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Size and age of the universe

  1. Feb 20, 2007 #1
    none of the forums seemed to be an exact match for this question, so i placed it in the most general forum.

    i dont understand why scientists think they have an accurate answer to these questions. i understand that they are using the doppler effect with regards to light waves.

    but according to their results the diameter of the universe is already more than double the speed of light, in light years.

    so it would seem that we would have to conclude that space itself is expanding faster than the speed of light. or at least was at some point in time.

    plus we know that the rate of this expansion is still increasing.

    and we would no doubt already have areas of space that are receding from us at faster than the speed of light, which means that we will never have access to those areas, assuming that we cant receive any information faster than the speed of light.

    so it seems to me that we may have very accurate numbers regarding the "observable universe", but we dont have even the slightest clue as to what percentage of this observable universe makes up the entire universe, and never will.
  2. jcsd
  3. Feb 20, 2007 #2


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    Without reading all you post, I suggest that you take note that scientists estimate the size of the visible universe, the word visible is always there.
  4. Feb 20, 2007 #3
    What up Physics Learner,

    You are for sure correct in saying that the Universe is expanding (which has been verified in support of various evidences) and that it is expanding faster than the speed of light, which is expected, according to General Theory of Relativity. Now, while you might very well be correct in stating that we can’t experimentally verify the diameter of the entire visible universe, we can extrapolate mathematics from current theories in conjunction with empirical observation to derive what we believe to be, a fairly accurate number (given the current parameters of technology and our understanding of physics and mathematics). We have peered back roughly, 13.6 billion light years into time, using the Hubble Deep Field Survey in accordance with a few other orbiting satellite telecsopes. I am not sure if you are having trouble visualizing this expansion or not, as your post seemed vague. However, if you are, perhaps I can describe a very simply analogy that might help you to understand. Keep in mind, this is not technical, merely a conceptual way to imagine this process.

    Imagine an infinite rubber sheet stretched to infinity, so that from your vantage point on the sheet, it extends into the horizon, in all directions. This can be visualized in similar fashion to being on a ship in the middle of the ocean, in all directions the ocean extends into where the sky meets the water. This is the fabric of the space-time background. Now, imagine that we place an infinite amount of people at fixed positions, so that when you look into the horizons, you see people distributed at fixed points. The people represent our galaxies. Now, we have constructed our very simple, ideal Universe.

    Imagine that the rubber sheet is pulled with equal force, evenly, in all directions. From your vantage point, you see the individuals at fixed points, recede away from you equally, as if they were moving. We continue to pull the infinite rubber sheet until everyone has receded away from you and from your vantage point, you appear at the center of the Universe. The center is an illusion and from each individual’s vantage point, they appear to be at the center of the universe, however, there is no center.

    We can continue to pull the infinite rubber sheet in all directions, causing a continuous recession between the individuals on fixed points. Theoretically, we could imagine this simulation run forever until everyone has receded infinitely away from one another. We have allowed for the movement of bodies faster than the speed of light, without violating the constant speed of light as the background itself expanded and everything ‘on it’, merely moved ‘with it’. If we add light into our thought simulation, we can see that as the rubber sheet expands, the light must also cover the distance of the expansion, whereas the bodies themselves, are fixed and move in reference to the background itself. This is how the background of space-time can expand faster than [tex]C[/tex] and why [tex]C[/tex] has to traverse this constant expansion and why bodies of matter do not.

    We can then, rewind this simulation, so that the infinite rubber sheet contracts continuously, until each individual is next to each other. We can continue to contract the infinite rubber sheet until each individual is stacked one on top of the other and this represents our 'cosmological singularity'. I am not sure whether or not there really was a 'singularity' as that seems physically impossible (in the sense that the singularity has some infinite property), however, it might help to visualize the process, I don't know.
    Last edited: Feb 20, 2007
  5. Feb 20, 2007 #4
    hi hootenanny,

    you mean like in this article -


    "visible" is mentioned rarely. people want to know how old the universe is and how big it is - not how old and big the visible part is to us.

    especially since the visible part could be only the tiniest fraction of it. how interesting is that ?
    Last edited: Feb 20, 2007
  6. Feb 20, 2007 #5
    hi complex,

    i do not disagree with anything that you said. and i suspect that our measurements could be accurate with regards to the "visible" universe.

    i was referring to the "entire" universe. i think we think we know a lot more than we do. LOL.
  7. Feb 20, 2007 #6
    Perhaps we should make a distinction about what defines the 'visible' universe and what constitutes the 'entire' universe, so that we can remain consistent, homie!

  8. Feb 20, 2007 #7
    i dont know that we can make any distinction. we know a lot about the "visible" universe. we dont know anything about whatever part of the universe is no longer visible to us. the entire universe is all that is in this universe. the visible part is that which we have access to, by means of light reaching us.
  9. Feb 20, 2007 #8
    Why do you presuppose the existence of a 'non-visible' universe and what are the contents of this region of the Universe? Quantum entities, strings, loops, particles?

    I understand that by 'visible' universe, you are referring to what we can directly observe by means of the electromagnetic spectrum (not necessarily just visible light, correct?), however, you assume the existence of some other aspect of the universe that we can not perceptually 'see' and treat it as more real than the region that we can see? As far as I know, a 'non-visible' universe exists only theoretically, to explain dark matter/dark energy and/or Quantum Mechanics, correct. I might be wrong about that though.
  10. Feb 20, 2007 #9


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  11. Feb 20, 2007 #10
    i certainly dont know anything for sure. but it seems likely that due to the rate at which space has already expanded, and that the rate is increasing, that most of the universe is expanding away from us at greater than C, so we can never have access to it again.

    why does that part of the universe have to be any different than our part ? i did not mean to imply that it was. in fact, i am thinking that it is no different. it consists of whatever we consist of. we just cant verify its existence with certainty, because it is expanding away at faster than C.

    but again, it seems likely - due to what we do know about the expansion of our visible universe. (yes, any sort em, not just light).
  12. Feb 20, 2007 #11
    thanks hoot,

    i know it is sometimes mentioned, but most of the time it is not. most people are interested in the entire universe, whatever that may be.

    if we were told that an infinitesimal portion of the universe is 156 billion light years in diameter and 14 billion years old (or whatever the latest numbers are), it would not be interesting. it would be like talking about the size of a grain of sand, but not knowing anything about the size of the beach that it lays on.
  13. Feb 20, 2007 #12
    i am having these same sorts of discussions with jesse over at relativity. from a personal desire, i want to know what is, not what we perceive things to be. i suspect that this is beyond our ability to ever do.
  14. Feb 20, 2007 #13
    The link provided by hootenanny, describes the 'visible' Universe as the region around the Earth, that has had enough time for light and radiation emitted from objects, to reach our vantage point, e.g. the surface of last scattering. It also describes the notion that at each distinct vantage point, a different 'visible' Universe emerges.

    So, what you are referring to then, is the ENTIRE universe, which we have no experimental verification of, other than indirectly. So, when we say the 'visible' universe is 90 billion light years in diameter (or whatever it is approximated at right now), we are only referring to objects that we can observe at the surface of last scattering.

    If this is logically sound, then we are correct in our approximations of the 'visible' universe as it only refers to this particular vantage point, unique to the Earth. The 'non-visible' universe you are referring to, would not be directly seen by us, even if we had the technology because there hasn't been sufficient time for the light to scatter.
  15. Feb 20, 2007 #14
    That would be very interesting indeed, though. If we said an infinitesimal portion of the universe was 156 billion light years in diameter and we had the ability to distinguish our portion as a 'grain of sand' buried in a 'beach', then we know that we live in an extremely large Universe. That is VERY interesting and has implications on our existence.

    The notion that our visible portion of the universe is approximated somewhere between 78 to 90 billion light years in diameter (those are the numbers I saw last), then we know our universe is already extremely large. Unimaginably large, in fact. This is interesting.

    The fact that we don't have access to the 'other portions of the universe' or the 'non-visible' portions, is irrelevant since all we can do is speculate that it even exists.
  16. Feb 20, 2007 #15
    ok if all thats true
    then there is far more mass outside our visible bit of the univerce
    then inside
    so could the mass outside pull us in the observed expansion
    without dark energy being the cause or part of the effect anyway
    could the so called dark matter be real matter too faraway to allow
    the light to reach us
    or how does real matter outside our vision limits effect the given percents of real matter vs dark matter and dark energy in the total univerce
  17. Feb 20, 2007 #16
    I am not sure I follow. I didn't make any truth statements regarding the validity of any theories, results, observations or experiments. The only number I quoted, was an approximation from memory regarding the diameter of the visible universe and the definition of 'visible universe'.

    What you quoted was my perspective regarding a hypothetical universe, in which we can transcend the barriers of perception and abstractly construct all of the axioms of our ideal universe. He was generating a universe, in which we know for certain, that we don't know about very much of our universe.

    Realistically, we are not certain if there exists anything beyond our visible range, other than what emerges mathematically or theoretically, although I am sure that will change very soon.

    I am not qualified enough to discuss dark matter and dark energy (nor is that my primary field of interest), so I will reserve that for someone else.
  18. Feb 20, 2007 #17
    hi complex,

    let me restate something, because i dont think you are grasping what i am intending to communicate to you.

    you said something about the light not having enough time to reach us.

    what i am saying is that this light will never ever reach us. we are running away from it at faster than the speed of light. so every second, that light keeps getting further and further away, even though it is travelling at the unbelievable speed of C.

    i say this seems likely because - from the article i posted, the diameter of the visible universe is 156 billion light years, while the age of it is only 14 billion years. space is expanding at greater than the speed of light, has been for 14 billion years, and the rate that matter is expanding from other matter is increasing. i didnt say that the distance from us is increasing, which it is. but i am EMPHATICALLY saying that even the rate is increasing. and that rate is some humongous number already, and has been for a very long time.

    so it seems very likely that there is a part of the universe which is hidden from us now, and always, unless we are unlucky to see it collapse. but you and i will be dead by a few billion years if that were to happen. LOL.

    so not only does it seem possible, it seems extremely likely that not only do we have a part of our universe outside the visible one, but that most of the entire universe may be outside of the visible one. and my example of a grain of sand may not be farfetched at all, when looking at all the other numbers.

    most people are interested in this type of stuff on some sort of spiritual level. we want to know what sort of world we live in, our purpose, etc. if in fact we only have access to a very small portion of our universe, it is startling to say the least. especially when we look at the astonishly, overwhelmingly size of the visible universe. LOL. it makes us look awfully insignificant.

    this is why i am interested in what is, and not what we perceive things to be.

    but unless we prove einstein wrong, and we have some way of getting info faster than C, we aint ever ever gonna know any more about what lies beyond the visible universe than we do currently - which is zero, today.

    that is disappointing to me, but i dont see any way around it - at least not in my lifetime. LOL.
  19. Feb 20, 2007 #18
    btw, i only saw the 7 physics forums when i made my original post. now i see the whole forum screen - very impressive. it reminds me of my college days when some of it made some sense - LOL.
  20. Feb 21, 2007 #19


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    Seeing objects receeding faster than the speed of light is not a problem - which includes all objects with a redshift [z] more than about 1.6. We routinely observe objects in visible wavelengths up to z~10. We also routinely detect photons in the microwave band that were emitted from the surface of last scattering [i.e., the CMB], which is receeding at z~1100. This is perfectly fine under the rules of GR. Take a look at:

    We can see as much of the universe now as we could ever have seen since one planck tick of time after the big bang - always have, and always will. Distant objects do not disappear from view, they merely redshift and time dilate into frozen obscurity.
  21. Feb 21, 2007 #20
    i looked at the site, but could not find it. is it in one of the faq's ?

    but it does not make sense to me that we could see light from a star moving away from us at faster than C.

    that does not mean that we cant see light from the star in the past, which is basically what we see when we look thru a distance. we are seeing light that was emitted sometime in the past.

    but the star, if it is still alive today, and moving away from us at faster than C, i dont think the light from that star emitted today will ever be seen by us.

    and if the universe is as large as i think it may be, then there would likely be stars that were newly formed in places where the space was already receding away from us at faster than C, and therefore never see the light from those stars, and have no idea whatsoever how large the entire universe actually is.

    in fact, it seems to me that if we could see light from any distance at any rate of expansion, there would be no need to use the term "visible universe".
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