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If our universe was fundamentally described by a theory of everything (let's say e.g. string theory) it would have a set of fundamental symmetries (which would correspond to the fundamental symmetries of that underlying theory of everything, like e.g. the fundamental symmetries of string theory). However, could there be any process or event where these fundamental symmetries would be broken (not spontaneously, but explicitly) or where they would not hold exactly (but only approximately)?

For example, if the universe underwent a vacuum phase transition, this could break the symmetries of the underlying theory, but only spontaneously (as far as I know), so the symmetries would actually still apply (at a fundamental level).

However could there be any other phenomenon or event (known in theoretical physics at least) that would change the fundamental structure and components of the universe so these fundamental symmetries would be broken explicitly (or at least would not hold exactly, but only approximately)?

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    $\begingroup$ In what sense would symmetries so broken be fundamental? $\endgroup$
    – J.G.
    Commented Feb 21, 2023 at 11:21
  • $\begingroup$ The fundamentals of physics are phenomena, as revealed in experiments and observations. Mathematical concepts like symmetry are useful model building tools, but they will never be fundamental. So, what experiment do you propose to test your hypothesis? $\endgroup$
    – John Doty
    Commented Feb 21, 2023 at 13:23
  • $\begingroup$ An explicitly broken symmetry is just not a symmetry. Also, there is a common belief that a proper theory of quantum gravity will not have any continuous global symmetry. $\endgroup$
    – Connor Behan
    Commented Feb 22, 2023 at 4:57
  • $\begingroup$ Related post by OP: physics.stackexchange.com/q/748785/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Commented Apr 6, 2023 at 12:30

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You know, if we allow explicit symmetry breaking, we can say that a theory has any symmetry we like, but that it's explicitly broken. So explicitly broken symmetries aren't a particularly useful concept, unless there is a sense in which the breaking is small, such that the symmetry was approximate.

This useful sense sometimes happens in anomalously broken symmetries. These are symmetries of a classical theory that are broken by quantum effects. Since quantum effects are (typically) small, these are approximate symmetries. One example of a proposed fundamental theory of this type is agravity. This is a theory with a classical scale invariance - it contains no massive scales and it is completely agnostic about scales - but this is broken at the quantum level.

There are other QCD-like models of this type (technicolor, composite Higgs), though they don't afaik make claim to be truly fundamental.

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