Pedagoguery

One of the puzzling aspects of our universe is how carefully tuned it appears to support life. Many physical constants seem finely tuned. For example, if gravity were significantly stronger, the universe would have collapsed in upon itself long before now. If it were significantly weaker, stars would not have been able to form and the universe would consist of a thin gas of hydrogen. Often times, this fine tuning was “explained” via the anthropic principle: if the universe were not turned to support life, we would not be here to comment on that fact. However, modern speculation on the possibility of a multiverse has led to conjectures on different sets of physical laws that would support life. The key is to potentially manipulate more than one physical parameter at a time. Below I present a couple of possibilities that have been developed, and how those universes would look.

In our universe, there are four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. You are both no doubt familiar with gravity and electromagnetism. The strong nuclear force is what holds quarks together to form baryons, and baryons together to form atomic nuclei. The weak nuclear force is more subtle. It moderates the transformation of neutrons into protons and vice versa. It is key in the processes of nuclear fusion in stars. The first step is generally when two protons collide, and the weak nuclear force operates to transform one of them into a neutron. It would appear that all four forces are necessary for life to exist, for without the process of nuclear fusion in stars, heavier elements like carbon, on which life depends, would not be possible. However, it is possible to describe a universe lacking the weak nuclear force where life is possible.

In such a “weakless” universe, you would have to tweak the initial matter/antimatter abundance in such a way that neutrons are more abundant at the start. Since neutrons could not decay into protons, as they do in our universe, that initial abundance of neutrons would remain. As such, a much greater abundance of deuterium (a hydrogen nucleus containing one proton and one neutron) would be formed in the initial nucleosynthesis. Stars would generate energy by the fusion of hydrogen and deuterium, which produces less energy than the fusion of hydrogen alone. As a result, stars would be smaller and dimmer. Carbon could still be formed, by a couple of different mechanisms. First of all, hydrogen and deuterium would form helium-3, and two helium-3 would produce helium-4 plus two protons. A second mechanism would be the direct fusion of two deuterium nuclei into helium-4. Once helium-4 is produced, the same mechanism that exists in our universe, the triple-alpha reaction, could form carbon-12. However, due to the lack of a way to produce free neutrons, elements heavier than iron would be virtually absent. There might be minute traces of elements up to strontium, but nothing heavier.

In our universe, there are two types of supernovae, essential for spreading the products of stellar fusion out into the universe. They are core-collapse and runaway fusion supernovae. A core collapse supernova would not be possible in a weakless universe, since in our universe, it depends on the massive production of neutrinos to stall the infalling matter and push it outward. Neutrinos are a product of the weak nuclear force. Instead, only runaway fusion supernova would be possible. This is the type that is produced when a white dwarf accretes enough matter to push it over the Chandrasekhar limit, generating a runaway fusion reaction in the star that blows it apart.

So, a weakless universe would conceivably support life. How about another scenario? A different group of scientists has examined the relationship between the masses of different quarks. In our universe, there are only two types of quarks with relatively small mass: the up and down quark. The up quark has a charge of +2/3 while the down quark is about twice the mass of the up quark and has a mass of -1/3. A proton is made up of two ups and one down, while a neutron is made up of two downs and one up. Computing the mass of the resulting baryon is not trivial, since the bulk of the mass is made up of the mass energy of the gluon cloud that surrounds each quark, but the result is the fact that the neutron is about 0.1% heavier than the proton, meaning that protons are stable, but neutrons decay into protons. What if the relative masses were changed? There are a couple of different possibilities.

If the mass of the down quark is slightly lighter than that of the up quark, then a proton would be about 0.1% heavier than a neutron. Thus, the lightest form of hydrogen would be deuterium, since s bare proton would decay into a neutron. In such a universe, it would be possible to form carbon, oxygen, an other heavier elements, so life would be possible.

What if we add a third quark to the mix? The next lightest quark in our universe is the strange quark, which is much like a down quark. If the strange and up quarks have about the same mass and the down quark is much lighter, then instead of protons and neutrons being the fundamental baryons, it would be neutrons and something called a sigma, which is made up of two downs and a strange quark. It would have a charge of -1, so in that universe, the equivalent of electrons would be positively charged. So, hydrogen in that universe would be a sigma particle with a positive electron. Heavier elements would be possible as well, allowing life to exist.

Other combinations of quarks do not work out so well. For example, if the up quark is the only light quark, then only one baryon is stable – the delta particle, made up of three up quarks and with a charge of 2. No heavier elements are possible. Likewise, if the up, down, and strange quarks all have relatively the same mass, then there are eight possible baryonic particles possible, but none of them can combine with each other to form heavier nuclei, so life is not possible in either of these scenarios.

There are other thought experiments out there, and this does not address all of the fine tuning in our universe. For example, none of them explain why the cosmological constant in our universe is so small. If it is too large and positive, the universe expands too fast for structures like stars to form, if it is too large and negative, the universe collapses in on itself if a tiny fraction of a second. Continued investigation into our physical laws may shed addition light on these mysteries.

Next time, some of the mysteries of star formation.