<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> </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>
A video made by me about space and the loneliness in the universe
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