# Why didn't the Big Bang create heavy elements?

In the case of a supernova explosion it is possible to create heavy elements through fusion. Supernovae have a tremendous amount of energy in a very small volume but not as much energy per volume as there was in our early universe. So, what is the major difference? Why didn't the Big Bang create heavy elements?

• The Big Bang was not an explosion, but Supernovae are. – Zach466920 Aug 10 '15 at 17:36
• It takes thousands of years to make carbon and allow for heavier elements to form in stars, which are needed for the production of very heavy elements during a supernova. The Big Bang Nucleosynthesis, in which all of the original elements were first formed, lasted only minutes. Not long enough to produce heavy elements. This is directly from Wikipedia – Jim Aug 10 '15 at 17:49
• – Kyle Kanos Aug 10 '15 at 17:55
• while that's true, it's not really an answer. Which is why he didn't post it as an answer :P – jhocking Aug 11 '15 at 13:42
• @jhocking but OP didn't even suggest it was an "explosion", so it wasn't really relevant to mention. Doesn't help explain the answer either. Was really just a "I'm smarter than you" comment that attracted a lot of upvotes. – Señor O Aug 11 '15 at 19:29

Heavy elements couldn't form right after the Big Bang because there aren't any stable nuclei with 5 or 8 nucleons.

Source: Wikipedia (user Pamputt)

In the Big Bang nucleosynthesis, the main product was $^4He$, because it is the most stable light isotope: 20 minutes after the Big Bang, helium-4 represented about 25% of the mass of the Universe, and the rest was mostly $^1H$. There was only 1 nucleus of deuterium and helium-3 for each $10^5$ protons, and 1 nucleus of $^7Li$ for each $10^9$ protons.

Given these abundances, the most probable reactions to yield heavier elements would be $^1H + {}^4He$ and $^4He + {}^4He$, but neither produces stable nuclei. So instead we have only $^2H + {}^7Li \to {}^9Be$ and $^4He + {}^7Li \to {}^{11}B$. This reactions are extremely unlikely, since lithium was so scarce. It is predicted that one of these nuclei was form for $10^{16}$ protons. Abundance of the previous elements and cooling of the universe prevented the formation of even heavier elements.

On the other hand, in the first stars carbon formed in the triple alpha process, which is only possible with the density and helium abundance found in stars, and takes a lot of time. Subsequent nuclear fusions create heavier elements up to iron, and the energy released in the supernova explosion allows the synthesis of even heavier elements.

## References

Alain Coc, Jean-Philippe Uzan, Elisabeth Vangioni: Standard big bang nucleosynthesis and primordial CNO Abundances after Planck JCAP10(2014)050 arxiv:1403.6694

• I think this diagram is very cool - is there a diagram like this for all possible elements and isotopes rather than only ones involved in the big bang? – Random832 Aug 11 '15 at 3:56
• @Random832 There's too many to fit in a diagram nicely; you can usually find anything particular quite easily, for example the fusion products in stars. Wikipiedia has many. – Luaan Aug 11 '15 at 9:15
• For some reason, "triple alpha process" sounds like some kind of certification scheme. – David Richerby Aug 11 '15 at 12:30
• My understanding of the processes in a star are that fusion relies on pressure as much as temperature, it's not enough to have atoms be near, you need lots of atoms being shoved together to overcome the improbability of fusion happening? – Kaithar Aug 11 '15 at 20:20
• @Kaithar it is also having the right types of atoms together. – user57109 Aug 11 '15 at 22:52

In the case of a supernova explosion it is possible to create heavy elements through fusion. Supernovae have a tremendous amount of energy in a very small volume but not as much energy per volume as there was in our early universe. So, what is the major difference? Why didn't the Big Bang create heavy elements?

I just want to point out, too much energy hurts the building of elements process, it doesn't help.

http://www.physicsoftheuniverse.com/topics_bigbang_timeline.html

Nucleosynthesis, from 3 minutes to 20 minutes: The temperature of the universe falls to the point (about a billion degrees) where atomic nuclei can begin to form as protons and neutrons combine through nuclear fusion to form the nuclei of the simple elements of hydrogen, helium and lithium. After about 20 minutes, the temperature and density of the universe has fallen to the point where nuclear fusion cannot continue.

So, over a billion degrees, protons and neutrons are too energetic to bind. Under a billion, they can begin to fuse and you begin to get hydrogen fusion into deuterium and helium.

But, there's a problem,

https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis

As the universe expands, it cools. Free neutrons and protons are less stable than helium nuclei, and the protons and neutrons have a strong tendency to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium. Before nucleosynthesis began, the temperature was high enough for many photons to have energy greater than the binding energy of deuterium; therefore any deuterium that was formed was immediately destroyed (a situation known as the deuterium bottleneck). Hence, the formation of helium-4 is delayed until the universe became cool enough for deuterium to survive (at about T = 0.1 MeV); after which there was a sudden burst of element formation. However, very shortly thereafter, at twenty minutes after the Big Bang, the universe became too cool for any further nuclear fusion and nucleosynthesis to occur. At this point, the elemental abundances were nearly fixed

• That last paragraph seems to really hit the nail on the head. – Alex Aug 10 '15 at 19:01
• GREAT ANSWER: many thanks !! I read Steven Weinberg's book, THE FIRST THREE MINUTES, several years ago, but forgot many of the details ... thanks for the refresher-course !! – PERFESSER CREEK-WATER Jun 27 '16 at 20:25

This question is answered in detail by the so-called "Big Bang Nucleosynthesis", the theory about the creation of the nuclei in the early Universe. Almost out of nothing, it allows one to determine that 75% of the nuclear mass was coming in hydrogen, 25% in helium, and some small traces of lithium appeared, too.

Even though Gamow used to think that all elements could have been created in the Big Bang, Alpher and Herman quickly showed otherwise. The reason why the heavier elements can't be created in the Big Bang is that elements with masses above 56 require neutron capture to be created.

Supernovae are a great environment for neutron capture. However, after the Big Bang, the density of neutrons goes down as the Universe expands; and after much more 10 minutes, the lifetime, the neutrons decay away. There isn't enough time to create the heavier elements.

So we're left with the composition created without processes like neutron capture. And those favor the energetically optimized nuclei such as the light three. The issue is really that it's not quite "high temperature" that is needed for the creation of heavy nuclei. The high temperature is "good" for the creation of energetically wasteful bound state; but it is also "good" for their destruction. The Big Bang is a process in which the temperature is going down so at the end, the energetically thrifty bound states (with higher binding energy) dominate.

• So if all atoms in the universe started out of lead atoms, would they extremely quickly be split to elementary particles (before the universe could cool down sufficiently)? – Peter Mortensen Aug 11 '15 at 20:44
• Dear Peter, yes, at temperatures much higher than 10 MeV (over $k$) or so, the high edge of the characteristic energy/temperatures of nuclear physics, it's so hot that complicated nuclei split into simpler ones, like hydrogen and helium, quickly. There was a lot of photons per baryon - lots of bullets that bombard nuclei to split all complicated nuclei to pieces. If the Universe gets through this hotter-than-nuclear stage, nothing is remembered about the initial isotopic composition. – Luboš Motl Aug 12 '15 at 8:19