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The redshift is how far/long ago?

  1. Jan 31, 2008 #1
    How far into space do we need to look before we can see the first signs of redshift?
     
  2. jcsd
  3. Jan 31, 2008 #2

    Wallace

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    In principle there is no lower limit, the redshift of an object is greater the further away it is so at some point objects are close enough to us that we can't really measure the redshift, but how close that is depends on the accuracy of the measurement. In addition, galaxies all have there own small random motions on top of the general expansion so for nearby galaxies the doppler shift due to these dominates over cosmic redshifting.
     
  4. Jan 31, 2008 #3
    thank you for that answer. so there is no discernible distance a t which the redshift effect "turns on" vs. gravity? it seems like they might be trying to compete. is there a clear connection between the two?
     
  5. Jan 31, 2008 #4

    Wallace

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    hmm, unfortunately your question is a little too confused to allow a straightforward answer.

    Redshift to a good approximation is simply due to the fact that galaxies are moving away from each other, this velocity causes a doppler shift that shifts spectral lines. That is all redshift is.

    There are some subtleties and when redshift gets large enough it becomes inaccurate to think of redshift as a doppler effect, but redshift is that a 'cause' that competes with gravity, rather it is an effect that is caused by motion and gravity.

    It sounds like it would be worth your while to read an introductory cosmology textbook to explain this more clearly step by step. It's hard to get a good clear overall picture in your head of these ideas just from ad hoc bits of advice on forums. Better to read a textbook or good website and use forums to clarify the bits you're not understanding.
     
  6. Jan 31, 2008 #5
    yes, thank you. i had lept forward in my head to a place that i did not communicate well. i meant that the effect of gravity seems to involve a contaction of the space between objects at a rate that is a function of the extent of that space (distance). the redshift implies that space between distant objects is expanding at a rate that is a function of the extent of that space. i wonder what connection might exist there?
     
  7. Jan 31, 2008 #6

    Wallace

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    Gravity does not "involve a contaction of the space between objects at a rate that is a function of the extent of that space (distance)". If you think of gravity as doing this you will lead yourself to all kinds of misconceptions!

    There are a lot of threads here that deal with the (much abused) concept of expanding space. Use search and have a read of them. To be very brief, the expansion of space is a intellectual shorthand, in other words it doesn't 'really' happen but it can be a convenient way of think about some things, in particular an expanding universe.
     
  8. Jan 31, 2008 #7
    so you are saying that gravity does not involve the acceleration of one rest mass towards another with the consequence that the distance between the two is reduced? And then isn't the changing rate at which this distance is being reduced the same thing as saying that they are accelerating towards eachother?
     
  9. Jan 31, 2008 #8

    Wallace

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    Yes. Two things can be accelerating towards each other but moving apart, this is what happens if you throw a ball in the air.

    Again, you are mixing the acceleration caused by gravity with velocity. They need not have the same sign.
     
  10. Jan 31, 2008 #9
    ok, i can see what you are saying. however, that requires kinetic energy, which may or may not have to do with gravity.

    I am just saying that gravitational attraction implies a direction and acceleration (even if the velocity of the body is for some other reason not following the acceleration vector, and when people say the universe is expanding at an accelerating rate, that also implies a direction and acceleration. The direction in this case seems to imply that distance between object and observer should be increasing.

    Of course one theory for this may be the big bang, but it seems like a lot of people have put some holes in certain parts of that theory. And still no graviton. It just seems like there is a problem there that people like to speculate about. I was just wondering if anyone had shown any connection between what we see as gravity and what we see as an expanding universe.
     
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