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

Age of the universe

  1. Sep 20, 2014 #1
    Hi all!
    how much the unobservable universe is taking into account when decide the age of the universe, weather cosmologists use the speed of galaxies, the temp of white dwarves etc....because no matter what you take from the observable the unobservable could change it AND if all material speeding in same speed from each other, why are there part we can't see and some part we can?
    thanks
     
  2. jcsd
  3. Sep 20, 2014 #2

    mathman

    User Avatar
    Science Advisor
    Gold Member

    The major clues for the age are the expansion rate and the temperature of the background. Go backwards until the universe is squeezed to a point. Thebackground temperature now tells us about how much it has expanded since the universe cooled down so that radiation could escape.

    The speed of expansion relative to us (or any place else) is proportional to distance (approximately).
     
  4. Sep 20, 2014 #3

    phinds

    User Avatar
    Gold Member
    2016 Award

    AAARRRRGGGHHHH! You are supporting the wrong model of the early universe and adding to the popular misconception that the big bang was indeed a point in space rather than a point in time.
     
  5. Sep 20, 2014 #4
    sorry but that is not good enough. the temp of the uni is almost absolute zero, who knows how to measure the rate of cooling . all we know is that there is backround radiation today. is the uni like a deordorant can as the gas expand outside the can gets colder? maybe/. but what about the unobservable uni? WOULDNT IT BE MORE ACCURATE TO SAY THAT THE AGE OF THE **observable** UNI IS 14 BILLION?

    yes it's proportional. but if all the galaxies came from a single point and started to spread like light from a torch than how come some photon/galaxies are VERY far and some are not that far. maybe it's because there is no symmetry (anti matter matter, spread of galaxies in the uni and thermal differences etc)
    when you turn on a torch and there is a wall in front of you all the pohoton arrive at the same time and create a perfect circle
     
  6. Sep 20, 2014 #5
    so how the glaxies were set before the explosion?
     
  7. Sep 20, 2014 #6

    Bandersnatch

    User Avatar
    Science Advisor

    You misundersood. The "temperature of the universe" is not the temperature of the interstellar gas, but of the background radiation. It's spectrum is such as would be emitted by a black body of temperature 2.7K.
    Gas in the early universe would have to be about 3000K hot to become plasma and be opaque to light. Yet the spectrum we observe is that of a 2.7K body, not 3000K. The wavelengths have been stretched.

    But you're right in that ultimately we always talk about the observable universe only. However, it's generally a good bet to assume things aren't that much different outside our viewing radius, without any different physics going on. It makes for a simpler model, and in the absence of means of probing beyond what we can see, it's the only sensible approach, really.


    Again, you misunderstood. Proportional means that the velocities increase the farther the galaxies are. Those closer move more slowly, those farther move faster. As they move farther, their speeds continually increase.
    They most certainly do not spread like light from a torch(whose velocity is not proportional but constant).

    Disregarding the fact that there were no galaxies early on, everything was set pretty much exactly as it is now, albeit proportionally closer. Nothing ever moved relative to anything due to expansion - it's only that all the distances increased. You should be able to see how there is no relative motion as long as all distances grow by equal factor.
     
  8. Sep 20, 2014 #7

    phinds

    User Avatar
    Gold Member
    2016 Award

    meni-ohana, you would likely find it helpful to read the 1-page discussion pointed to in the link in my signature. You have some fundamental misconceptions that are very common.
     
  9. Sep 21, 2014 #8

    Drakkith

    User Avatar

    Staff: Mentor

    Galaxies didn't exist before the big bang/expansion of the universe. galaxies didn't form until at least several million years after the big bang.
     
  10. Sep 21, 2014 #9
    how you american say that? ohhhh boy!! :) [english is NOT my mother toungue and i dont want to check too much and think about how i write so sorry for that)
    you didn't get me.
    i didnt say glaxies are like light in the sense of speed. i was talking about how the big bang radiate/expand/proprigate as light from torch in geometrical sense

    i never talked about the gas, anyway though -- i'll flow with you, lets say the stick yard is the color shift of light. it tell us the speed of the space streching everywhere - fine. how would you know for how fast the edges of the universe (which are not observable to us) strech?
     
  11. Sep 21, 2014 #10

    phinds

    User Avatar
    Gold Member
    2016 Award

    There IS no "edge of the universe" unless you mean the observable universe. I suggest again that you read the discussion in the link in my signature.
     
  12. Sep 21, 2014 #11

    Drakkith

    User Avatar

    Staff: Mentor

    There are no edges to the universe that we know of. The most likely scenarios are that the universe is either bounded, meaning that you could continue in a straight line and eventually come back to your starting point, or that the universe is infinite and unbounded, so that you could continue in a straight line forever without ever coming back to your starting point.

    As for expansion, the rate of expansion is approximately 73 km/s per megaparsec in distance. (A megaparsec is 3.26 million light years) This means that objects 1 Mpc apart will recede from each other at 73 km/s. At 10 Mpc apart, they will recede from each other at 730 km/s. This recession velocity continues to increase without end as the distance between objects increases.
     
  13. Sep 21, 2014 #12
    ok since i dont want to make this too shallow discussion and i dont have much time right now to read and dive into this ill sadly end it, thanks though!
     
  14. Sep 22, 2014 #13

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    There's no reason to do this -- there is no such thing as a stupid question. Ask anything you want to try and learn!
     
  15. Sep 23, 2014 #14
    i dont have time to read what i should read right now. i will return later about that
    thanks everyone!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




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

  2. Age of universe (Replies: 5)

  3. Age of the Universe (Replies: 2)

  4. Age of the Universe (Replies: 2)

Loading...