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How can we calculate universe diameter at a given time

  1. Mar 21, 2015 #1
    How can we calculate universe dimater at a given time ?
  2. jcsd
  3. Mar 21, 2015 #2


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    The universe is not known to HAVE a diameter so it would be tough to calculate.

    The observable universe has a diameter, currently about 90+ billion light years, centered on your left eyeball (when you have your right eye closed).
  4. Mar 21, 2015 #3
  5. Mar 21, 2015 #4


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    You need to be more specific. I'm not going to read that entire web site to try to figure out what your question is.
  6. Mar 21, 2015 #5
    I have already gave the spesific part. In bottom theres a table and theres horizon size How they calculate it ?
  7. Mar 21, 2015 #6


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    No, you had not given a specific at all, but now you have, although your statement "In bottom theres a table" was very misleading since the table is in the middle of the long page, not at the bottom.

    It is a reasonable question, but I don't know specifically how how they calculated those horizons. I can tell you in general that many of the calculations after what they call decoupling were based on estimates of the age of the universe derived from red-shift and other information and extrapolated back from the known age of the universe.
  8. Mar 21, 2015 #7


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  9. Mar 21, 2015 #8


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    The horizon size is a function of the combination of the speed of light with the expansion history.

    If we wait forever, a photon that leaves the Earth today will only barely reach the matter that is currently at our cosmological horizon. We can never communicate with any matter that is currently further than this.
  10. Mar 21, 2015 #9

    Thanks for the link, Quarlep; I have been searching various sources for WEEKS for a tabular chart showing this information. Although there ARE a few gaps and I am confused by some of the data, it is a big help and I appreciate your sharing.

  11. Mar 21, 2015 #10


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    That only accounts for the size of the observable universe. It does not take into account the possibility the universe may have always been spatially infinite, nor that it may be much larger than its particle horizon.
  12. Mar 21, 2015 #11


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    Quarlep, do you know what the Hubble radius is? Would you be satisfied to know how to calculate that distance, at any given time.
    It is sometimes referred to a a cosmic "horizon", so maybe you have been reading about the Hubble radius and thinking of it as a measure of size of universe. I think in that "Yuki" UCBerkeley material you linked to, the Hubble radius was at one point called the horizon.

    The Hubble radius increases over time and gradually converges to a distance beyond which signals we might send cannot reach and from beyond which galaxies' light cannot reach us.

    The Hubble radius at any one given time is the size of distances which are increasing at speed of light. So if a galaxy is today at that distance and sends us a flash of light, that flash would at least for the time being not make any progress. Because the distance it would have to travel would be growing at the same speed as the light was advancing.

    At present that R is 14.4 billion light years. It has been growing throughout history and will continue as far as we know.

    It can be calculated for any given year of universe time--is that what you are asking about?

    I don't think anyone knows the actual diameter of the universe, it might not have a diameter because it is infinite. That is now well-defined and nobody bothers with it. When they use words like radius and diameter they are usually talking about stuff like the Hubble radius, i.e. horizon type stuff.
  13. Mar 22, 2015 #12
    Thanks for adding to the conversation, my friends; I found your comments very helpful.

    I have a few questions, but I think as a preface, I need to define my views regarding a few concepts concerning the diameter of the universe:

    Subjective View # 1: I don’t accept ANYTHING as being “infinite.” Maybe I’m just being anal retentive (I’ve been called that more than once!), but I find the concept of an infinite universe as being unacceptable; I like “order” in my world, and an infinite cosmos (to ME, anyway) flies in the face of a clearly defined universe.

    Subjective View # 2: I understand that there is a difference between the “observable” universe and the (as yet) “UN-observable” (beyond our light cone) universe.

    Subjective View # 3: I do not accept the parallel universes concept (Everett’s many world’s interpretation of quantum physics), the term meaning that there are an infinite number of side-by-side universes with carbon copies of me in them, differing only in minor details (i.e. occupation, hair color, etc, ad infinitum). To me, hat’s just too “messy,” with a vast overabundance of realities!

    Subjective View # 4: HOWEVER, I am a big believer in the Multiverse, each universe differing in physical properties and constants, mainly because this neatly solves the anthropic problem of why our universe is so “finely tuned for intelligent life.”

    Now, on to my questions:

    Question # 1: On Yuki’s excellent “How has the universe evolved” chart, his column # 5 is labelled “Horizon Size.” I am assuming that this is the diameter of the OBSERVABLE universe. Am I correct in this assumption?

    Question # 2: In the “Horizon Size” column, diameters are given in meters and light years… with one exception: Row 5 (End of Inflation), he gives the diameter simply as “1” with no units. “1” what?

    Question # 3: Assuming Yuki’s chart gives the diameter of the OBSERVABLE universe. Is there a chart, graph, or simple formula that gives the diameter of the UNobservable universe, as well? The reason I ask this question is that I usually think of the diameter of the universe after inflation in the commonly referred to terms as “being 10 centimeters in diameter, about the size of a grapefruit.” However, I recently viewed a YouTube video by a practicing astrophysicist as giving the diameter of the universe after inflation as being .16 light years in diameter; B-I-G difference. So I am assuming this latter figure is for the UNobservable universe; am I correct in my assumption?

    Question # 4: Regarding the Multiverse… We have the OBSERVABLE universe, and the UN-OBSERVABLE universe (which is always larger). So where would the bubbles of the multiple Multiverses reside? Would they occupy space outside of the UN-observable universe?

    Thanks in advance, Chronos and Marcus, for your time and consideration,

    Last edited by a moderator: Mar 22, 2015
  14. Mar 22, 2015 #13
    I was asking in early universe when Inflation happened I see somewhere that we can calculate it to think a light cone as somebody say above.Inflation theory happened that time 10-35 but at that time universe diameter is 10-35xc =10-27m. In early universe I guess we can calculte universe diameter but know we cant cause we can see only part of the universe.
  15. Mar 22, 2015 #14
    I am very happy If helped you
  16. Mar 22, 2015 #15


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    Many scientists share your revulsion at the notion of an infinite [or infinitesimal] anything. It usually leads to paradoxes - e.g., Olber's Paradox. The universe, however, is a special case and we have no observation that prohibits it from being infinite. A finite universe is, in fact, an oxymoron that provokes illucid questions like - what lays outside the universe?
    Your objection is logical. An infinitude of parallel universes is, IMO, unaesthetic and almost surely paradoxical.
    Yes, column 5 is the causal, or particle horizon of the universe.
    Within the context of this presentation, I infer it to mean 1 meter.
    Yuki displays a chart of physical size vs horizon size of the universe further down the page, but, beware, it can be highly misleading. None of these figures have observational support, and subtly incorporate the assumption the universe originated as a singularity.The size of the universe following inflation depends heavily on assumptions. The inflation model was intended to resolve problems in a universe without inflation - like the horizon problem, flatness problem, etc. Inflation is an effective theory. For it to be viable, it must result in a universe that approximates the one we observe today. To achieve this goal, the universe must have expanded by an enormous amount in a very, very short interval of time. That number turns out to be about 60 e-folds [a factor of about 10^27 in lay terms]. Again, different models predict a different number of e-folds, but, it is generally agreed it cannot be much less that about 60 e-folds.
    In string theory, the answer would be the 'bulk' - a hypothetical higher dimension wherein these 'bubbles' float around. Again, I caution we have no tangible evidence of any universe outside our own, so that should be taken with a solar mass grain of salt. To avoid confusion or bias, I elect to decline further comment.
  17. Mar 23, 2015 #16
    Does that mean my left eyeball can see parts of the universe that my right eye can't see. What happens then when both eyes are open.
  18. Mar 23, 2015 #17


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    When both eyes are open your observable universe is centered on the bridge of your nose. If you are cross-eyed, you have two observable universes.
  19. Mar 23, 2015 #18


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    That seems is left right, according to my observables... lol
    Last edited: Mar 23, 2015
  20. Mar 28, 2015 #19

    Thanks, Chronos, for your feedback on my thoughts and questions; it cleared up some of my muddled thinking.

  21. Apr 5, 2015 #20
    We cannot, the universe is infinite in the sense that we cannot measure, the terms observable universe and universe go hand in hand with each other, we may be able to calculate the diameter of the observable universe, but it will always be larger, an alien on Andromeda galaxy for example is further away from us and therefore sees further out into space and the same thing applies to us, we can observe parts of the universe that he cannot. The universe is humongous, to say the least.
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