After Big Bang: Hydrogen & Helium Formed, But Why Didn't They Burn?

[Einstein]

Hydrogen and Helium were the main constituents formed soon (thousands of years?) after the big bang. Why did they not burn (as they are flammable), I am sure the universe was hot enough then to ignite it?

 

[marcus]

Originally posted by Einstein
Hydrogen and Helium were the main constituents formed soon (thousands of years?) after the big bang. Why did they not burn (as they are flammable), I am sure the universe was hot enough then to ignite it?

hydrogen and helium are the main constituents of the sun at the present time

and the sun also is hot

so one way you could think about the problem you have raised
is to ask why the sun does not immediately burn up
Wouldn't you say the sun was remarkably stable?
It seems so to me anyway.
Whatever it is doing it seems to be doing it very very slowly,
only a little bit (percentagewise) at a time.

Perhaps whatever regulates the sun and prevents it from
immediately consuming itself in some reaction
also (by some analogous mechanism) regulated the
universe a long time ago

 

[quantum]

Also, helium isn't flammable.

 

[russ_watters]

Originally posted by Einstein
Hydrogen and Helium were the main constituents formed soon (thousands of years?) after the big bang. Why did they not burn (as they are flammable), I am sure the universe was hot enough then to ignite it?

Hydrogen burns if you combine it with oxygen. There was not then nor is there now a large amount of oxygen (percentagewise) in the universe.

 

[Tyger]

Originally posted by Einstein
Hydrogen and Helium were the main constituents formed soon (thousands of years?) after the big bang. Why did they not burn (as they are flammable), I am sure the universe was hot enough then to ignite it?

When Hydrogen fuses in the Sun the process involves the release of neutrinos, which are very rare events, so the Sun uses its Hydrogen very slowly. The process is slow enough that the Universe could cool down before significant quantitites of Hydrogen were converted after the Big Bang.

 

[Simfish]

Hydrogen, Helium, and small quantites of Lithium were the only elements that lasted from the Big Bang to until stars first formed. All elements with the exception of these three elements were formed from the fusion of atomic nuclei in stars, and before stars existed, these were the only elements that existed.

Additionally, burning is a chemical reaction that requires the exchange of electrons to occur. Up until 300,000 years after the big bang, electrons were free particles, and there was too much energy for electrons to be brought into orbits around hydrogen, helium, and lithium nuclei. If an electron encountered a nuclei, it would simply collide but would not be placed in orbit around the nuclei of the hydrogen/helium/lithium atom. However, when the universe cooled 300,000 years after the Big Bang, electrons then started orbiting hydrogen, helim, and lithium nuclei; and the universe became transparent. However, oxygen would never form until stars came into being.

Additionally; helium is a noble gas; it does not combine naturally.

 

[Phobos]

Welcome to Physics Forums, Einstein & Simfishy!

Good topic, everyone.