Please expand my understanding of the early universe.
OK, we start with the big bang producing an expanding sphere of super heated energy. As it expands, it cools. E=MC squared says that matter starts to form in the cosmic soup, eventually producing stable hydrogen.
Is this a very basic but essential correct understanding of the early universe?
If yes, does this mean that we have to wait for stars to form before helium appears? I’ve been told that no elements heavier than iron can form without a sun’s super-nova explosion. Is this true? Could we have found heavier elements in the early universe?
No. Current theory says some helium was produced before stars. It’s called primordial helium. Supernova do produce heavier elements, but recently it’s been found that collisions of neutron stars do as well.
Here’s a Wikipedia article on big bang nucleosynthesis. It gives the proportion of He4 as about 25% based on several different models. And a very small amount of Lithium. But note that the proportion of Lithium produced then is a goodly amount of all the Lithium around now.
It’s fascinating to realize that the conditions inside a supernova are more conducive for heavier element production than the first 20 minutes after the big bang.
Up to the first 20 minutes, even though the temperature is hot enough for proton-proton (p-p) fusion to occur, there just isn’t enough density for much helium-helium (He-He) fusion, a.k.a the triple alpha process, and no heavier elements to support alpha process or heavy element fusion. The period after this, from S+20 minutes to S+240 kyr (sometimes called the “Photon Epoch” or “Opaque Era”) is dominated by photon pressure which prevents ionized hydrogen and helium ions from capturing electrons, and thus electrostatic repulsion exceeds gravitational attraction and there is virtually no fusion to power nucleosynthesis. At some point in this era matter starts to form the origins of the large scale structures we see today for reasons not fully understood but attributed to weakly interacting “dark matter” that is not affected by light pressure but at ‘local’ scales is too homogenous to form stars and galaxies.
After the universe cooled sufficiently from expansion and becomes transparent, electrons joined with the primordial hydrogen and helium ions to form stable matter that is gravity dominated, and permitted the accumulation of gigantic metal-poor stars with short lifetimes whose hypernovae produced the the bulk of the heavier elements in the universe, specifically carbon, nitrogen, oxygen, which make Earth life possible today, as well as neon, silicon, zinc, nickel, iron, er cetera which allow rocky planetoids to form. It used to be though thst this era of Population III stars only formed several hundred million years after the singularity but several current hypothesis place the origins of heavy matter and stellar firmation further back, possibly starting only a few tens of millions of years after transparency. The bulk of elements heavier than iron, however, had to wait for the second generation of stellar formation and other processes (neutron star collision, cosmic ray spallation, other mechanisms) and the formation of galaxies and galactic clusters to condense matter into tight formations.
While it was previously assumed that most heavy metals were produced directly by supernovae nucleosynthesis, more recent modelling has shown that much of the r-process production occurs in supernova remnants (neutron stars with their associated shock wave and in the accretion discs of black holes) and s-process in asymtopic giant branch stars. So while supernovae produce the necessary conditions for high mass element nucleosynthesis, most of the element production occurs after the actual supernova event, although a lot of elements up to iron are produced by silicon burning as an immediate precursor to core collapse and Type II supernova explosion.
It should also be mentioned that the relative abundances of the isotopes of hydrogen, helium, lithium, and I think beryllium (all of which except for He-4 were formed predominantly in the Big Bang) can be used to determine very well just what the density of baryons (protons and neutrons) was in the early Universe. And that density can be used to calculate their density today, which is how we know that most of the matter in the Universe is not baryonic.