By Gary Felder, Smith College
One second after the big bang, the ratio of protons to neutrons was steadily growing. If nothing had happened to preserve the neutrons, they would all have been gone 10 to 20 minutes later. But an important event did intervene, about a minute after the big bang. It was the process we call the big bang nucleosynthesis.
One cannot talk about matter in the early universe without understanding the role of the neutrinos. These particles fill the universe even today, but pass through solid matter with almost no interactions.
However, for the first second after the big bang, the universe was so dense that even neutrinos couldn’t pass through. Neutrinos constantly collided with other particles.
Starting about one second after the big bang, though, the matter wasn’t dense enough to stop neutrinos anymore. We say that at one second, the neutrinos ‘froze out’. That term doesn’t mean their temperature changed at that moment; it means they almost entirely stopped interacting with other matter.
Since that moment, those neutrinos have been flying freely through the universe. Although they haven’t been interacting with matter, the neutrinos have slowed down as a result of the expansion of the universe.
This article comes directly from content in the video series The Big Bang and Beyond: Exploring the Early Universe. Watch it now, on Wondrium.
Primordial Neutrinos Still Fill the Space
Those neutrinos, left over from the first second of the universe, still fill all of space today. Calculations showed that these primordial neutrinos should be coming to us equally from all directions, at a temperature of about 2° above absolute zero.
In 2015, for the first time, scientists detected this cosmic neutrino background and measured its temperature at 1.96 above absolute zero, beautifully confirming our understanding of what was happening in the first second of the universe.
One Second after the Big Bang
The next important events had to do with the protons and neutrons. Protons and neutrons can transform into each other in a process called beta decay. When quarks first combined, they produced equal numbers of protons and neutrons, and for a while, they each transformed into each other at equal rates.
But as the temperature dropped, it became more likely for the slightly heavier neutrons to decay into the slightly lighter protons than the other way around.
By about one second, the decay of protons into neutrons had stopped, but the other way around—neutrons turning into protons—was still happening. So the ratio of protons to neutrons was steadily growing.
If nothing had helped to preserve the neutrons, they would all have been gone 10 to 20 minutes later. However, about a minute after the big bang, protons and neutrons began combining into light nuclei in the process we call big bang nucleosynthesis, or just nucleosynthesis.
The key to nucleosynthesis is that protons and neutrons all attract each other via the strong force. When quarks combine into groups of red-green-blue like protons or neutrons, the strong forces between two of those groups mostly cancels out.
But suppose two protons get very close to each other. Some of the quarks in each proton are closer to the other protons, so the forces between the quarks don’t perfectly cancel. The result is that there’s still a residual strong force between the two. The same logic holds for two neutrons, or for a neutron and a proton.
All of those particles attract each other with a strong force that is much weaker than the force between individual quarks, because those quark-on-quark forces mostly, but don’t perfectly, cancel out. But that residual strong force between protons and neutrons is still strong enough to hold nuclei together.
Helium-4: End of the Chain
As with individual quarks, the attraction between protons and neutrons can’t hold them together if they are moving with too much energy. For protons and neutrons to stick together, the temperature had to drop below about a billion degrees, which happened about a minute after the big bang.
Once the temperature was low enough, one proton and one neutron could combine to form a nucleus called Hydrogen-2, also called deuterium. Two deuterium nuclei could then combine into Helium-4, two protons plus two neutrons.
And for the most part, that was the end of nucleosynthesis. One can imagine combining Helium-4 with a single proton or neutron, or a deuterium, or another Helium-4, but none of those happens to produce a stable nucleus, so for the most part Helium-4 is the end of the chain.
If nucleosynthesis had started with equal numbers of protons and neutrons, then essentially all the nuclei would have ended up as helium-4. Conversely, if there had been no neutrons, then no nucleosynthesis would have happened at all and all nuclei would have ended up as hydrogen.
But when nucleosynthesis started at one minute, there were about seven protons for each neutron. So each set of 14 protons and two neutrons produced one helium nucleus and left 12 bare protons, which are hydrogen nuclei.
Deuterium, Confirming the Big Bang
After nucleosynthesis, helium made up about 8% of the nuclei in the universe. Because helium nuclei are heavier than hydrogen nuclei, that meant helium made up about 25% of the nuclei by mass.
Nucleosynthesis didn’t start before the one-minute mark because the universe was too hot for nuclear fusion. Nucleosynthesis stopped a few minutes later because the universe became too cold.
At that point there was still some deuterium left that had not fused into heavier nuclei, and since nucleosynthesis was no longer possible, that deuterium just stayed deuterium. But after nucleosynthesis, deuterium accounted for only about one in a billion nuclei.
Nonetheless, measurement of that trace deuterium in exactly the amounts predicted by the big bang model has been one of the great experimental confirmations of the model.
Common Questions about the Big Bang Nucleosynthesis
By about one second after the big bang, the decay of protons into neutrons had stopped, but the other way around—neutrons turning into protons—was still happening. So the ratio of protons to neutrons was steadily growing.
The attraction between protons and neutrons can’t hold them together if they are moving with too much energy. Therefore, for protons and neutrons to stick together, the temperature had to drop below about a billion degrees, which happened about a minute after the big bang.
If nucleosynthesis had started with equal numbers of protons and neutrons, then all the nuclei would have ended up as helium-4.