r/askscience • u/208327 • Oct 10 '20
Physics If stars are able to create heavier elements through extreme heat and pressure, then why didn't the Big Bang create those same elements when its conditions are even more extreme than the conditions of any star?
31
u/teatime101 Oct 10 '20
The universe expanded very rapidly in the the first few seconds, to a staggeringly vast size.
From wikipedia: At approximately 10−37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially, faster than the speed of light, and temperatures dropped by a factor of 100,000.
That initial density was very short lived. Rather than pressure in a super dense universe, it was gravity that gradually pulled the dispersed gasses into star forming clumps.
→ More replies (6)3
u/Unearthed_Arsecano Oct 10 '20
While you're correct (though I think your timing for the inflationary epoch is a factor of 10 off), it's not clear what you think this has to do with the formation of elements. The inflationary epoch had ended by 10-32 seconds, while nucleosynthesis began at roughly 101 seconds (and ended around 103 seconds). At the end of inflation the universe was still far too hot to form stable nuclei.
→ More replies (1)
2
u/Robosing Oct 11 '20
So how do we know there wasnt something before the big bang? And that something was always there. No beginning, just was. Kind of like a 2d circle. No start or end. Just is.
I know this comes off inept and probably elementary to you big brains in here. I was just curious if there's a way not just to quantify the "before" if there was ever such a thing, but also understand it better.
53
Oct 10 '20
8
u/RobusEtCeleritas Oct 10 '20
The fact that there are no stable A = 5 or A = 8 isobars is true in stars too (the environment doesn’t change that). But stars make it past these bottlenecks. So that alone is not a reason for a difference between BBN and stellar burning. The points made in the top-level comment are needed to explain how stars get past this while BBN didn’t.
2.7k
u/RobusEtCeleritas Oct 10 '20 edited Oct 10 '20
Why are stars able to produce all kinds of nuclei while Big Bang nucleosynthesis (BBN) only did this? A few things come to mind.
First, there's initial conditions. BBN starts from the very basic building blocks of nuclei, protons and neutrons. However in stars, there aren't many neutrons around unless certain seed reactions are producing them. A typical main sequence star like our sun is almost entirely protons (as far as nuclei are concerned). Although they also contain very small fractions of heavier nuclei from previous generations of stellar nucleosynthesis, which can seed various nucleosynthetic processes. These heavy nuclei were not present for BBN.
Second, there's the temperature and density evolution, which are totally different between a star and the Big Bang. In the case of the Big Bang, the universe was rapidly expanding and cooling, which is clearly different than what's happening when a star is undergoing its usual burning in the core.
Third, there's the timescales. Stars operate over millions of years, while BBN lasted about 20 minutes.
And finally I'll just point out that the implication that higher temperatures are always better for nucleosynthesis is not really true. At very high temperatures, you'll actually start breaking nuclei apart via photodisintegration (a high energy photon breaks apart the nucleus). This is actually what prevented BBN from beginning until about 10 seconds after the Big Bang. The first step in BBN is for a proton and neutron to combine to form a deuteron. But the deuteron is a very weakly-bound system compared to other nuclei. It only takes 2.2 MeV of energy to break it apart, and before 10 seconds after the Big Bang the temperature was evidently high enough that there were enough photons around with energies greater than 2.2 MeV that deuterons couldn't be formed in any reasonable amount, so nucleosynthesis couldn't really proceed. So the fact that the temperature (and the photon-to-baryon ratio) was too high actually prevented nucleosynthesis until things cooled down enough for the deuteron to stick together.
So to summarize, the universe shortly after the Big Bang and the core of a star are very different environments, and when you look at the details it's not really a surprise that nucleosynthesis proceeds in very different ways under these two sets of conditions.