kind of guy It is because of dubious entries like this in popular science that I am employed (on an incredibly part-time basis) to help edit the UKs best selling dictionary of science on matters of astrophysics, cosmology, quantum theory, particle physics and fundemental theoretical physics. In other words... trust me and not Wikipedia This message has been edited by cavediver, 01-03-2006 04:32 AM

You'll appreciate them all the more when you see my reply to Crash over in "When a tree falls..." in defense of you Oh no, quite the opposite. It's just teaching from the ground up. There's no need to overcomplicate things. I have said many times that the way to learn this stuff is to understand the original closed big bang model in all its 4d wonder. You are then reasy to appreciate all of the other non-finite models. It's right on the edge. Recent measurements show it to be possibly just over critical density, but I would not be surprised if that changes. Inflation drives the universe towards Criticality from either side. But the main reason we will not recollapse is the observed acceleration of the expansion driven by a cosmological constant type device.

No, only that it looks very very flat. Which is what we expect from inflation. The question is, is it very very flat because:1) it is closed with staggeringly large radius2) it is open with vanishingly small curvature3) it is actually perfectly flat1) and 2) and what we expect from inflation, starting with a closed and an open universe respectively. 3) is the peculiar one.The flatness of the universe is a prediction of inflation confirmed by observation.

Yes, it absolutely does, and no, it is not a different thing at all. Plug in the mass distribution into GR and it tells you the shape of space-time. That is what it does. The singularity at t=0 is just the shape of the universe at that point. To put it simply, GR predicts that the universe is spherical (hyper-spherical) with a radius R and it shows us that R ranges from 0 at t=0 (the initial singularity) to some maximum value at t=0.5 and then reduces back to 0 (the final singularity) at t=1. When you see the R=0 at t=0 prediction, you say "wow, R=0 implies that there is an infinite density point at t=0"This message has been edited by cavediver, 01-04-2006 05:32 AM