
#37
Feb2812, 03:27 PM

P: 2,896





#38
Feb2812, 05:52 PM

PF Gold
P: 3,075





#39
Feb2912, 03:21 AM

P: 2,896





#40
Feb2912, 04:59 AM

Sci Advisor
PF Gold
P: 9,183

TD, it appears you are implying the universe is infinitely old and all the evidence accumulated to date strongly suggests we do not reside in such a universe.




#41
Feb2912, 05:17 AM

P: 2,896

Also note that the words infinitely and old can't be logically put together. 



#42
Feb2912, 02:32 PM

PF Gold
P: 3,075





#43
Feb2912, 04:40 PM

P: 2,896

This is the condition of the exercise. 



#44
Mar112, 08:29 AM

PF Gold
P: 3,075

And this is the answer to the exercise: if you do not give an age, then it makes no difference what the spacetime is (static or expanding), you can never get an H/He unless the latter has reached a steadystate value. I'm sorry, that's just perfectly obvious. So you have two choices, even within a static spacetime:
1) specify the age of the universe, and derive H/He from that. If the age is short (along the lines of our current age), you cannot answer it because it depends on the initial value assumed, since stellar nucleosynthesis hasn't had enough time to do much. If the age is very long, you'll have all iron. If the age is somewhere in between, stellar nucleosynthesis rates, and the age given, will determine H/He. 2) use an effectively infinite age, which is tantamount to the last possibility of #1. That is the answer to your exercise, and it's all been given above. I'm afraid I don't know what else you are looking for. 



#45
Mar112, 12:15 PM

P: 2,896





#46
Mar112, 02:39 PM

PF Gold
P: 3,075

Ah, typo I meant the second case in #1, not the last case. If the age is long enough to reach a steady state, then age doesn't matter, and that is equivalent to an infinite age, in regard to the question you are asking. The bottom line is, if a question is posed that does not specify the age, one must assume the age doesn't matter, which is always equivalent to assuming a steady state, which is always equivalent to an infinite age, which means the answer is "all iron."




#47
Mar212, 03:21 AM

P: 2,896





#48
Mar312, 04:20 PM

PF Gold
P: 3,075

We don't necessarily the "all iron" is not guaranteed in an expanding scenario, the density might eventually drop too low to make stars, and the H/He at that point would be "frozen in" for all time following, much as the H/He ratio was "frozen in" in the original Big Bang nucleosynthesis. So "all iron" is only the static noagegiven answer, whereas "maybe all iron, maybe some frozenin value of H/He" is the expanding answer. Some even think expansion might get so severe as to rip matter apart. So it's not clear what the asymptotic behavior of the expanding scenario actually is, because of the changes in the background spacetime.




#49
Mar412, 04:52 AM

P: 2,896

It turns out the answer "all iron" is wrong for a static spacetime because that would require time evolution of the universe which is a feature static spacetimes don't have globally. Thanks Ken G anyway, at least you tried.




#50
Mar412, 10:17 AM

PF Gold
P: 3,075

1) You are wrong. You are saying that because the spacetime is static, no time evolution in any physical variable is possible. Which theory does that come from? 2) Your original question is meaningless. You asked for the static H/He ratio, and now you are saying that no evolution of that ratio is possible. If you believe that, then obviously the H/He ratio in a static universe is set by the initial condition, which you did not specify. So take your pick your question has no answer, or has a simple answer that you don't believe. What a waste of time. 



#51
Mar412, 02:54 PM

P: 2,896





#52
Mar512, 01:04 AM

PF Gold
P: 3,075





#53
Mar512, 03:36 AM

P: 2,896

Yes, the cores of stars as isolated systems tend to p>n, so in the hypothetical static spacetime some mechanism should be compensating this, I guess. 



#54
Mar512, 08:09 AM

PF Gold
P: 3,075




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