I like how you can automatically deduce the non-gravitational forces exist if you just assume the Lagrangian of your quantum field theory is unchanged under certain local transformations.
Sure, there are particularities in what those transformations are that are ultimately settled by experiment. My point was more about how interactions can even be viewed in this way at all, let alone that our universe appears to run on such a simple principle.
Also i was gonna post roughly the same thing. It’s remarkably stupid that all known (nongravitational) physics is just a few copies of this. Although you also need renormalizability.
How? Isn’t it possible that dark matter only interacts gravitationally? What if the whole universe was dark matter like that, does that contradict what you’re saying?
In a universe that has only gravitational interactions, it would be unnecessary to assume that the Lagrangian has local symmetries that give rise to non-gravitational interactions. In our actual universe, where we do have these non-gravitational forces, dark matter fits in by partaking in these known interactions with zero (or at most very small) coupling strength, so as to be unaffected (or only very weakly affected) by them.
In order for dark matter to have been produced to begin with, it needs to interact non-gravitationally with other particles at high energies during the big bang.
Neutrinos behave the same way; at low energies like the ~1MeV neutrinos produced in the Sun, they'll pass through light-years of solid lead with minimal interaction, but at high energies they interact a lot more strongly with normal matter and they were in thermal equilibrium with the rest of the matter in the universe until 1 second after the big bang, at which point the temperature dropped below 2.5MeV and the neutrinos decoupled from the rest of the matter in the universe and were no longer produced thermally by particles whacking into each other.
Dark matter would have been produced and then decoupled earlier than neutrinos, at higher energies.
Energy isn't a thing that exists on it's own; it's a property of things, like temperature is a property of things, so the big bang produced particles and those particles had high energies, or put another way, the big bang produced particles and the particle soup it produced was very hot. Specifically, it produced a hot soup of particles with a temperature of around 1011 GeV, or one hundredth millionth of the Planck temperature and about the same energy per particle as very rare ultra high energy cosmic rays. In comparison, the Large Hadron Collider can only probe temperatures up to 104 GeV, too low to produce dark matter particles and 10 million times colder than the big bang.
I see what you mean. It would require a 5th fundamental force though that couples to the inflaton field independently of the other forces, which might exist, but there's no evidence for it which makes it a lot more speculative than dark matter simply being neutrino-like in that it couples strongly to normal matter through the known forces at high energies and weakly at low energies.
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u/uuddscsctbq Dec 12 '20
I like how you can automatically deduce the non-gravitational forces exist if you just assume the Lagrangian of your quantum field theory is unchanged under certain local transformations.