If the strong nuclear force were just 2% stronger, the neutron would be a stable particle instead of having a half life of about 13 minutes. What difference would that have made to Big Bang nucleosynthesis, to the growth of structure, to the formation of stars, nucleosynthesis in stars?
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1$\begingroup$ IMO this is too open-ended for SE. The FAQ says, "If you can imagine an entire book that answers your question, you’re asking too much." Answering this would require rewriting all of astrophysics, planetary science, and biology. $\endgroup$– user4552Commented Nov 25, 2011 at 21:58
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$\begingroup$ I think this question has no propose or end... The "how would... if..." are not very good questions in physics $\endgroup$– Jorge LeitaoCommented Nov 25, 2011 at 22:04
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1$\begingroup$ In response to the critical comments I've narrowed the question. It is not clear to me if it would make any difference in big bang nucleosynthesis since I assume neutrons and protons would be created equally and neutrons have no time to decay. $\endgroup$– FrankHCommented Nov 25, 2011 at 22:46
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1$\begingroup$ I don't think this question is open-ended at all: The circumstances where neutron stability plays a part are either easily enumerable or don't. The question is very precise in my opinion. The answer might be hard, but if it exists, it might fit (or an overview of it) in 3 paragraphs $\endgroup$– lurscherCommented Nov 26, 2011 at 5:50
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Assuming that the proton is heavier than the neutron, by more than the mass of the electron (plus the mass of a neutrino, plus the ionization energy of hydrogen), this is easy to answer, it would just make hydrogen unstable to decay to a neutron an an electron positron pair, so that a mostly hydrogen universe will decay into neutrons and electron-positron pairs, which wil annihilate into photons. So I will assume that the difference between neutron mass and proton mass is less than the mass of the electron, so that both the proton and the neutron are stable.
The most drastic effect of this is on big-bang nucleosynthesis, where two new stable species can be created, neutrons and tritium, and He-3 would be unstable to inverse beta-decay into tritium. So you would produce hydrogen, deuterium, tritium, helium, lithium, and neutrons. The initial conditions are mostly neutrons, not mostly protons, because the mass is inverted, and so you would get a lot of He-4, very little H-1, and most of the universe's mass would consist of stable neutrons and alpha-particles. These neutrons might collide to form neutron clusters, which would then beta-decay to protons once the binding energy was greater than the neutron proton mass difference.
There wouldn't be stars, but there might be gravitationally bound neutron clusters. Neutrons are neutral, and find it hard to dissipate energy, but the time scales are long, so they might be able to eventually settle down into neutron-star-like objects.
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2$\begingroup$ +1: Thanks so much Ron. Your answer shows how wrong the two critical commenters (to the question) were to assume that question was not appropriate for Physics SE. I will accept it as THE answer in a day or so to see if any other answers come in... ((PS: second line of 2nd paragraph has neutrons listed twice)) $\endgroup$– FrankHCommented Nov 26, 2011 at 3:41
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$\begingroup$ @anna v: Yes, I have often wondered if Dark Matter isn't like this--- complex like ordinary matter. It would be hard to know without a detailed measurement of the exact distribution of dark matter around galaxies, to get a sense of how squashed the distribution is. $\endgroup$ Commented Nov 28, 2011 at 8:57