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I Observable Universe Size in Different Perspectives

  1. Aug 22, 2017 #26
    We never were.
     
  2. Aug 22, 2017 #27

    phinds

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    Gold Member

    Shows how useful commas can be.
     
  3. Aug 23, 2017 #28
    Science Advisor,
    Thank you for the plots. Visuals always appreciated in a physics discussion.
    Feedback:
    If the universe is expanding a constant rate, as can be seen from nearly the entire history of the expansion, then the expansion is not slowing down. If you viewed a galaxy moving away from you at say 6 Gy and then again at 10 Gy it would be receding from you at the same rate. A galaxy twice as far away would still be receding at twice that velocity. This would also be true of the event horizon. A better way to understand this is to consider the perimeter of the circle. The observable universe exists along that perimeter. The perimeter of the circle scales linearly with the radius. The co-moving coordinates describe the expansion in this manner.
     
  4. Aug 23, 2017 #29

    PeterDonis

    Staff: Mentor

    No, that's not what is seen from the entire history of the expansion. The entire history of the expansion shows change in the rate of expansion--it was decelerating in the early universe, then a few billion years ago it started accelerating.
     
  5. Aug 23, 2017 #30
    A wrong quote
    >"Now we are not in the center of the universe ...."
    - that was the quote from the original post not (not mine).

    Actually, I'm in the center of universe; any other observer may make the same claim...
     
  6. Aug 23, 2017 #31
    Ooops sorry, don't know how I did that.
    but yes everyone is at the center of what is observable for them.
    The whole Universe may or may not be infinite, and need not have a center anywhere,
    just as there is no place on the surface of a sphere which could be called a center.
     
  7. Aug 23, 2017 #32
    We dont even know the universe is sphere. I dont think theres any meaning calling "center of the universe", in this sense its just meaningless
     
  8. Aug 23, 2017 #33

    phinds

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    Gold Member

    Absolutely false. You are in the center of the Observable Universe, NOT "the universe"

    EDIT: OOPS ... I see rootone beat me to it.
     
  9. Aug 23, 2017 #34

    phinds

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    Actually, we know that is is NOT a sphere. If it were, it would have a center and it does not.
     
  10. Aug 23, 2017 #35
    Well, it is not false; in fact, the existence of "the universe" if it is not an observable one (in broad information sense) is matter of fate and believe...
     
  11. Aug 23, 2017 #36

    PeterDonis

    Staff: Mentor

    Only in the sense that we can't directly observe it. But if you want to reject the belief that there is universe beyond the part we can observe, you have to claim that the laws of physics suddenly change at the boundary of our observable universe, for no apparent reason. Because if they don't, then the universe beyond what we can observe must be there, since that's what the laws of physics that we see in our observable universe imply.
     
  12. Aug 23, 2017 #37
    Well, if it is not provable then it is matter of assumption what is totally fine to make, pretty much the same as with different kind of multiverses if such assumptions help some brains to construct theories with better predictive power then it definitely has its merits...
     
  13. Aug 23, 2017 #38

    PeterDonis

    Staff: Mentor

    Yes, but saying that the universe just stops at the boundary of our observable universe is also an assumption--and one which violates Occam's Razor. That's why the assumption that the universe continues past the boundary of our observable universe seems preferable--because it doesn't require claiming that something changes at that particular boundary for no apparent reason.
     
  14. Aug 23, 2017 #39
    >"but saying that the universe just stops at the boundary of our observable universe"

    Pls quote where I made such silly assertion ;o)
     
  15. Aug 23, 2017 #40

    PeterDonis

    Staff: Mentor

    You made it implicitly here:

    It's only a matter of "faith and belief" if you start with the assumption that the universe stops at the boundary of our observable universe, for no apparent reason. If instead you start with the assumption that the laws of physics don't stop at a particular point for no apparent reason, just because that point happens to be the boundary of our observable universe--i.e., if you start with Occam's Razor--then it doesn't take any "faith and belief" to see that there must be universe beyond the boundary of our observable universe.
     
  16. Aug 23, 2017 #41
    I've made an explicit explanation of my point:

    SB>“if it is not provable then it is matter of assumption what is totally fine to make, pretty much the same as with different kind of multiverses if such assumptions help some brains to construct theories with better predictive power then it definitely has its merits... ”

    You are definitely free to make your “implicit” interpretations I just see no relation to my point to discuss.

    Thanks,
    SB
     
  17. Aug 23, 2017 #42

    PeterDonis

    Staff: Mentor

    What is "provable" depends on what assumptions you start with. You start with the assumption "anything we can't observe we have to take on faith"--but that requires you to also assume that the laws of physics suddenly change at the boundary of our observable universe, for no apparent reason. Only with that additional assumption--which violates Occam's Razor as well as common sense--can you say it isn't "provable" that there is more to the universe than the part we can directly observe.

    Whereas I start from the assumption "the laws of physics stay the same everywhere", which allows me to prove immediately that they don't change at the boundary of our observable universe for no apparent reason, and therefore there is more to the universe than the part we can directly observe.

    In other words, your apparent belief that only I have to make "assumptions" and you don't, in order to support our respective viewpoints, is not correct.

    Perhaps the above will help to explain its relevance.
     
  18. Aug 24, 2017 #43
    Thanks, That is what I thought.

    Some of the other answers were confusing, at least to me.
     
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