# The acceleration of the expansion of the universe

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#### jimg44

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<p>I have heard that by studying the redshift of the light of galaxies and other distant objects, astronomers have determined that, not only is the universe expanding, but the rate of expansion is accelerating. That the farther the galaxy is from us, the faster it is moving away from us.</p><p>But as we look at galaxies that are farther and farther away, aren't we also looking farther and farther back in time? So if&nbsp; they are moving faster and faster the more we go back in time, how is that acceleration?&nbsp; It sounds like deceleration to me. </p><p>If the expansion of the universe was accelerating over time, it seems to me that that objects closer to us would appear to be moving faster than objects more distant. </p><p>Why isn't this the case?</p><p>&nbsp;EDIT: I just noticed the thread about redshift, galaxies and the expanding universe. Don't know how I missed that. Maybe I should read that first.</p>

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#### MeteorWayne

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I have heard that by studying the redshift of the light of galaxies and other distant objects, astronomers have determined that, not only is the universe expanding, but the rate of expansion is accelerating. That the farther the galaxy is from us, the faster it is moving away from us.But as we look at galaxies that are farther and farther away, aren't we also looking farther and farther back in time? So if&nbsp; they are moving faster and faster the more we go back in time, how is that acceleration?&nbsp; It sounds like deceleration to me. If the expansion of the universe was accelerating over time, it seems to me that that objects closer to us would appear to be moving faster than objects more distant. Why isn't this the case?&nbsp; <br />Posted by jimg44</DIV><br /><br />Well, it's a little more subtle than that. In a nutshell, the galaxies far away (back in time) are moving away from us just a very small bit different than they should be. The acceleration of the expansion is very small, it's basically a small change in the slope of the distance/recession speed line. (Hubble's constant) <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I have heard that by studying the redshift of the light of galaxies and other distant objects, astronomers have determined that, not only is the universe expanding, but the rate of expansion is accelerating. That the farther the galaxy is from us, the faster it is moving away from us. <br /> Posted by jimg44</DIV></p><p>Both your sentences are quite correct, but they aren't related to each other in the way you think they are. In a universe that were expanding at a <strong><em>constant speed</em></strong>, the further away a galaxy is, the faster it would be receding. This is due to the nature of expansion being a cumulative effect. Think of it like this... if 1 light year were to double in size over a given period of time, then all lightyears would double in size over that time.</p><p>If, over 1 billion years, a universe were to expand to double its original size, then an object that was 1 million light years away would recede to 2 million light years, and an object that was 1 billion light years away would recede to 2 billion light years.</p><p>So, the closer object has receded at 1 million light-years per billion years, while the further object has receded by 1 billion light years per billion years. The further object has receded a lot faster than the closer one did.</p><p>And that is with a constant rate of expansion. But, as you say, if the rate of expansion is accelerating, then closer objects would be receding at an even faster rate when compared to more distant objects than they would be with a constant rate of expansion.</p><p>For a long time, our measurements of the redshifts of different galaxies light gave us an overall picture of expansion that was decelerating. The galaxies light was being "stretched" by the expansion of the universe and the longer the light had been travelling the more it had been stretched. When we compared distant objects to closer ones we found that the rate seemed to have been decelerating throughout time. But we can only use redshift as an indicator of cosmic expansion over relatively large distances - the closer we look, the more an objects redshift is "contaminated" by its actual inertial motion relative to us.</p><p>This is because there is more than one cause of redshift. As galaxies swirl around each other, due to their gravitational attraction, their light is subject to Doppler effect, in the same way as the sound of a train changes as it approaches or recedes from you. Our closest neighbouring galaxy, Andromeda, is blue-shifted, as it is emitting its light towards us as it is also moving towards us.</p><p>All the galaxies close around us show differing redshifts indicating differing directions of motions. The galaxies are clustering together due to their gravity. But over a certain distance we only measure redshift for galaxies and the amount of redshift increases with distance.</p><p>The upshot of all this is that as we look at objects that are closer to us in time, redshift is less effective as an indicator of the amount an object has receded. We had a picture of the early universe having a fast rate of expansion and that the expansion was decreasing until around 6 billion years ago, and we had assumed that the expansion would have continued to decelerate.</p><p>Then, we noticed something. A certain type of supernova, known as an SN1a or type 1a, has been observed to always burn for the same length of time at a given distance. The distance is determined by its brightness or magnitude. The expansion of the universe stretches the light from these supernova so that we can determine their distance from us by their duration - an SN1a with a duration of 10 days is twice as close as an SN1a with a duration of 20 days. These supernovae are known by astronomers as "standard candles" and they are useful for "calibrating" our observations.</p><p>Well, we noticed that when we compared some very close (in cosmological terms) type1a supernovae with more distant ones, the closer supernovae were dimmer than they should be, for their duration. The closer supernovae were further away than they should have been, if the rate of expansion had continued to decelerate as we had assumed it would. In the end, we found out that the rate of expansion had slowed until around 6 billion years ago, where it had levelled out and then started to accelerate.</p><p>Sorry for the rambling post, I hope you get something from it. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" /> </p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### MeteorWayne

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Posted by SpeedFreek</DIV><br /><br />You said it much better than I did. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>A certain type of supernova, known as an SN1a or type 1a, has been observed to always burn for the same length of time at a given distance. The distance is determined by its brightness or magnitude. The expansion of the universe stretches the light from these supernova so that we can determine their distance from us by their duration - an SN1a with a duration of 10 days is twice as close as an SN1a with a duration of 20 days. These supernovae are known by astronomers as "standard candles" and they are useful for "calibrating" our observations.Well, we noticed that when we compared some very close (in cosmological terms) type1a supernovae with more distant ones, the closer supernovae were dimmer than they should be, for their duration. The closer supernovae were further away than they should have been, if the rate of expansion had continued to decelerate as we had assumed it would. In the end, we found out that the rate of expansion had slowed until around 6 billion years ago, where it had levelled out and then started to accelerate.Sorry for the rambling post, I hope you get something from it. &nbsp; <br /> Posted by SpeedFreek</DIV></p><p>I don't think the duration of a type Ia supernova has anything to do with it.&nbsp; Generally speaking, they all explode with the same energy and follow a predictable light curve.&nbsp; In other words, they explode, achieve a peak luminosity and fade at the same rate.&nbsp; By measuring their luminosity, we can judge how old the supernova is.&nbsp; Knowing that their ages and luminosities follow a predictable curve, they can be used as a standard calibration tool by judging their apparent magnitude and comparing it to expected redshift based on distance.</p><p>Knowing the age of the supernova, we know what spectral emissions to expect to be recieved by our instruments and compare that to what is actually recieved... this is how the redshift is determined.&nbsp; With redshift and being aware of the Hubble parameter, you can accurately judge distance.&nbsp; &nbsp;</p><p>It was found that the apparent magnitude of more distance supernova were dimmer than what the redshifts indicated.</p> <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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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I don't think the duration of a type Ia supernova has anything to do with it.&nbsp; Generally speaking, they all explode with the same energy and follow a predictable light curve.<br /> Posted by derekmcd</DIV></p><p>Yes, you are correct here and I am glad you pointed it out as I can get a little carried away when composing these posts and forget vital points like that. I was adding the observed time-dilation for the Type 1a Supernovae into the mix, which does indeed increase in a manner consistent with their redshift. I tend to think in terms of cosmic scale factor and was using the concept where we observe a "standard candle" supernova over a longer duration than it was emitted for, as a way to illustrate the increase in scale factor.</p><p>As you say, it was the dimness when compared to their redshift that told us that the rate of expansion was accelerating, and I did actually say that, although I said it was their dimness compared to their duration, rather than their redshift. Seeing as their apparent duration increases alongside their redshift, I wasn't actually wrong as such, but I was certainly putting it the wrong way in the context of the story I was telling. I was trying to put too much into it (I never know when to stop, as one subject leads to another!), and I failed to emphasise that it is the absolute magnitude or brightness of a SN1a that is always the same and the amount we see it dimmed by tells us how far away it is.</p><p>I simply cannot resist mentioning the observed time-dilation of these supernovae too, you see, as that is the part that, to me, really drives home the realisation of the expansion of the universe for someone. When we look at a distant galaxy or star, over a period of time, we only really have spectral analysis, apparent magnitude and angular diameter to work with.</p><p>The dimness of an object is a measure of it's distance. That's easy to understand but to calibrate that dimness we need something to compare it to.&nbsp;</p><p>The wavelength of the light from a distant object is increased by the expansion of the universe as that light makes its journey towards us, so that absorption lines within its spectrum are moved towards the red end and this allows us to measure the increase in the scale factor of the universe during that journey. It makes sense but it's kind of abstract as a comparison for the layman.<br /> </p><p>But if I emphasise that we see these supernovae burn with the same brightness and the same duration too, which we do at a given redshift, and then we measure more distant supernovae that we assume actually burned for the same duration, we find that the duration over which we observe them is stretched too. We are watching the event happen in slow motion, as the expansion of the universe has also expanded the apparent duration of those events as their light made its journey towards us. With the stretching of an objects light we find redshift due to that stretching and we find time-dilation too, so we are actually seeing that object in slow motion, but we can only really see it with standard candles.</p><p>Sorry if I misled anybody, I'm getting forgetful but hopefully I'm not putting too many misconceptions into peoples heads!&nbsp;</p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I was adding the observed time-dilation for the Type 1a Supernovae into the mix, which does indeed increase in a manner consistent with their redshift.<br /> Posted by SpeedFreek</DIV><br /></p><p>Thanks for clarifying that!!&nbsp; </p><p>I completely missed the correlation you were making.&nbsp; I guess I'm just not used to seeing it put that way.&nbsp; It's definitely not the standard way of explaining it, but you are completely correct.&nbsp;</p> <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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#### jimg44

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<p>Thank you for your information!</p><p>&nbsp;I am so happy to have found this forum, you have now answered two questions that have kept me up many nights! </p>

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