If objects are accelerating, are we measuring the systems getting smaller?

  1. What the title implies, with the discovery that the universe is expanding, are we observing the systems becoming harder to see? You'd think that with the speed of the objects and the distances involved, things like brightness and actual size would be getting reduced...
  2. jcsd
  3. Chronos

    Chronos 10,268
    Science Advisor
    Gold Member

    What observational test would you propose to affirm that hyposthesis?
  4. Chalnoth

    Chalnoth 5,614
    Science Advisor

    Well, yes, but on human time scales, the differences are so miniscule as to be inconsequential.

    Consider, for instance that we imagine that a galaxy that is one billion light years away is traveling at 0.1c away from us (this is just a number I pulled out of my backside...it's probably way, way wrong, so don't take it seriously).

    Now, at this distance, if we wait for ten years, the galaxy will be a whole extra light year further away. One light year out of a billion. So sure, it's getting further away, and therefore dimmer and smaller. But it's just not going to be noticeable.
  5. I find it hard to understand how the change is insignificant and can't be tested. Say for example, one of our planets fell out of orbit and started flying out of the solar system. We'd noticably see the object getting smaller and fainter, the further it got from the Earth. Now, if these distant objects are large enough and bright enough to be seen, and are accelerating away from us at such high speeds, why can't it be detected exactly?

    If I take a picture of an apple, I can use the ammout of pixels in the image to determine it's apparent size in relation to another image of the same apple in which it is further away. Can't that same idea apply to this concept given that the image was at a high enough resolution to mark out noticable change?
  6. marcus

    marcus 24,058
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    Gold Member
    2014 Award

    The Hubble Law change in distance amounts to 1/140 of one percent every million years.

    We can't measure distances to far-off galaxies with an accuracy of 1/140 of one percent, instruments and techniques are not even close to that level of precision.

    But if we could determine distances to 1/140 of one percent, then your idea would work. We could just wait a million years and see if the distance changed (by brightness and size clues, just as you suggested!) But that is obviously impractical.

    There are better ways to test Hubble Law expansion than what you propose (waiting for a measurable change in distance to show up). I hope you don't find this so hard to understand now. Your idea is impractical by a factor of about a million, to put it simply.
    Last edited: May 8, 2009
  7. I see, thanks. It's hard to comprehend how insignificant it actually is, when we're talking about enormous bodies, at enormous distances and enormous speeds. You'd think you'd see at least something, puts into perspective just how BIG this universe is...
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