|MadSci Network: Physics|
Hi Tom Sorry for the longish wait in replying to this. When I first started looking into the backgruond to this question I thought the answer was cut and dried. I have since found that this may not be the case. As you say, objects further away are seen to have greater velocities. The increase in velocity follows a linear relationship, that is, somewhere around 45 Km per second per Megaparsec. The exact value is not yet known but appears to be around this figure. But does this imply an acceleration? Oddly it does not. It does imply the universe is getting bigger but not necessarily that objects further away from us are accelerating. For this to be true it would imply that we occupy some privileged position in the Universe, that we are stationary and everyone else is accelerating away from us. For various reasons the concept of `privileged observors' is not allowed in Cosmology. The trick is to remember that we are part of the same Universal expansion as everything else and then see how this explains what we observe. The way to imagine how the Universe expands is to draw several dots on a piece of elastic. Now stretch the elastic. Points close to each other move apart from each other slowly. Points at each end of the elastic move apart very quickly. Now if you stretch the elastic at a constant speed there is no acceleration yet the points furthest from each other are seen to move apart faster than points closer to each other. If we know stretch (pun intended) this analogy to a ballon what do we see. Take a deflated ballon and draw a number of dots on the surface. Blow the ballon up. The same thing is seen, points close to each other move apart slowly, points far from each other appear to move apart quickly. Yet they are all moving at the same speed, not accelerating. Stretching the analogy further these points are Galaxies and other objects, the surface of the balloon is the 4 dimensional spacetime that is the Universe. It is worth remembering how things look to the objects we see moving at a fraction of light speeds at the limit of our telescopes observing power. From their perspective they are stationary and it is we who are moving at a large fraction of the speed of light. The above is basically the standard model used to explain the acceleration but I suspect you will still have a few obvious questions that I will try and answer now. If the Universe is expanding then at some point it must have expanded from a point. This is the basic argument that fuelled research into the Big Bang. The idea here is that before the Big bang NOTHING existed, not even a primordial black hole that exploded. No space, no matter, no time, no Universe, nothing existed. Then for some reason spacetime was created in an infintessimal volume, then expanded to something close to it's current size in a fraction of a millionth of second. The explanation for how this happens requires a few more chapters, please feel free to contact me if you are interested in the explanation. The byproduct of this sudden inflation is what we see as the current exapnsion of the universe. Depending on the mass (density in fact) of the universe this expansion could in fact be retarding, that is, slowing. This is a very active area of research where I do not believe a firm consensus has yet been reached. So in a way you are right to ask where did the source of the energy come from that drives the expansion, it came from the big bang and an effect called "spontaneous symmetry breaking of the higgs field". This drives an initial acceleration which when the energy is removed the Universe continues to expand at a constant velocity. I hope this answers the your question and raises a few more. Certainly it made me question some long held truths and made me re-examine what i thought I knew. If you want to know more I would suggest looking at , Ned Wrights Cosmology tutorial. It is worth investigating all of Neds pages for more information. John Baez's homepages Both of the above people are researchers in this area. I would also reccomend looking at The public pages of the Department of Applied Maths and Theoretical Physics at Cambridge" I would also reccomend reading some general level text books like "Black holes and Time Warps" by kip s. Thorne, "The New Physics" edited by Paul Davies.
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