Primordial Nucleosynthesis ?

[Widdekind]

The binding energy per nucleon of Lithium is ~2MeV greater than for Helium. And, observations show that Lithium is scarce. Do these facts imply, that when primordial fusion was occurring, the ambient temperature was <2MeV (so that the "energy hill" from Helium to Lithium was not crossable)? The binding energy of Deuterium, the first step for all future fusion, is ~2MeV per nucleon. "Naively", Deuterium would begin to be stable, when the ambient temperature dropped below kT ~ 2MeV. Qualitatively, all the "naive numbers" point to ~2MeV as the relevant temperature value.

 

Now, 2MeV ~ 20e9 K. And, if primordial fusion occurred at giga-Kelvin temperatures, then allot less Lithium would have been produced. So, would "hot primordial fusion" be able to account, for the scarcity, of Lithium?? The presence of "more neutrons" during primordial fusion would be able to accommodate observations (the .ppt document). And, at giga-Kelvin temperatures, more neutrons, relative to protons, remain stable.

 

Also, the neutron-proton mass-energy difference is ~1MeV. So, "all the numbers seem to point to the 1-2MeV regime". If so, then neutrons would have been (nearly) as abundant as protons. Could "hot neutron-rich" fusion account for observations??

 

http://www.astronomy.ohio-state.edu/~dhw/A682/p6.pdf

http://www.astro.rug.nl/~hidding/ao/energy_big.jpg

http://www.ioffe.ru/astro/QC/img/abund_t9_big.gif

insti.physics.sunysb.edu/~meade/phy599/vonhippelslides.pptx

Edited October 6, 2012 by Widdekind

[Widdekind]

[math]p^{+} + e^{-} + 1MeV \rightarrow n^0 + \nu_e[/math]

 

Ergo, when the sum of thermal (KE) energies of the particles on the LHS, can no longer supply the ~1MeV of energy, then the reaction becomes quenched, and the neutron:proton ratio is frozen in. If two particles must supply 1MeV, then the quenching temperature would be about kT~1/2 MeV. At that temperature, the Boltzmann factor e^-E/kT would be about e^-2 ~ 1/7. And the observed n:p ratio is ~1/7. So, again, do not all the seemingly relevant energies point towards a Primordial Nucleosynthesis temperature regime, of ~1/2-2MeV?

 

Seemingly interestingly, the initial deuterium binding energy (~2MeV) forestalls fusion at higher temperatures. But then the "Lithium hill" (~2MeV per nucleon from He to Li) blocks any fusion that does occur, from hurdling that hill, and fusing on to CNO. Meanwhile, once fusion onsets (~2MeV), neutrons are commonly formed, until ~1/2MeV, so that fusion occurs in a neutron-rich environment. Below ~1/2MeV, neutron (and positron pair) production are quenched. Inexpertly, higher temperatures combined with more neutrons would both reduce Lithium production, bringing theory closer to observations.

[Widdekind]

If primordial fusion did happen to occur at ~1MeV; then primordial fusion occurred in a temperature regime not since repeated, by any natural systems. For, even the most massive stars, which burn Silicon into Nickel (which decays into Iron), operate at temperatures <1/2MeV. Thus, if Big Bang fusion occurred at ~1MeV, then Big Bang nucleosynthesis occurred at "supra-natural" temperatures.

 

http://en.wikipedia.org/wiki/Silicon-burning_process

[Enthalpy]

Primordial nucleosynthesis was not an equilibrium. That's why only light nuclides were produced, instead of just iron. So you can forget all arguments comparing temperature with reaction energy.

 

I suppose the part of the big bang hot enough was too short to achieve any equilibrium - but ask a specialist.

 

...the "Lithium hill" (~2MeV per nucleon from He to Li) blocks any fusion that does occur, from hurdling that hill, and fusing on to CNO. Meanwhile, once fusion onsets (~2MeV)...

Fusion from helium to carbon and oxygen does not pass through lithium in stars. There are easier paths.

Fusion happens widely before 2 MeV in stars. 15 MK in a normal yellow star makes 1.3 keV.

Edited October 7, 2012 by Enthalpy

[Widdekind]

Primordial nucleosynthesis was not an equilibrium. That's why only light nuclides were produced, instead of just iron. So you can forget all arguments comparing temperature with reaction energy.

 

I suppose the part of the big bang hot enough was too short to achieve any equilibrium - but ask a specialist.

 

 

Fusion from helium to carbon and oxygen does not pass through lithium in stars. There are easier paths.

Fusion happens widely before 2 MeV in stars. 15 MK in a normal yellow star makes 1.3 keV.

 

First, the number ratio of H:He is approximately 12:1. Triple and quadruple "alpha particle" (He) collisions would have been unlikely. And, in the absence of any accumulated C,O, the catalytic CNO cycle cannot run. Are those the "easier paths" to which you refer?

 

Second, temperatures after the Big Bang dropped from "super high"; so the word "before" would apply, to higher temperatures, not lower. Perhaps you are suggesting, that fusion could have continued to occur, on down until colder later epochs?

 

Third, primordial plasma was only "singed", not "burnt", in the sense that >90% of the H remained unfused (thereby remaining for future generations of stars).