Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I The Age of Our Universe

  1. Dec 29, 2016 #1
    At the big bang, 'we' (an inside observer) were travelling at great speeds. Does this mean to say that time was 'slower' and thus the value (distance traveled in time) of time within 1 second, experienced by an inside observer, was much larger than our modern, perceived, value of 1 second?
    If so, how is this phenomenon accounted for in our predictions of the age of the universe?
     
  2. jcsd
  3. Dec 29, 2016 #2

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    No, this is not correct. Comoving observers have no speed relative to the background medium.
     
  4. Dec 29, 2016 #3
    Please explain...
     
  5. Dec 29, 2016 #4

    phinds

    User Avatar
    Gold Member
    2016 Award

    The concept of a comoving observer is very basic to cosmology. How about you do some research and come back with a specific question if there is something you don't understand.
     
  6. Dec 29, 2016 #5

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Which part of the statement was unclear? It is impossible to explain further if you do not specify which part you did not understand.
     
  7. Dec 29, 2016 #6

    Dale

    Staff: Mentor

    This may be a source of confusion. The adjective "inside" doesn't apply here. All observers are inside the universe, and there are no outside observers in modern physics.
     
  8. Dec 30, 2016 #7

    Ibix

    User Avatar
    Science Advisor

    @Jacob Gawel - there are a number of misconceptions in your post. You are applying special relativistic concepts such as time dilation to a scenario where special relativity does not apply. You are treating speed as an absolute quantity, which isn't correct even in Newtonian physics. And you are not allowing for the complexity of defining speed in a curved spacetime. The end result, unfortunately, is that your question makes no sense. To explain why, however, would mean teaching you general relativity - and I don't have a couple of years free to do that right now.

    Suffice it to say that there is a clear meaning to the statement that the universe is 13.9 billion years old, which is that a clock that sees the universe as isotropic and has existed since the Big Bang would show 13.9 billion years having passed. Other clocks could show other things, but it would be harder to defend them as "the age of the universe".
     
  9. Dec 30, 2016 #8
    By the term 'inside observer', I was referring to an observer who is travelling at the same 'speed' as the expansion of the universe at the big bang, as opposed to an observer in a rest frame. I now know that this, essentially, is incorrect and I will do my research on 'comoving observers'.
     
  10. Dec 30, 2016 #9

    Dale

    Staff: Mentor

    Several speculative and off topic posts have been removed. Please don't try to post speculative or off topic ideas in response to this thread.
     
  11. Dec 30, 2016 #10

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    A starting misconception is that there is any such thing as a rest frame. In cosmology, not only is this concept impossible, it is even impossible to define anything resembling an inertial frame of SR covering more than a (cosmologically) small region.
     
  12. Dec 30, 2016 #11

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    Coarse-graining over a large scale of space leads to the description of the spacetime as a space with maximally symmetric spaces for a socalled "comoving observer". I.e., there exists a reference frame (physically determined as the frame according to which the temperature of the cosmic microwave background radiation is isotropic), for which an observer, at rest relative to this frame, considers space at any fixed (coordinate) time as homogeneous and isotropic. This leads to the Friedmann-Lemaitre-Robertson-Walker solution of the Einstein field equation with an energy-momentum tensor consisting of a universe filled with homogeneous and isotropic matter and radiation:
    $$\mathrm{d} s^2=\mathrm{d} t^2 -a^2(t) \left [\frac{\mathrm{d} r^2}{1-kr^2} + r^2 (\mathrm{d} \vartheta^2 +\sin^2 \vartheta \mathrm{d} \varphi^2) \right].$$
    with ##k \in \{-1,0,1\}##. In this sense there's indeed a kind of "rest frame", physically specified by the homogeneity and istropy of the CMBR wrt. to this frame.
     
  13. Dec 30, 2016 #12
    Misconceptions notwithstanding, I've been thinking along a similar vein. Having learned here that there is no universal now (or when or while or center) it would seem to me that there is no universal age. E.g., to an observer under massive gravitational forces (or traveling near c relative to earth's [and the above-mentioned "rest"] frame of reference) it's going to be a different age and size/shape than it is to us. And it's probably safe to assume that gravitational forces were huge following the Big Bang, which would have dilated time, which brings me back to the poster's question.
     
  14. Dec 30, 2016 #13

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Yes, but this coordinate system has little in common with an SR rest frame. Its closest analog in SR is coordinates completely unlike Minkowski coordinate; specifically, Milne coordinates are the closest flat spacetime analog; and in these (Milne) coordinates, world lines of constant coordinate position have unambiguous relative velocity.
     
  15. Dec 30, 2016 #14

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    It might be helpful to provide unambiguous examples of what the OP may be getting at:

    1) Suppose there is some non-comoving observer (that sees anisotropy). If they compare their clock to clocks of comoving galaxies as they pass them, will their measure of age of universe be different? Of course. In this sense, as already noted, the age (hypothetically) measured by comoving observers (they all get the same thing, by isotropy and homeneity) is just a definition of 'age of the universe'. However, it seems a far more useful definition than any other.

    2) Suppose there is a specific non-comoving observer that travels from one galaxy to another, setting their clock to agree with the comoving clock of that starting galaxy. When they arrive at a different galaxy, how will their clock compare to the comoving clock of the destination galaxy? In general, their clock will be behind the destination comoving clock when they arrive.
     
  16. Dec 30, 2016 #15

    Ibix

    User Avatar
    Science Advisor

    Gravity isn't a force. And the basic problem with your premise is: time is dilated with respect to what? With a black hole I can park a clock close to it and one far away from it and exchange light signals to compare clock rates. How am I going to exchange signals with a clock 13.9 billion years ago? I can't. So there isn't really a meaningful way to ask whether clocks ticked at the same rate or not.

    13.9 billion years is what a clock would read if it was at rest in the comoving frame. It's the length of the longest time-like "straight line" I can draw on a map of 4d spacetime that has one end at me now and one end at the Big Bang.

    As I said before, other clocks could read other things. But they're harder to defend as measuring the age of the universe.
     
  17. Dec 30, 2016 #16
    Just googled "What is gravity?" and got lots of links, almost all beginning with "gravity is a force..." E.g.:

    grav·i·ty
    ˈɡravədē/
    noun
    1. 1.
      PHYSICS
      the force that attracts a body toward the center of the earth, or toward any other physical body having mass...

    But whatever the correct semantics, it impacts time. Mightn't whatever material changes were occurring after the Bang be considered a clock? Might one ask "With respect to what?" re the size of the early universe as well?
     
  18. Dec 30, 2016 #17

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    It's very hard to figure out what you are getting at. If you are possibly wondering whether there could be some observer that would measure the universe to be older than the standard definition, the answer is no. Any hypothetical clock in the universe that existed from the Big Bang to some 'now', that follows some world line different from a comoving clock, will show a younger age for the universe compared to the now coincident comoving clock.
     
  19. Dec 30, 2016 #18
    Thank you, PAllen, this is all I, and I believe the poster, were trying to establish.
     
  20. Dec 30, 2016 #19

    Nugatory

    User Avatar

    Staff: Mentor

    That definition soooooooo doesn't apply at cosmological scales.
     
  21. Dec 31, 2016 #20
    A dictionary is probably a poor place to look for a physics definition.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: The Age of Our Universe
  1. Age of the universe (Replies: 2)

  2. Age of the universe (Replies: 12)

  3. Age of universe? (Replies: 4)

Loading...