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What does When the Universe was one minute old mean?

  1. Apr 1, 2010 #1
    What does "When the Universe was one minute old" mean?

    I understand time in terms of references such as how long the second hand takes to travel around a clock, or the definition in terms of the cesium atom, or other references such as planetary motion and heartbeats, but without such references for the early universe, what exactly does a one minute old, or a one second old universe mean? What does the age of the universe mean at a time t when none of our present references (AFAIK) make sense?
     
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  3. Apr 1, 2010 #2

    chroot

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    Re: What does "When the Universe was one minute old" mean?

    Welcome to PF, John Bleau. It's an excellent question. I don't think it's particularly troubling to not have references -- time still exists even if you have no clock. I am more troubled by the fact that GR eliminates the concept of a universal time coordinate.

    If GR dictates that it makes no sense to define an 'instant' in time (the same instant everywhere in the universe, like the Newtonian model of time) then it surely makes no sense to define a 'minute' after the big bang, either. The same argument applies -- does that declaration of time apply everywhere in the nascent universe?

    Perhaps someone with more experience with models of the early universe can answer.

    - Warren
     
  4. Apr 1, 2010 #3

    DevilsAvocado

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    Re: What does "When the Universe was one minute old" mean?

    Everything is relative, as good old Albert told us, and so is the second hand and the cesium atom (e.g. GPS satellites are corrected for relativistic effects). The only constant is the speed of light 299.792.458 m/s.

    Good question though, and I can't really tell you how they for example measure the Planck epoch (10–43 seconds).
     
  5. Apr 1, 2010 #4

    marcus

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    Re: What does "When the Universe was one minute old" mean?

    Cosmology is based on GR, but is a separate discipline. In cosmo there is a widely used universal time coordinate variously known as universe time or Friedmann etc model time.

    The vast majority of cosmo papers use the Friedmann equations as their basic model of the universe. These have a 3+1 split, a time coordinate, and a scale factor a(t) which is an increasing function of time. The Friedmann equations govern the increase in a(t).

    They are simple differential equations which are derived from GR. Cosmologists do not use the full GR eqn as a rule. Almost everything is done with the simplified model. So universe time has a meaning.

    The meaning of universe time is observationally linked to the CMB. Light from the ancient matter while still a hot cloud of gas. Observers are at rest relative to CMB if they see no doppler dipole. Two observers, both at rest, are synchronous if they both measure the same CMB temperature. This is only an idealized approximation but you get the idea.

    Universe time is the time experienced by all the observers who are at rest relative to Background.

    ======================

    When one studies the early universe, basically what one is talking about is the Friedmann model of the early universe.

    This comes equipped with universe time. Using the model we can tell how the temperature and density rise as we crank back in time, because they depend on the scalefactor a(t).
    And we know what particle reactions occur at high temperature and density so we can talk about the matter and radiation that would prevail at one hour, one minute, one second, of universe time.

    But as they approach the start of expansion, using the Friedmann model, people become dissatisfied and mistrustful of the model. So they modify it.
     
    Last edited: Apr 1, 2010
  6. Apr 1, 2010 #5

    bapowell

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    Re: What does "When the Universe was one minute old" mean?

    As Warren mentions, there is no universal time in GR. When we talk about the history of the universe, for example, when we say that the CMB formed 300K years after the big bang, we implicitly mean that this is the time as measured by an observer that is comoving with the expansion of the universe -- an observer locally at rest but moving along with the expansion. This is typically the cosmological time that people refer to.

    NOTE: Looks like marcus beat me to it. To connect our replies, comoving observers are at rest relative to the CMB.
     
  7. Apr 1, 2010 #6

    marcus

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    Re: What does "When the Universe was one minute old" mean?

    Sorry Powell, if I'd known you were answering I wouldn't have. Over and out to you. I've been seeing you giving the straight dope on all this stuff. Hopefully the OP will ask more questions and you will respond.
     
  8. Apr 1, 2010 #7

    DaveC426913

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    Re: What does "When the Universe was one minute old" mean?

    I am not sure we can take for granted that the speed of light in the early moments of the BB was the same as what it is now. 'twas in those few moments that those very laws of the universe were being writ.
     
  9. Apr 2, 2010 #8

    Chronos

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    Re: What does "When the Universe was one minute old" mean?

    We can safely say it was probably very different from what we now see.
     
  10. Apr 2, 2010 #9

    DevilsAvocado

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    Re: What does "When the Universe was one minute old" mean?

    You’re absolutely right. Matter and the laws of physics was of course created during the early moments of the BB, and furthermore – the photons/light was 'trapped' until the last scattering (~400,000 after BB).

    So when would it be accurate to talk about a constant speed of light? Dark ages, Recombination, Photon epoch...?
     
  11. Apr 3, 2010 #10
    Re: What does "When the Universe was one minute old" mean?

    I'll be studying your answers, everyone, thanks. I'll post later in the coming week.

    chroot, thanks for your welcome.

    JB
     
  12. Apr 7, 2010 #11
    Re: What does "When the Universe was one minute old" mean?

    Sorry about the time between my posts, life intervenes...

    If I understand correctly, there is disagreement and uncertainty as to how to quantify time in the very early stages of the universe (say, the Plank epoch).

    Re the "simplified model" marcus refers to (thanks, marcus): could someone post the differential equations or refer me to a website that has them?

    Re two observers at rest: not sure what that means. Could someone explain how two observers might not observe the same CMB temperature? Would it be by one of them accelerating to an extremely high velocity as compared with the other?

    ===============

    I'd like to describe how I've had to vulgarize the Big Bang for myself so that more knowledgeable posters here might tell me where my conception clashes with current theory. Then I might be able to incorporate the notion of time into it.

    Is it reasonable for me to conceive of the Big Bang as a singularity like a black hole (a white hole, actually)? To picture the history of the universe, I use the balloon analogy, in which a balloon expands from a singularity. I imagine myself as a two-dimensional creature on top of the balloon (its North Pole) and all my lines of sight (through 360°, which is why the CMB is observed all around us) go back in time, curving around a smaller and smaller balloon, back to the singularity. I believe this analogy is very commonly used. The measurement of time here would seem quite straightforward, as it would be the length of the curve divided by the speed of light. Except that as we approach the singularity, its density affects time and space. When we reverse the arrow of time, this is similar to falling into a black hole, isn't it? Maybe I read wrong, but I read somewhere that falling into a black hole actually takes forever. If that is the case, wouldn't the age of the universe be infinite?
     
  13. Apr 7, 2010 #12

    bapowell

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    Re: What does "When the Universe was one minute old" mean?

    Sure, or even just moving at constant velocity. If you are locally at rest with respect to the CMB (comoving with the expansion) you will measure a uniform temperature (modulo intrinsic anisotropies on the order of 1/100000). Now, if you are moving relative to the CMB, then the CMB photons in front of you will be blue shifted relative to those in the back, leading to a temperature profile like this:

    dipole-s.jpg

    This "dipole" anisotropy is still small relative to the average temperature of the CMB. For the Earth, moving at around 625 km/s relative to the CMB, it gives an effect on the order of 1/1000.
     
  14. Apr 7, 2010 #13

    DevilsAvocado

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    Re: What does "When the Universe was one minute old" mean?

    http://en.wikipedia.org/wiki/Friedmann_equations" [Broken]
     
    Last edited by a moderator: May 4, 2017
  15. Apr 7, 2010 #14
    Re: What does "When the Universe was one minute old" mean?

    Thanks both.

    bapowell, it seems that this gives meaning to "absolute motion." Intuitively, I would have thought that the apparent source of the CMB would have shifted slightly for the "moving" observer, making it appear as homogeneous as for the "at rest" observer. Has the "dipole anisotropy" been observed experimentally and reliably? If so, then this is a pretty major revelation for me.
     
  16. Apr 7, 2010 #15

    bapowell

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    Re: What does "When the Universe was one minute old" mean?

    Sure. The picture I included above is the actual dipole as seen by NASA's COBE satellite. However, we're still not talking about 'absolute motion' -- the CMB merely serves as a convenient frame of reference. Research "CMB dipole" or any of the CMB experiments (COBE, WMAP, etc) for more information.
     
  17. Apr 7, 2010 #16

    DevilsAvocado

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    Re: What does "When the Universe was one minute old" mean?

    Very interesting, does this in any way 'neutralize' the SR Inertial frame of reference?
    If the "CMB frame of reference" is available and the same everywhere in the universe, why can’t we use this as the "absolute standard reference frame"?
     
    Last edited by a moderator: Apr 25, 2017
  18. Apr 7, 2010 #17

    bapowell

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    Re: What does "When the Universe was one minute old" mean?

    Well, the SR inertial frame is not applicable in cosmology, due to the presence of gravity. In a homogeneous universe, however, observers comoving with the expansion are inertial observers in that their worldlines follow geodesics.
    It sort of has become the standard reference frame, since, as we have mentioned above in this post, it has become a common frame to use when talking about the age of the universe and other observer-dependent quantities. However, I still would avoid the word 'absolute', because, although convenient, the rest frame of the CMB is by no means physically preferred over an other frame.
     
  19. Apr 7, 2010 #18

    DevilsAvocado

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    Re: What does "When the Universe was one minute old" mean?

    Okay, thanks for info bapowell.
     
  20. Apr 7, 2010 #19

    DevilsAvocado

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    Re: What does "When the Universe was one minute old" mean?

    Don’t know if this is of any value to you (it’s not exactly the balloon analogy), but it definitely helped me to put my own little life in perspective to the CMB and the rest.

    The Known Universe Scientifically Rendered For All to See
    https://www.youtube.com/watch?v=<object width="640" height="505"><param name="movie" value="http://www.youtube.com/v/17jymDn0W6U&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/17jymDn0W6U&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="505"></embed></object>

    Go to http://www.youtube.com/watch?v=17jymDn0W6U&hd=1", it’s amazing.
     
    Last edited by a moderator: May 4, 2017
  21. Apr 7, 2010 #20

    DaveC426913

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    Re: What does "When the Universe was one minute old" mean?

    It's cool but I am still a fan of "Cosmic Zoom", even though it's a good 30 years old by now and needs to be updated.

    What was really cool about it was it went the other direction as well, down to the subatomic level.

    (In case the name doesn't ring a bell, the start and end of the film was at a human scale - of a mosquito on the arm of a person at a picnic in a park.)
     
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