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No need for complications - Simple clear steps

  1. Feb 12, 2014 #1
    Hi All,

    I am new to this website, I have read allot on this forum without subscribing previously.

    I have a simple question: if the universe age is around 13.7 B years and if it started from a singularity and expanded at the same rate in all direction, and C is the speed limit of anything. So can't we deduce that the maximum radius of the universe should be = distance = max speed x Universe Age (time) = (3x10^8 m/s) x 13.7 B x( 365 days x 24 hours x 60 minutes x 60 seconds) = 1.3 x 10^17 meters?

    or in other words we can say that the distance between the farthest 2 galaxies from each other is less than or equal : 2.6 x 10^17 meters?

    I have read other threads saying that there is no thing such as center of the universe, but I was not convinced, my view about this is that, we know that the visible universe is surrounded by dark matter and the dark matter is surrounded by dark energy, so for me it is very logical that the singularity is inside the observable universe ( not in the dark matter or the dark energy) and more precisely in its center.

    Thanks, looking forward for your replies!
  2. jcsd
  3. Feb 12, 2014 #2


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    Imagine a white balloon with red dots on it.
    The surface of this balloon has no boundary.
    When inflated, the balloon expands, and the distance between the dots increase.
    There is no expanding boundary, no boundary at all.

    This analogy shown that expansion does not imply a boundary.
    This is what happens to the expanding universe.
  4. Feb 12, 2014 #3
    Thanks for your reply maajdl, maybe I didn't communicated my question accurately. I am not suggesting a boundary for the universe, when I calculated the radius I meant as of today ( after 13.7 B years). so my proposition ( or question) is that today, after 13.7 B years of its creation ( or occurrence), the universe cannot have a diameter ( assuming it is spherical) mor ethan 2.6 x 10^17 meters). i.e the diameter is less than or equal 2.6x 10^17 meters.

    I hope my question is now mor eclear.

  5. Feb 12, 2014 #4


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    Hi Reptile,

    c times age of the universe gives you the radius of a static(non-expanding) universe only. That is, if the universe popped up into existence already infinite(or just very large) and then started to emit light, while neither expanding nor contracting.

    Since light from the farthest objects has to move through expanding space, the source of light emitted 13.7 billion years ago has already receeded to about 46-ish billion light-years.
    It's similar to walking on a rubber band that is being stretched. If you start 100 metres away from your target, and walk at 2m/s, while the rubber band under your feet is being stretched by 1% every second, it will take you more than just 100/2 seconds to get to your destination.

    Also, the rate of expansion isn't really constant, and varied throughout the history of the universe. If you make a forum search for that term, you'll find plenty of discussions about the nuts and bolts of it.

    Finally, dark matter and dark energy are completely different things that you imagine them to be. These are not somewhere outside the univese(and indeed, it's meaningless to talk about the outside of the universe), surrounding it, but are a part of the universe we live in. Dark matter is floating around just as the regular matter does, and dark energy is a property of space.
  6. Feb 12, 2014 #5
    Thanks Bandersnatch that was helpful. in this case how do we know that is the is some sort of a rubber band that is expanding the universe and giving extra speed to objects and lights and make them faster than the speed of light? how can we tell that this is really happening, and not that simply objects can go faster than C.

    in relation to dark matter, are these regular matter but we cannot see anymore because they in the part of the universe that is expanding faster than the speed of light?
  7. Feb 12, 2014 #6


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    As for dark matter, have you heard about neutrinos? Those very elusive particles that get emitted in nuclear reactions, in especially high quantities during supernovae explosions?
    Dark matter is similar to those, albeit even less prone to interaction with other matter. As of today, we know that it interacts gravitationally, and otherwise can pass through anything on its way. A neutrino could theoretically pass through a 100 light-years thick lead slab before interacting with anyting on its way. It appears that dark matter can pass through much more without interacting, but there are experiments set up here, on Earth, to detect it all the same.

    It has got nothing to do with being outside the observable universe. It's 'dark' precisely because it's so weakly interacting that it's been so far impossible to detect through any other means than analysing gravitational attraction it exerts on and within galaxies.

    And how do we know it's expansion? There's a number of reasons. Observations support c being the maximum allowable speed, and the theory of relativity builds upon it to produce a very predictive and robust framework for understanding the reality we live in. One of its predictions is that the geometry of the universe can change(it can expand, or contract), and further observations of redshift of distant object(and the discovery of background microwave radiation), as well as time lag in certain types of supernovae explosion duration all corraborate the model. Everything fits in nicely so far, which is a good indication that we've got the right idea about how it works.
    There's basically over half a century of cosmology behind it, and it would be difficult to cover it all in a single post, even if I had the knowledge to do so.

    Try perusing the FAQ in the cosmology section. The "Effort to get us all on the same page" sticky thread in the same section of the forum might be helpful, even though it has grown so large that it's daunting to approach.
    This article has got a more detailed analysis of the observational evidence for the expanding universe: http://arxiv.org/abs/1204.5493
    It's a bit technical at times, but you may be able to get the general idea.
    This article: http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
    has got a nice, layman-oriented, and very informative approach to explaining the expansion through the balloon analogy.
    This site: http://www.talkorigins.org/faqs/astronomy/bigbang.html also has got a good section on the evidence for the big bang as we understand it.
  8. Feb 12, 2014 #7
  9. Feb 12, 2014 #8

    There is a simple proof you can perform yourself to see why the universe has no center.

    draw on a piece of paper several circles, one within the other. Add as many contained circles as you like 2 will suffice. Draw lines at every 45 degrees. Where the circles cross the lines apply a coordinate letter assignment. Yo will quickly notice or should notice that two coordinates near the center of the circle will not increase in distance at the same rate as two coordinates near the outer circle.

    In fact two coordinates at the inner circle will increase in distance at a slower rate than two coordinates at the outer circle.

    In cosmology all coordinates separate from all other coordinates at the same rate. Much like coordinates drawn on the ( surface only) of a balloon.

    in terms of density, the inner regions will be more dense than the outer regions in the first example.

    in the Balloon analogy all points on the surface has the same density, only time varies the diameter of the balloon so as you go back in time the density increases.
    the first example has no easy means to describe the time factor so it would be confusing to describe lol. At least I can't think of a way in the first example lol.

    Thankfully that isn't important to understand how easy it would be to determine a center if there was one.
    Last edited: Feb 12, 2014
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