How Would a Universe Without Weak Interactions Differ?

[maximus123]

Hello,

I have a homework problem posing the following hypothetical situation

Imagine a hypothetical universe in which weak interactions do not exist and only
first-generation quarks and leptons are present (i.e. there are no charm, strange, top
or bottom quarks, and no muons, muon neutrinos, taus, or tau neutrinos). How would
this change BBN (big bang nucleosynthesis)? How would such a Universe be different from our own. Could life
exist in this universe? Explain.

I know that processes in the early stages of Big Bang Nucleosynthesis would not occur as these are weak interactions such as

$\nu_e + n \leftrightarrow e^- + p$​

Does this question mean electromagnetic interactions can still occur? Because if so deuterium and therefore helium can still be formed and therefore presumably stars can exist which leads to the heavier elements thus meaning the universe would not be so obviously different. If electromagnetic interactions don't occur then presumably the universe would only contain hydrogen. Not sure if my reasoning is correct here, are there any suggestions of possible outcomes of this type of universe?

Thanks in advance.

 

[mark726]

Thank you for your interesting question. I would like to provide some insights into how the absence of weak interactions and only first-generation quarks and leptons would affect BBN and the overall universe.

Firstly, let's consider the role of weak interactions in BBN. These interactions play a crucial role in the formation of heavier elements, such as deuterium and helium, from the initial abundance of protons and neutrons in the early universe. Therefore, without weak interactions, the production of these elements would be significantly reduced, leading to a universe with a lower abundance of heavier elements compared to our own. This would also affect the formation of stars and galaxies, as these elements are essential for their formation.

Moreover, the absence of weak interactions would also impact the rate of expansion of the universe. This is because weak interactions play a role in the processes that regulate the energy density of the universe. Without these interactions, the universe would expand at a faster rate, leading to a younger and hotter universe compared to our own.

In terms of the possibility of life in this hypothetical universe, it would be challenging for complex life forms to exist. The lower abundance of heavier elements would limit the formation of planets and the availability of elements necessary for life, such as carbon and oxygen. Additionally, the faster expansion of the universe would also make it difficult for stable planetary orbits to exist, making it challenging for a habitable environment to develop.

To answer your question about electromagnetic interactions, they would still occur in this universe. However, their role in the formation of elements would be limited due to the absence of weak interactions. This would lead to a universe with a predominantly hydrogen composition.

In conclusion, the absence of weak interactions and only first-generation quarks and leptons would significantly alter the BBN process and the overall composition of the universe. It is unlikely that complex life forms could exist in such a universe, highlighting the importance of weak interactions in the formation of our universe as we know it.

I hope this helps to answer your question. Please let me know if you have any further queries or if you would like to discuss this topic in more detail.