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Expansion of the universe

  1. Nov 18, 2003 #1
    some HW question.Thanks for your help...

    1)decribe the DIFFERENCE between the FINAL states of the universe a)if the average density of the universe is greater than the critical value b)if it is equal to the critical value

    2)if the velocity of recession of the galaxies were not proportional to the distance (v=Hr) but rather proportional to the distance squared v=Hr^2 or proportional to some other power of the distance, then our galaxy would occupy a preffered, central spot in the universe?

    3)if you look at the night sky, you see that the distribution of stars is not uniform.What main deviation from uniformity do you see, an to what is this deviation due?

    4)the value of the hubble constant that hubble had deduced from the available data in 1936 was 1.6*10^5 (m/s)/(million light years) the corresponding expansion time is 1.9*10^9 years. how does compare with the age of the earth and with the age of the globular clusters? with what problem was hubble faced?
  2. jcsd
  3. Nov 18, 2003 #2


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    I probably should shoot this over to the Homework Help forum, but I think the regulars in this forum will provide more thorough answers.

    (3) Assuming you have mostly light-pollution free skies and the proper time of night to view, most of the starlight you will see will be within a broad band that stretches all the way across the sky. This is the Milky Way galaxy viewed, of course, from our perspective on the inside. The Milky Way is a spiral galaxy in which most of the stars are configured into a flattened disk (as opposed to some other galaxies which are more like elliptical clouds of stars). Our view of this disk is edge-on, thereby resulting in the majority of starlight being focused within the band. Of course, we are not at the extreme edge of the disk...more like 2/3 the way out, and our region is still several thousand light years thick. So, we also see nearby stars (within a few hundred light years) that are "up/down" from us with respect to the overall disk. More distant stars are too faint. More distant stars within the disk are also too faint, but there are so many that they add up to the faint Milky Way band across the sky. Check it out with binoculars and you'll see many more stars in that region of the sky than in other regions. If the Milky Way is not visible to you (light pollution or whatever), then you're seeing an essentially random distribution of the brightest nearby stars...with a few star clusters here and there (a result of star formations from within the same original nebula).
  4. Nov 18, 2003 #3


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    You should really show us what you've done so far, but here are few very brief answers/pointers:

    1) a)Big crunch b)finite size

    2) Think about a co-moving sphere, can the points on the surface of the sphere all be moving apart at a velocity proportional to r2?.

    3) Phobos has pretty much answered that one for you

    4) It's about the same age as the Earth and much less than the age of the global cluster, the problem is obvious.
  5. Nov 18, 2003 #4
    yes it is clear i made a fatal error. in case of problem 1 a)if the density of the universe is lower than the critical value...

    Now the universe will keep expanding...I think that the end of the universe won't be different in both cases.When I asked for a hint about the question to the assistant he told me to think as escape velocity.So what happens? If you send a rocket with a speed equal to the escape velocity of greater than that, the rocket will leave the planet?

    And in the last question what I really want to learn is "what was the problem Hubble faced"?
    Last edited: Nov 18, 2003
  6. Nov 19, 2003 #5


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    1) a) becomes the universe will contiune to expand forever (look-up heat death)

    4) the problem as I said is obvious, his value for the age of the universe was in disagreement with other values.
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