# A question that I'm sure has a simple answer

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#### MannyPim

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One thing that has occurred to me which I ponder occasionally has to do with the acceleration of the expansion of the Universe.<br /><br />What we see is that the further out we look at distant objects in the Universe, teh faster they seem to be receeding from us as indicated by the doppler redshift.<br /><br />Now here is the question: Since we are not just looking far away but we are also looking far back in time, is it possible that the Universe was expanding at a faster rate in the past and has in fact been slowing down? <div class="Discussion_UserSignature"> <font size="2" color="#0000ff"><em>The only way to know what is possible is to attempt the impossible.</em></font> </div>

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#### SpeedFreek

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You are confusing 2 separate issues here, but don't worry, it's an easy mistake to make.<br /><br />Firstly, the universe was indeed expanding at a faster rate earlier on, and the expansion was slowing down. Until recently, that's all we knew.<br /><br />But let's take a moment to consider the expansion itself. The metric expansion of space is a process where the metric that defines distance changes over time. This means that any given unit will change by the same factor as any other given unit, over the same time period. This is not as straightforward as it might seem at first.<br /><br />For example, take a distance like 1 light year, and say it doubled to 2 light years in 5 billion years. That means that all distances would double in that same time period, so if 1 meter becomes 2 meters, 5 billion light years becomes 10 billion light years! If this happens over a period of 5 billion years, then we can see that at small scales, the expansion is very slow (it took 5 billion years for a distance of 1 meter to become a distance of 2 meters), but at large scales the expansion is at the speed of light (it took 5 billion years for a distance of 5 billion light years to become 10 billion light years).<br /><br />What this tells us is that, with a <b>constant</b> rate of expansion, the further away an object is, the faster is it receding from us. So this doesn't help us understand how the rate of expansion might have changed.<br /><br />When we plot the redshifts of the galaxies we find that the plot of distance versus age is not a straight line, but curves away sharply with our most distant observations, telling us that the expansion was faster earlier on, and was decelerating towards the present day. But we can only measure cosmological redshift (the type that shows the expansion of space, rather than the inertial movement of objects) for objects over a few billion light years away.<br /><br />With closer objects, their redshifts are dominated by the doppler shift caused by their i <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### MannyPim

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Greetings Speedfreek,<br /><br />Thank you for the explanation.<br />I'll have to think about it for a while.<br />Still not sure how the time factors into this.<br /><br />I'll be able to formulate better questions soon (I hope).<br /><br /> <div class="Discussion_UserSignature"> <font size="2" color="#0000ff"><em>The only way to know what is possible is to attempt the impossible.</em></font> </div>

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#### weeman

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Hmmm...I guess you could call that simple <img src="/images/icons/laugh.gif" /><br /><br />I never really thought of the expansion of space working like how you described. It indeed makes it sound as though the expansion is very, very slow! <br /><br /><font color="yellow"> What this tells us is that, with a constant rate of expansion, the further away an object is, the faster is it receding from us. So this doesn't help us understand how the rate of expansion might have changed. </font><br /><br />I've often had a hard time visualizing this. It seems to me that this is more of an optical illusion. It also seems to be somewhat paradoxical. Shouldn't it be, that if you were standing in one of these far off galaxies it wouldn't appear that you are traveling any faster than the Milky Way? It appears that distant galaxies are traveling faster because they've covered more distance in the same amount of time as nearby galaxies.<br /><br />1-2-3-4 <------ early universe<br /><br />1----2----3----4 <------ present universe<br /><br />Lets say that each number represents a galaxy. Galaxy 1 would argue that galaxy 4 moved away at a faster rate than galaxy 2 because it covered more distance in the same amount of time. However, according to galaxy 4, they moved at a slower velocity while galaxy 1 moved away at a faster rate.<br /><br />Am I understanding this at all? Do I just sound confused more than anything? <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>

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#### derekmcd

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It would be more like this:<br /><br />Galaxy 1's perspective.<br /><br />1-2--3----4--------5<br /><br />Galaxy 5's perspective.<br /><br />5-4--3----2--------1<br /><br /> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### SpeedFreek

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Yup, that's a good description. <img src="/images/icons/smile.gif" /><br /><br />I'm going to bring out my expanding ping-pong balls now!<br /><br />Imagine all empty space is filled with ping-pong balls touching each other, and they are each expanding <b>incredibly</b> slowly, all at the same rate.<br /><br />The centres of each ping-pong ball are ever so gradually getting more distant from the centre of the ball next to them. Now wherever you locate yourself, the further away the ball you look at, the faster the centre of that ball is moving away from you. At a certain distance you will come to a ball that has a centre moving away from you at the speed of light, and beyond that all the centres are moving away faster than light. <b>Wherever </b>you locate yourself, the balls next to you are expanding really, really slowly and the furthest are moving away superluminally, but nowhere is anything <i>actually</i> moving faster than light.<br /><br />There endeth the balls. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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