Big bang question

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bjn
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Big bang question

Post by bjn » Fri Dec 17, 2021 6:18 pm

I have a question about the big bang. Why didn't all the stupidly densely packed matter fuse into elements heavier than lithium? The temperatures and density would have been much hotter than a super-nova, so surely we should be seeing primordial elements of all masses? What key bit of science don't I understand here.

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basementer
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Re: Big bang question

Post by basementer » Fri Dec 17, 2021 6:20 pm

It wasn't anything that you'd recognise as matter at that time.
Money is just a substitute for luck anyway. - Tom Siddell

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shpalman
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Re: Big bang question

Post by shpalman » Fri Dec 17, 2021 6:37 pm

bjn wrote:
Fri Dec 17, 2021 6:18 pm
I have a question about the big bang. Why didn't all the stupidly densely packed matter fuse into elements heavier than lithium? The temperatures and density would have been much hotter than a super-nova, so surely we should be seeing primordial elements of all masses? What key bit of science don't I understand here.
Big Bang nucleosynthesis produced very few nuclei of elements heavier than lithium due to a bottleneck: the absence of a stable nucleus with 8 or 5 nucleons.
This deficit of larger atoms also limited the amounts of lithium-7 produced during BBN. In stars, the bottleneck is passed by triple collisions of helium-4 nuclei, producing carbon (the triple-alpha process). However, this process is very slow and requires much higher densities, taking tens of thousands of years to convert a significant amount of helium to carbon in stars, and therefore it made a negligible contribution in the minutes following the Big Bang.
Also bear in mind that any kind of reaction which is favoured, i.e. leads to a lower energy state than the one it started from, needs to be able to get rid of that energy. Let's say you had two particles in a perfectly sealed box and when they react they join together and emit a photon. Well, then the photon bounces around in the box and eventually hits the joined particles and gives them the energy they need to split apart again.

All that temperature is a bit useless if you don't have anywhere you can dump the entropy.
molto tricky

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bjn
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Re: Big bang question

Post by bjn » Fri Dec 17, 2021 6:42 pm

Thanks!

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