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Some facts / figures

  1. May 18, 2013 #1
    Hi there

    Am to teach a lesson on stars and galaxies to my year 7 class on Monday...
    Want to brush up on a few things... so would be grateful for answers to these questions:

    1.Is the current thinking that the universe is infinite or finite? (I thought the consensus was that it was finite but I am not entirely sure if thinking hasn't changed... )

    2.when we say the "observable universe" does this refer to a universe that is 13.8 billion light years in size? ie. the Planck telescope could view back as far as when the universe became opaque which was 13.8 billion years ago......

    3. When we say that there are about 100 billion galaxies in the universe, does this refer to observable universe?

  2. jcsd
  3. May 18, 2013 #2
    Its considered finite or infinite there is no consensus on either.

    The observable universe is 47 billion years radius. Age 13.78 billion years
    the number of galaxies are probably the observable portion.
  4. May 18, 2013 #3
    thanks a lot!
    Can I also ask a follow up question?!:

    I was asked in my last lesson where most of the meteors that strike Earth come from.... I said that most of them came from the asteroid belt and it was the gravitational pull of Mars that deflected them in towards Earth.... this was a guess on my part. Can you tell me if I happened to be right there?
  5. May 18, 2013 #4


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    The gravitational influence of Mars on the asteroid belt is small - Jupiter is more important, together with asteroid/asteroid interactions and close encounters of (non-asteroid-belt) objects with the inner planets (Mars is an inner planet, but here we have asteroids which are not in the asteroid belt).
  6. May 18, 2013 #5


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    Most meteor showers arise from Earth passing through the path of comets. Many short period comets come from the Kuiper belt, which lies out beyond the orbit of Pluto.
  7. May 18, 2013 #6


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    If you want facts and figures you need an idea of distance. Very often in cosmology one uses the concepts of "universe time" and "proper distance"---that is distance at a particular instant of universe time with the expansion frozen or paused, so that you can measure distances without them changing.
    Hubble law, the universe's overall pattern of expansion (as far as we know) is stated in terms of universe time and proper distance.

    Because the percentage rate of expansion has varied a lot over history, there is no simple correspondence between the time a flash of light has been traveling and its distance from point of origin. Expansion helps the light cover bigger distances than it could by itself alone in a static nonexpanding space.

    So the ancient light of the CMB which we see today was emitted by matter that is now over 45 billion lightyears from us. but the light has only been traveling for almost 13.8 billion years. Expansion has helped to distance it from its matter of origin.

    When we talk about the currently "observable" portion of the universe---the radius is estimated to be around 46 billion lightyears (proper distance).
    Get some of your 7th year students to try this:
    It gives a history of the expansion from the origin of the CMB ancient light in year 373,000 (when distances were 1/1090 present size) to the present (the S=1) row of the table, and on to year 92 billion when distances will be 100 times present size.

    There are blue information dots beside all the quantities which give information and explanation. The student can just move the cursor to a dot he wants and the info will pop up.

    Increase the number of steps from 10 to 20 or 30 to get longer tables.

    The radius of the observable portion is technically called the "particle horizon" and denoted Dpar in the table. You can see that it is now about 46 billion ly and you can see how it is expected to grow in future. But to get Dpar to appear as a column in the table you must select it and press calculate again. Open "Column Definition and Selection" menu and check Dpar. Then press calculate
    Last edited: May 18, 2013
  8. May 19, 2013 #7


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    The observable universe is definitely finite. The 'unobservable universe' is not necessarily finite. I fail to see the scientific relevance of speculating beyond observables. If there are detectable observables beyond the boundaries of our universe, that would be marvelous. Otherwise, I consider it nonsence.
  9. May 19, 2013 #8


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    I know you do, Chronos. You have often expressed this lack of understanding. The point is not to "speculate" beyond the limits of observation, but to make the simplest assumptions that allow one to construct a mathematical model based on GR which fits the available data.

    By far the simplest math model we get from GR has no spatial boundary. It would simply not work to model only the currently observable portion.
    So one makes a point of AVOIDING speculation and assuming SAMENESS so as to have the minimal complete entity that one can model using the simplified version of GR which has been found to work.

    This is how one best can, in effect, treat the data represented by the observable portion of the cosmos and as much as possible avoid "making up stuff".
  10. May 19, 2013 #9


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    marcus, you have again expressed your inability to comprehend my point without turning it into support for some ridiculous assertion.
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