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#### SpeedFreek

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<p>A lot of what you are asking boils down to a simple question - how straight is a straight line?</p><p>On the surface of the Earth, if you draw a long enough straight line you end up back where you started. This is because your straight line is being drawn on a curved surface. So what if the universe were "curved" in the same way?</p><p>On a local level we think of the ground as being flat, but if we examine that ground at a large enough scale we notice it is part of a curved surface and the same might be true of the universe. The surface of the Earth is two dimensional, but that surface is wrapped around a three dimensional object - a sphere.</p><p>But what if we take this principle and step it up a dimension? What if the three spacial dimensions of our universe were "wrapped" around a fourth dimension? If that fourth dimension was perfectly flat then any straight line in our three spacial dimensions would continue to infinity, but what if that fourth dimension had some curvature? If our three spacial dimensions were wrapped around a four dimensional sphere, then any straight line across our universe would actually be part of a large curve and there would never be an "edge" to the universe, just as there is no edge to the surface of the Earth. </p><p>So far astronomers have thought of a few methods to look for evidence of curvature and have found that the observable universe seems to be within 2% of being flat. That 2% represents a margin of error, but also allows us the possibility that our observable universe is a very small part of a curved universe, where the radius of that curvature is far larger than our observable universe.</p><p>The accelerating rate of expansion means that light will never be able to "circumnavigate" the whole universe, but there remains the possibility that the path light takes through the universe is ever-so-slightly curved and that if we were able to follow that path, we would end up coming back to the origin of that path from the opposite direction, rather like the way that pac-man exits the videogame screen on one side and enters on the other.</p><p>This is just one of the possibilities.</p><p>To the question of light being the fastest thing in the universe, a key point to remember is that <strong>nothing ever overtakes a photon</strong>. Distant galaxies may recede from this point in space faster than light, but in their local area they could consider themselves to be at rest, just as we do here. From their point of view it would be our Milky-Way galaxy, seen as it looked billions of years ago, that is receding from <em>them</em> faster than light, but we are not overtaking photons here! </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### brellis

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<p>hi Speedy</p><p>Once again, you explain the cosmic stuff so very well.&nbsp; Thanks! </p> <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>

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#### SpeedFreek

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I think Derek deserves more applause for his post than I do for mine, but thanks for the thanks! <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-embarassed.gif" border="0" alt="Embarassed" title="Embarassed" /> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### brellis

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yes, I agree, sorry Derek! <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>

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#### neilsox

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<p>
&nbsp; or race on an alternate planet elsewhere in the universe is at the centre of&nbsp;THEIR own Visible Universe.5) I've also been told that the visible Universe from our 'bubble' isn't special in any way shape or form. That its just like a horizon view on a ship at sea, that the person can only view a certain and equal</p><p>Unless the theory has changed, the Universe is receding from us at the speed of light, 13.8 billion&nbsp;light years away, so we can't know what is beyond, and won't know later as light coming our way from 13.8 billion light years will never arrive, unless the&nbsp; universe changes from expansion to big crunch = thought unlikely. There could, therefore be lots of universe beyond 13.7 light years in some directions and very little in other directions. We are likely 13,7 light years or more from an edge, but possibly not close to the center. In some geometries, the Universe has neither center nor edge, but I can't claim to understand that.&nbsp;&nbsp; Neil</p>

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#### SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Unless the theory has changed, the Universe is receding from us at the speed of light, 13.8 billion&nbsp;light years away, so we can't know what is beyond, and won't know later as light coming our way from 13.8 billion light years will never arrive, unless the&nbsp; universe changes from expansion to big crunch = thought unlikely. There could, therefore be lots of universe beyond 13.7 light years in some directions and very little in other directions. We are likely 13,7 light years or more from an edge, but possibly not close to the center. In some geometries, the Universe has neither center nor edge, but I can't claim to understand that.&nbsp;&nbsp; Neil <br /> Posted by neilsox</DIV></p><p>You almost got it right (current theory), but the horizon where on object is receding from us at the speed of light (the Hubble Distance) is not a barrier that stops us seeing any further. Consider that, to a galaxy just beyond that conceptual barrier, the space around it acts pretty much like the space around our own galaxy. Nothing is moving at the speed of light there, except for light itself and the light from that distant galaxy can easily start heading in our direction.</p><p>If the light from a distant object beyond the Hubble distance is close enough to that horizon, it will easily cross it and we will eventually see it.</p><p>The horizon you are looking for is the "cosmological event horizon". Here, any <span style="font-style:italic">light</span> that has been emitted in our direction is receding from us at the speed of light. The light itself is moving away at the speed of light and will never reach us, due to the acceleration of the expansion of the universe.</p><p>The cosmological event horizon is currently thought to be around 16 billion light years away and we will never see any event that happens <em>now</em>, if that event happens at a distance further than 16 billion light years away.</p><p>Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe</p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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