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Thread: Timeline of early universe

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    Timeline of early universe

    When astrophysicists speak about the timeline of the early universe, is time dilation considered? For instance, it took roughly 3 minutes to go from the big bang to when the first protons developed. What is the reference of that 3 minutes? Is that 3 "Earth" minutes or 3 minutes in that specific environment?

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    Time dilation is a relative thing. It refers to what happens in one frame of reference as observed from another frame of reference. It isn't really meaningful to try to apply it to what is essentially the entirety of the universe.

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    Welcome to the forum Josh G, and the answer to your question is actually "neither", but it's an astute question all the same. The kind of time that is generally used to talk about the history of the universe is the time that would be registered on a clock that is "comoving" with the general matter in the universe around it, so basically the time your awareness would register were it possible for you to have been around since the beginning. So it's not 3 Earth minutes (those are minutes your awareness registers in the very different gravitational environment that is the Earth) and it's not 3 minutes in the gravitational environment where nuclei are forming (that environment is also changing), it's 3 minutes elapsed over the changing environments from "the beginning" to the formation of nuclei.

    Now, if you think about this and object to the idea that any kind of clock could exist in environments at or near "the beginning", then you are a good scientific thinker indeed-- we really don't know how to extrapolate the concept of time I just described all the way back into environments for which we have no physical understanding, and that's why the entire notion of "the beginning" is rather nebulous. So instead, we "start the clock" at whatever age of the universe we think we can support a concept of a clock (i.e., whenever we think we understand the basic physics of elapsing time in the environment in question), and that time is very very short indeed, so 3 minutes of elapsed time is a concept we think we understand pretty well even in those high energy environments.

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    The time used in these discussions is called cosmic time - the time that would be recorded by clocks at rest in the Hubble flow in a homogeneous Universe (so moving with expanding space, and embedded in a region of average density for that epoch).
    You could, in principle, compare these clocks with (say) a clock that was moving very quickly relative to the Hubble flow, or a clock that was sitting very close to a mass concentration nearby, and see the effect of time dilation. But there's no way you can take one of those clocks and bring it to here-and-now to compare it a present-day Earth clock. So there isn't a way to compare elapsed time then and now.

    Grant Hutchison

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    Quote Originally Posted by Ken G View Post
    So instead, we "start the clock" at whatever age of the universe we think we can support a concept of a clock (i.e., whenever we think we understand the basic physics of elapsing time in the environment in question), and that time is very very short indeed, so 3 minutes of elapsed time is a concept we think we understand pretty well even in those high energy environments.
    It may be helpful to restate it as beginning with a specific event (i.e. proton formation) and see about how long it takes to continue backward in time until reaching close to t=0. I assume the physics' models include t such that it equates to about 3 minutes to get to those first few Planck units of time just before the wheels go flying off the model. Is this right?
    Last edited by George; 2019-Dec-04 at 03:38 PM.
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    It may be helpful to restate it as beginning with a specific event (i.e. proton formation) and see about how long it takes to continue backward in time until reaching close to t=0. I assume the physics' models include t such that it equates to about 3 minutes to get to those first few Planck units of time just before the wheels go flying off the model. Is this right?
    Yes, though I'm not sure the wheels don't fly off before you get to the Planck time. That's only where we know the wheels fly off, but there could be problems we don't know about when the energies get way past anything we've seen. Indeed, inflation is just one such effect.

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    Quote Originally Posted by Ken G View Post
    Yes, though I'm not sure the wheels don't fly off before you get to the Planck time. That's only where we know the wheels fly off, but there could be problems we don't know about when the energies get way past anything we've seen. Indeed, inflation is just one such effect.
    Yes, but in the 3 minute realm, don't most the models' wheels roll smoothly say to t = 1 sec, though I think most Inflation models have that phase ending at around 10-32 sec. If Inflation can be eliminated, perhaps the time for that phase of expansion would be greater but not by much, probably, right?
    We know time flies, we just can't see its wings.

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    Oh yes, that's what I meant by the time we feel we understand being much much less than 3 minutes. There should be no problem going back to a fraction of a second after the beginning.
    Last edited by Ken G; 2019-Dec-05 at 05:06 PM.

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    Quote Originally Posted by Josh G View Post
    For instance, it took roughly 3 minutes to go from the big bang to when the first protons developed.....
    Um, no, I believe the first protons were created well before 3 minutes. The 3 minute mark is when the Universe was cool enough for protons/neutrons to stick together after colliding, forming atomic nuclei (mainly helium nuclei).
    Everyone is entitled to his own opinion, but not his own facts.

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    According to wiki, protons started forming after about one second, and nucleo-synthesis started around two minutes.

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