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Homework Help: What is the reason we don't "see" the Universe's expansion?

  1. Jan 31, 2017 #1


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    1. The problem statement, all variables and given/known data
    Ok, so if the universe is expanding, and the scale of space itself is changing, then that means that even humans and the earth is expanding right?
    But is the reason why we don't notice this effect the slowness of it?
    So v=Hd, where H is Hubble's constant, which has a value of about 70 kms-1Mpc-1
    So if we convert that to ms-1m-1-->it becomes (70000/(3.1x10^16))-->2.26 x 10^-12, that means a point one metre away from me is moving away from me at a speed of 2.26 x 10^-12 ms-1?
    So if we take an entire human life to be, say, 100 years-->speed * time=2.26x10^-12 x 100 x 365 x 24 x 60 x 60=0.00713 metres-->is this why we do not "see" the expansion?

    2. Relevant equations

    3. The attempt at a solution
    (See above)
  2. jcsd
  3. Jan 31, 2017 #2


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    If our rulers are expanding at the same rate as we are expanding, how will we be able to measure any difference? :smile:
  4. Jan 31, 2017 #3


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    I think your figures are a bit out. The lifetime expansion per meter is nearer ##6*10^{-9}m##. Which would be hard to detect.

    A better example is the Earth's orbit round the Sun. The expansion of space would amount to about ##1km## in a lifetime. But, the Earth and the Sun are gravitationally bound, which means that gravity counteracts the minute expansion and keeps the two in the same orbit.

    A more extreme example would be an object sitting on the ground. A slow expansion of space could not counteract gravity and lift it slowly off the ground.

    Similarly the intermolecular forces that keep things together prevent you from expanding, even slowly!
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