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Author Topic: Why didn't the big bang produce heavy elements?  (Read 6937 times)

Nathaniel Toothaker

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Why didn't the big bang produce heavy elements?
« on: 23/05/2011 08:30:03 »
Nathaniel Toothaker asked the Naked Scientists:
Hi Naked Scientists,

I downloaded every podcast you guys have put out and have been listening to 2 episodes each day every day at work for months!  I'm currently through September of 2009, so I might not hear the answer to this question for quite some time, but this is what I was wondering...

According to the model I looked at, the big bang produced giant clouds comprised of almost exclusively hydrogen and helium, and from these clouds formed galaxies and the other celestial objects.  

I know that heavy metals are formed when a sufficiently large star goes supernova because of the intense heat and pressure which is necessary to form these elements.  

Here's my question.  

The big bang had to have been the hottest and densest of any explosion in the history of the universe, so why did it form primarily hydrogen and helium instead of instantly fusing most of these elements together to form heavy metals?
Nathaniel S Toothaker
Lewiston, ME

What do you think?
« Last Edit: 23/05/2011 08:30:03 by _system »


Offline yor_on

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Why didn't the big bang produce heavy elements?
« Reply #1 on: 27/05/2011 09:23:23 »
Another tricky question.

"Hydrogen is the most common element in the universe, 90% of all matter exists in the form of hydrogen. Helium constitutes the other 10% of matter, which is created from hydrogen through nuclear fusion reactions. It is the fuel of the stars as well as the element from which all other elements are created."

So Hydrogen should have came before Helium. Hydrogen contains one proton and one electron, making it is the most simple atom. It is expected to have been created in the first three minutes after the Big Bang and all elements heavier than helium are made in stars.

We think the reason how particles came to be has to do with the spontaneous pair production at high energies where two photons can collide to form a proton-antiproton pair. Due to some 'cosmic constant' possibly it seems as if there is a slight bias for conserving protons and as the temperature had cooled to below 10 trillion degrees Kelvin the spontaneous pair production stopped leaving us with lots of photons, a few protons, and no anti-protons.

The numbers looks like this, for each billion 'pairs' created you will get one proton left in the annihilation, created as the proton anti-proton interacts. The same goes for the Neutrons, they too was created around those temperatures and 'freeze out' not being created any more as the temperature sinks under 10 trillion degrees Kelvin. Electrons and positrons, only 1/2000 as massive as a proton, are still produced though and annihilated continuously until the universe drops to the much lower temperature around 6 billion degrees Kelvin. And the same number seems to rule in the electron positron pair production, of 1 billion pairs annihilation you will get one electron left. And as the temperature keeps falling those elements will start to fuse into nucleus's.

The change comes about around below 1.2 billion degrees Kelvin where we expect it to have gone around 2 minutes from the Big Bang. At that temperature the energy keeping the deuterium (hydrogen nucleus) apart is not enough any longer. So the proton and neutron starts to fuse together by the strong nuclear force.

So the reason seems to rest in how elements gets created and at what energies/temperatures they can come to be. The rest of the elements are then created in the stars created from this hydrogen with helium becoming the first element created in there. And looking at the oldest stars we know, we can see that the chemical composition seems to agree with what we would expect, consisting of 75% hydrogen and 25% helium, with only traces of other elements.

There is one thing more, those very old star, made out of hydrogen primary should all be dead by now. As they die they run out of hydrogen fuel and start to fuse together heavier and heavier elements, with some really big stars in their death throes going super-nova exploding and throwing those elements far and wide. The second generation of stars, as our sun might be, get those elements from the start as they condense from the enriched gas clouds. Our sun contain traces of iron which it can't have created by itself making it into a descendant from those first stars.
« Last Edit: 27/05/2011 11:23:30 by yor_on »

Offline Soul Surfer

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Why didn't the big bang produce heavy elements?
« Reply #2 on: 29/05/2011 23:31:11 »
The simple answer to this question is that there was not enough time for the nuclei to fuse to create the heavy elements.  the big bang expansion produced a small amount of deuterium which was being used up almost as fast as it was being produced quite a lot of helium and a tiny amount of lithium.  There was probably absolutely minuscule qualities of slightly heavier elements but by that time everything had cooled off too much.  This was a few minutes into the expansion.  Fred Hoyle did all the classic calculations and this is one of the strongest clues that the big bang model is correct.

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Why didn't the big bang produce heavy elements?
« Reply #2 on: 29/05/2011 23:31:11 »


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