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tea_and_biology t1_iuljyvz wrote

Well, first off, the big bang wasn't an explosion. The rapid expansion of space-time and condensed material and a legit boom-boom explosion are rather different things. Further, the (much) higher energy density in the moments immediately after the Big Bang hindered the process of element formation, rather than helping it.

At over a billion or so degrees kelvin, both protons and neutrons are too energetic to bind. Only once things had begun to expand and cool below this threshold, after about ~2 minutes, could they fuse, resulting in hydrogen (^(1)H) nucleosynthesis, and subsequent fusion into deuterium (^(2)H) and helium-4 (^(4)He). But if it gets too cool, fusion and nucleosynthesis stops altogether - so really, there was a critical period between approximately ~3 and ~20 minutes just after the Big Bang for all original element synthesis to happen.

After this short window, everything has cooled off, and you have a universe where ~25% of the mass is ^(4)He, and the other ~75% mostly ^(1)H - with teeny weeny dribs and drabs of ^(2)H, ^(3)He and lithium (^(7)Li) here and there. Given this composition, the only reactions that could form any heavier elements therefore include:

>^(1)H + ^(4)He → ...

>^(4)He + ^(4)He → ...

... but neither produces stable nuclei. There's only:

>^(2)H + ^(7)Li → ^(9)Be

>^(4)He + ^(7)Li → ^(11)B

But given lithium was so scare, these reactions were incredibly unlikely. Trying to build any heavier elements now becomes essentially impossible - the universe is too cool, and the stuff that's in it isn't super useful.

We needed to wait a helluva' long time for the first stars to begin forming things up to carbon (via the triple alpha process, over tens of thousands of years: ^(4)He + ^(4)He → ^(8)Be + ^(4)He → ^(12)C), subsequent fusion to get up to iron, and then supernovae for everything else. The secret ingredient here was time - stars can afford to wait and build up their cupboard of ingredients to get the fun recipes going, something the primordial universe lacked.

In short: During the Big Bang, it was too hot and dense for anything to form beyond the simplest gases, and then quickly became too cool for anything else to appear. Stars, by contrast, have plenty of time on their hands.

References & Further Reading:

Coc, A., Uzan, J.P. & Vangioni, E. (2014) Standard big bang nucleosynthesis and primordial CNO abundances after Planck. Journal of Cosmology and Astroparticle Physics. (10)

"Big Bang nucleosynthesis". Wikipedia article.

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omigodd OP t1_iull6rx wrote

Wow. Thank you for such a detailed reply. Makes a lot of sense

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NEYO8uw11qgD0J t1_iun3kwm wrote

So if the cooling process were longer, we'd have more heavier elements outright rather than waiting for stellar evolution to provide the rest, correct?

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Shufflepants t1_iun7vlf wrote

Would have to be longer specifically within the right temperature and pressure range, but yes.

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nivlark t1_iuowfau wrote

Not necessarily, because the restrictions they described would still exist: lithium would still be incredibly scarce, and the conditions simply aren't extreme enough to get the triple-alpha process running, counter-intuitive as that may sound. The problem here is that fusing two ^(4)He nuclei produces ^(8)Be, which has an astonishingly short half-life of one ten thousand trillionth of a second. Only inside the core of a massive star is the reaction rate high enough to fuse a third helium nucleus to make stable ^(12)C before the ^(8)Be falls apart.

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omigodd OP t1_iuo0qcp wrote

Alternatively, if Beryllium-8 was stable, we could have had heavier elements

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Sector-Feeling t1_iuq6q38 wrote

It's been a while since I've learned anything about the Big Bang, could you refresh me as to what occupied space before the Big Bang?

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brigandr t1_iuqbiwg wrote

That’s where the explosion analogy breaks down. At the start of the Big Bang, the universe was in an incredibly hot and dense state. If (as is commonly suspected) the universe is infinite, it was already infinite at that moment. The “bang” in question was an extremely rapid expansion of space in that hot, dense state.

The same stuff occupied the space in the universe then as it does now, just space has expanded by an incredible amount so that everything is farther apart than it used to be.

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palemon88 t1_iuo0pew wrote

Thanks. I got an irrelevant question after reading your answer though. How can you say something after big bang took X minutes if the time bends with gravity and such. Wasn’t the whole universe there after 3 minutes of big bang?

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nivlark t1_iuox49u wrote

Gravity measurably affects the passage of time for observers at different gravitational potentials. So a clock at the centre of the Earth would run slower than one on the surface. But the early Universe was almost perfectly uniform, so while it was extremely dense, the gravitational potential was equally close to being uniform and so there was no time dilation.

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z3n0mal4 t1_iuoldc5 wrote

Ok, from mammals and fish fins and fully aquatic dinosaurs to this … hats off to you, sir

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[deleted] t1_ium0avm wrote

[removed]

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Sharlinator t1_ium2pgz wrote

We're talking visible, baryonic matter. Dark matter and energy are irrelevant here.

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