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Expanding universe and age of light

  1. Jan 13, 2016 #1
    OK I might be stupid and english isn't my native language, so i'm sorry in advance.
    So:
    The red shift is bigger, the further away a galaxy is (they move away faster), and the closer it is, the shift is smaller and it goes to blue for Andromeda (towards). Everyone knows that.
    BUT
    When you consider that the light from the far galaxy is very very old, and from the close galaxy is new, don't you think that the universe WAS expanding long time ago and now it's collapsing?
    It's so simple I can't believe i might be right. I must be wrong. Please tell me where i go wrong...
     
  2. jcsd
  3. Jan 13, 2016 #2

    Orodruin

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    It is not what you describe. The expansion is about space itself growing and not really about the galaxies "moving". The redshift is not really a Doppler shift but an effect of space (and therefore the light wavelength) stretching out.

    To add to this, the galaxies also do move, but generally in random directions. This causes an additional Doppler effect which when a galaxy moves towards us can give a net blueshift. You have to take all effects into account properly.
     
  4. Jan 13, 2016 #3
    Well i'm a little confused right now. What exactly is "space" then (if not the distance between two things). Am I getting bigger right now, my molecules and atoms, or is the space between them getting bigger? And relative to what?
     
  5. Jan 13, 2016 #4

    Orodruin

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    You should be, these are really not concepts which let themselves be well explained with words.

    Space is generally a part of space-time. It describes a background in wich physics occur. A measure of space is distance, but it is possible for this distance to increase without two objects "moving" with respect to each other. I say moving with quotations because in general relativity this requires some additional definitions if the objects are not at the same place.

    If you have not seen the balloon analogy, I recommend you look it up (and remember that "moving" means moving relative to the rubber in the balloon, which represents space).

    No, you are not getting bigger. The forces involved in holding you together are far stronger than the expansion could overcome.
     
  6. Jan 13, 2016 #5
    First I want to thank you for responding. I was looking for answer for months but somehow didn't get the right article to read.
    I know the balloon analogy, but I thought it was simle Doppler, therefore my question about the age of the red shift. Now I know it is cosmological redshift (thank you), and the space universally is stretching the wavelenght, not the movement, therefore the age of light doesn't matter.
    One more question: In that case, (if the space is constantly stretching) is this a constant shift (same speed), or moving toward red/blue (accelerating/decelerating) for far and near galaxies. I can't find any information. If you have links, please share. Thank you again.
     
  7. Jan 13, 2016 #6

    Orodruin

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    The redshift affects all light in the same way regardless of the source. The factor with which the wavelength changes is simply the scale factor today divided by the scale factor at the time the light was emitted. In some sense, it just compares the size of the universe at the different times.
     
  8. Jan 13, 2016 #7
    I understand that.
    But if the wavelenght (red shift) of all galaxies changes over time (from the past 10 years readings for example) toward red/blue, that means space expanding speed is changed, right?
     
  9. Jan 13, 2016 #8

    phinds

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    This whole "space is expanding" is really not good terminology (yes, I know you see it EVERYWHERE in pop science). Things just get farther apart, and yes, they are getting farther apart ever faster. That's what it means that the recession is accelerating
     
  10. Jan 13, 2016 #9
    So when we observe galaxy 100 mil ly away with some speed (red shift) is this the current speed of the galaxy, or the speed 100 mil years from the past?
     
  11. Jan 13, 2016 #10

    phinds

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    We see things as they were in the past. We also have enough information, having observed LOTS of galactic recessions, to have a good idea what it's doing "now". I put now in quotes 'cause that's a bit of a tricky subject.

    This is how we know that the observable universe is about 13+ Billion years old and yet the observable universes has a radius "now" of about 50 Billion light years.
     
  12. Jan 13, 2016 #11
    Thank you, people.
    Sorry if i annoyed you with my ignorance.
    I have more questions about how the expansion is stretching the wavelenght, but I'll read more (now i know where).
    bb
     
  13. Jan 13, 2016 #12

    phinds

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    Hey, that's what we're here for. Glad to see you are learning this stuff. :smile:
     
  14. Jan 13, 2016 #13

    Chalnoth

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    The average distance between galaxies is getting larger over time. There are some galaxies that are close to one another that are locked in their mutual gravitational attraction (such as the Milky Way and Andromeda), but most galaxies are moving away from one another in a pretty uniform fashion (so that a galaxy that is twice as far away is, on average, receding at twice the speed).
     
  15. Jan 13, 2016 #14

    mathman

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    The mutual gravitation seems to hold for clusters of galaxies and maybe superclusters.
     
  16. Jan 13, 2016 #15
    my original question was "the red shift that we see today: is it old (outdated) data from millions years ago?" because light need time to get to us, and what we see now is actually what was long time ago. thats all i'm asking.
     
  17. Jan 13, 2016 #16

    Chalnoth

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    Yes, this is true. So when we look far away, we see the expansion history of the universe. The rate of expansion has been slowing over the last few billion years, and it appears to be approaching a constant expansion rate (given by the cosmological constant).
     
  18. Jan 13, 2016 #17

    phinds

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    That's a very confusing way to express it to a noobie. The rate of the ACCELERATION is slowing to a constant but since the recession is still accelerating and will continue to accelerate, the expansion is getting faster and faster and will continue to get faster still.
     
  19. Jan 13, 2016 #18

    Jorrie

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    IMO, this is even more confusing. I thought that we have agreed some time ago (will have to look it up) that the (fractional or %) expansion rate is represented by H(t) and that it is dropping to a constant value. The other value is the recession rate of specific galaxies and it is increasing today and in the future due the constant H.
     
  20. Jan 14, 2016 #19

    Orodruin

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    I believe this is confusing based on what the OP wrote earlier. While it is true that you can infer the expansion history, it is not true that seeing further objects redder implies that they were going away faster.
     
  21. Jan 14, 2016 #20

    phinds

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    Yes, but I'm talking about how it looks to a noobie who doesn't yet understand those terms.
     
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