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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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Nebahera t1_iuloqgx wrote

Additionally, the term Big Bang was coined by Fred Hoyle who was sternly denying the theory of an expanding universe that had a beginning. He mocked the theory and dissmissed it as a "big bang".

As more and more evidence suggested that it indeed seems to be true, cosmologists around the world adapted Hoyle's mocking term, as it actually had a catchy cling to it.

So the big bang is actually an ironic name for the phenomenon it theorizes.

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palemon88 t1_iuo0pew wrote

Replying to tea_and_biology (#416,392)

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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MarcusMacG t1_iuo84ub wrote

In the original paper on Big Bang Nucleosynthesis by Alpher, Bethe, and Gamow all the elements were formed in the Big Bang. As Stellar Nucleosynthesis became popular it grabbed the creation of heavier elements. Lithium hung around for awhile, but eventually BBN took it because SN didn't want it. There is no distinct reason for larger atoms to not be primordial other than our model is tweaked to fit with other models. Our model predicts population III stars, ones made out of hydrogen and helium devoid of heavy elements, but we don't see any so our model is wrong.

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the-channigan t1_iuoluhy wrote

Replying to MarcusMacG (#420,464)

Just a reminder that in the paper referenced Bethe didn’t really do anything. Gamow thought it would be fun to add him in to create a whimsical reference to the three types of nuclear radiation. Alpher, the junior researcher on the paper, was not best pleased with this as he foresaw he would lose credit with two more senior eminent physicists on the author list.

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nivlark t1_iuowfau wrote

Replying to NEYO8uw11qgD0J (#418,505)

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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nivlark t1_iuox49u wrote

Replying to palemon88 (#420,116)

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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brigandr t1_iuqbiwg wrote

Replying to Sector-Feeling (#424,194)

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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