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How do we know the universe is expanding?

  1. Nov 12, 2011 #1
    Im very new to astronomy and astrophysics, so this may be a very simple question.

    I read somewhere that astronomers observed that galaxies further away were moving away at a faster speed than galaxies closer to us. Hence, the universe is expanding at an accelerated rate. However, when we observe the galaxies that are further away, we are actually observing those galaxies as they were millions of years ago. Doesn't that mean that the speeds we observed were the speeds millions of years ago and not today? Could it be that those same galaxies are moving slower today?
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  3. Nov 12, 2011 #2


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  4. Nov 14, 2011 #3
    I have much the same problem. The further away something is the faster it is moving and the greater the redshift but the further back in time we are looking.
    Can anyone tell me please
    1. How do we know when light has been travelling 10billion years when in that 10b years that redshift occurred.
    2. How fossil light has any relevance to the universe today. e.g. fossil dino bones exist but have little relevance to the earth today unless you go 70 light years away and look back, in which case the dinos still rule the earth.
    3. Has redshift been measured on a close galaxy over a period of time and still found to be increasing as against measuring it over different ages and distances.
    4. The CMB has a redshift of 1100 what does this mean in terms of speed, or how fast is it moving away from us?
  5. Nov 14, 2011 #4


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    No! What this observation implies is that the universe is SIMPLY expanding! No acceleration is implied by the statement that the velocity of the galaxies is proportional to their distance. In fact, it is precisely because this proportionality doesn't quite hold at large distances that we can observe the acceleration of the universe!
  6. Nov 14, 2011 #5


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    The distance to the matter that emitted the CMB is increasing at the rate of 3.3 times the speed of light.

    That is not like regular motion because when you have uniform increase of distances, nobody gets anywhere. So there is no theoretical limit on the rate distance can increase, the way there is for ordinary familiar motion (e.g. towards some goal).

    Google "cosmos calculator" or use the "morgans" link in my signature which also gets you there.
    At the start of each session put the numbers .27, .73, 71 in the boxes on the left for matter density, cosmo constant, and Hubble rate.

    Then you can put in 1100 for redshift and it will give you the answer 3.3 c.

    I'd suggest playing around with the calculator as a way of getting used to the standard cosmo model.
  7. Nov 15, 2011 #6
    Do you have any comments on my other questions?
  8. Nov 15, 2011 #7


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    Hi John15,
    actually not at the moment. I'm hoping you will follow up on my suggestion and get some familiarity with the standard model cosmos that is built into that calculator.

    In cosmology, to paraphrase McLuhan the great Canadian Sage of the Sixties, "the model is the message."
    what we are talking about is a simple math model that is fit to data. The words we use are just somewhat imperfect and not completely consistent or reliable interpretations from the model.


    You can shortcut a lot of confusion if you go directly to the model and get hands-on familiarity.

    Another calculator that doesn't take priming with parameters and does much the same thing is what you get if you google "wright calculator". The format is not so clean and simple, there is a bit more jargon, but it is convenient to use because you don't have to start by putting in .27, .73, 71. Those numbers are already put in for you, in the wright calculator.

    How about giving it a try?

    Your "how do we know" questions are actually related to understanding where the model comes from and how the 3 main parameters are adjusted to get the best fit to the data. (the data is millions of numbers collected by observation and careful measurement). The equations built into the model, that it runs on, are derived from the 1915 Law of Geometry/Gravity. This is an extensively tested equation law explaining (the best we know so far) how gravity behaves and why geometry is the way we experience it in our surroundings. It is a law that allows geometry to change in response to matter.
    When you work with either of those two calculators you basically are getting hands on experience of that equation law of geometry/gravity fitted (as best we know how) to the main three batches of astro data.

    So how about, for starters, giving today's cosmo model a try?

    Try redshift z = 1.5, and 1.8, and 6, and 1100, for example. See if it raises any questions in your mind. Notice anything funny?
    Wright calculator has more decimal place accuracy. If the other one frustrates you because too much rounding off, you can always switch. Personally I don't mind rounded answers.

    BTW your name reminds me of the Gospel of St John. Are you familiar with this passage from John 15?
    " 5 “I am the vine; you are the branches. If you remain in me and I in you, you will bear much fruit; apart from me you can do nothing. 6 If you do not remain in me, you are like a branch that is thrown away and withers; such branches are picked up, thrown into the fire and burned. 7 If you remain in me and my words remain in you, ask whatever you wish, and it will be done for you. 8 This is to my Father’s glory, that you bear much fruit, showing yourselves to be my disciples."
    Last edited: Nov 15, 2011
  9. Nov 16, 2011 #8
    Interesting passage from the bible. Such branches are picked up thrown into the fire and burned.
    Sounds like many of the questions I have tried to find answers to on such sites as this.
    Will look at your link.
  10. Nov 16, 2011 #9
    Looking at the speeds now and then the speed now is always lower than the speed then.
    I may be getting this wrong but it is what I would expect of a universe slowing down from a period of high speed. High redshift (cmb/ early universe) as we come out of inflationary period with lower redshifts (speed ) as we come to present, like taking your foot off accelerator at maximum speed in a car and coasting to a halt.
  11. Nov 18, 2011 #10
    Marcus not sure if you are still there but on further thought, on a graph using distance and speed you will get acceleration over distance but using time and speed you will get a deceleration over time. The question is then which is the correct way of looking at it?
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