Size of the Universe

My laptop just bluescreened!! Haven't seen one of those in an age. And I lost my message So let's try again... No, expansion is not a velocity so you cannot compare it to the speed of light. Expansion is a measure of how much distance is gained over a set distance per time interval, e.g 10cm per kilometre per second. This has units of T^-1 and is clearly not a velocity. If we pick a distance of 10¹⁰km, then this distance is expanding at a rate of 10⁹m/s, which is indeed much larger than c, which is 10⁸m/s. BUT, just because they have the same units does not mean they are the same thing!This is all a consequence of curved space-time. Two object, situated 10¹⁰km apart, each consider themselves stationary with respect to their surroundings. Yet the distance between them is increasing at a rate of 10⁹m/s !! However, it would not be particularly sensible to describe them as moving away from each other at 10 x the speed of light. You cannot naively compare the relative velocity of two objects widely separated in a curved space.This is equally true on the surface of the Earth. Imagine two speed boats on the equator, one at 10^oE travelling east at 60knots, and one at 170^oW travelling at 60knots west. What is their relative velocity?Edited by cavediver, : No reason given.

No, for several reasons. Hoof Hearted's 156 billion is not correct, nor is your article, which doesn't help...We can only see back as far as recombination (about 400,000 years after the Big bang) as the the Universe is opaque before this time. We see photons today in the Cosmic Microwave Background Radiation that were emitted at recombination, and these define the observable edge of the Universe. This edge is currently about 46 billion light years away. Using observationally consistent values for the expansion model, we can estimate that the photons were emitted at a distance of around 36 *million* light years from us (or where we will be many eons later!)A photon received *today* from the edge of the Observable Universe, started out 36 million light years from us, and has taken 13.7 billion years to reach us because space has been expanding the whole time. This edge is *now* 46 billion light years away, but that is largely irrelevant to us.So the photons have travelled either 36 million light years, or 13.7 billion years, but not 46 billion light years, nor 76 billion light years, and most emphatically NOT 156 billion light years

In flat space, yes. In a curved expanding space, it will take much much longer, or quite possibly never actually be able to reach us. But as I explained above, the photons started out only 36 million light years away, and have taken 13.7 billion years to reach us because of the expansion slowing their prgress to us.

The photons in the CMBR are not from star-light. They are spontaneous emission from the plasma that was about to recombine. So they are indeed from 13.7 billion years ago.ABE first starlight would date to 13.6 billion years ago...Edited by cavediver, : No reason given.

quote:

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we live in a universe with topology scale smaller than 24 Gpc.

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You have to understand what this means. In my case, I was working on this particular area two decades ago, years before Neil started looking into it and talking to me about it Edited by cavediver, : No reason given.

Typical popular science screw-up. Neil is looking at potential topological compactifcation of the Univese (my first major research area), where the actual Universe is smaller than it appears and what we see in deep field images are possible multiple loops around the Universe. So we may actually be seeing ouselves at earlier and earlier times as we look deep into the Universe.Neil is continuing the work I was performing which is analysing our furthest data for signs of this repetition. When I was working on this, we only had the deep galaxy and quasar catalogues to work with - this was before COBE. Neil has COBE and WMAP observations of the CMBR is study. His results, like mine, do not show any sign of repetition but he can put a lower confidence bound on the size of the smallest repetition cell. This bound is the 24 Gpc quoted, which is the 78 billion light years quoted. This is not a radius. But confusion leads to thinking it is, so it is doubled to 156 billion light years!! It is also smaller (for obvious reasons!) than the observable Universe. If the size of the smallest cell is 78 billion light years, then the identified sides of the cell lie 39 billion light years away from us in opposite directions. The edge of the observable Universe *NOW* is 46 billion light years away. So the edge of any possible repeating cell is only just smaller than the observable Universe itself. So we are getting closer and closer to ruling out an observable multiply connected universe. Which is a shame.Pictures to follow...Edited by cavediver, : No reason given.

Sorry about the ascii pics but I hate uploading pictures and then have them vanish after a short while...Ok, our Observable Universe looks like this:

?  ?  ?  ?  ?  ?  ?

                          *********
                       ***         ***
                     **               **
                    *                   *
                   *                     *
                  *                       *
         ?        *                       *       ?
         ?        *           O           *       ?
         ?        *                       *       ?
                  *                       *
                   *                     *
                    *                   *
                     **               **
                       ***         ***
                          *********
                  
                    92 billion light years

The O is us Now, what I was looking for is evidence that the Universe is actually much smaller than this, and our sight wraps around the Universe multiple times, giving rise to this:

|     |     |     |     |     |
               |     |     |     |     |     |
               |     |    *********    |     |
            ---+-----+-***-+-----+-***-+-----+---
               |     **    |     |    **     |
               |    *|  O  |  O  |  O  |*    |
               |   * |     |     |     | *   |
            ---+--*--+-----+-----+-----+-*---+---
               |  *  |     |     |     |   * |
               |  *  |  O  |  O  |  O  |   * |
               |  *  |     |     |     |   * |
            ---+--*--+-----+-----+-----+--*--+---
               |   * |     |     |     |  *  |
               |    *|  O  |  O  |  O  | *   |
               |     **    |     |    **     |
            ---+-----+-***-+-----+-***-+-----+---
               |     |    *********    |     |
               |     |     |     |     |     |
               |     |     |     |     |     |

                           
                          Smallish

What we could see is multiple images of our own cluster of galaxies (and maybe the Milky Way istelf!) repeated in the night sky. This idea that the Universe could be wrapped around on itself is not that strange. In all of our higher dimensional theories, we consider the extra dimensions compactifed on some scale, such that they wrap up tightly. Thus, it is entirely reasonable to consider the possibility that our larg-scale dimensions are also compactified, just on a larger scale.I carried out statistical tests, looking for possible correlations in the coordinates of the farthest galaxies and quasars. To discover this would be fanatstic as it would enable us to *see* the evolution of galaxies and clusters, as we could see the same galaxy at different times. Sadly, we do not see any eidence of this in our deep field images of the Universe.Neil et al have examined the Cosmic Microwave Backgorund Radiation (CMBR) for possible correlations. It too is suggesting that there is no repetition, or at least, the size of the repeating cell is almost the size of the Observable Universe itself - depicted here:

|                 |
                     |                 |
                     |    *********    |
                     | ***         *** |
            ---------**---------------**---------
                    *|                 |*
                   * |                 | *
                  *  |                 |  *
                  *  |                 |  *
                  *  |        O        |  *
                  *  |                 |  *
                  *  |                 |  *
                   * |                 | *
                    *|                 |*
            ---------**---------------**---------
                     | ***         *** |
                     |     *********   |
                     |                 |
                     |                 |
                     
                        24Giga Parsecs
                              or
                    78 biliion light years

Hope this clears it up Edited by cavediver, : No reason given.

The discrete nature of the images is a function of the diameter of the basic "cell"-size of the Universe. The multiple images are created by light wrapping around the cell multiple times. So, light from the Sun comes straight to us in 8 minutes. Some of the light goes straight past us across the entire Universe to the cell edge*, wraps around the Universe and comes back towards us from the opposite far edge of the Universe and we see a second image of the Sun, only this time aged by the time it took the light ray to traverse the entire Universe. There will be another image where the light wraps around two times, three times, etc. But you need an integer number of wrappings around the Universe for each image.*there is no defined edge of course. If you think of us in the centre of the Universe, this is simply the point where you are equidistant from home in either direction.