Is there any form of matter, particle, etc, that could persist indefinitely, even after the universe reaches its final state (like heat death, big crunch, or other ultimate fate)?
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5$\begingroup$ Perhaps electrons, protons, and other fundamental particles and fields. Many have no known mechanism to decay into something else. $\endgroup$– RC_23Commented yesterday
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2 Answers
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Electrons, neutrinos (all flavors), and photons have no known decay mechanisms under the Standard Model. However, for protons, some theories predict extremely long lifetimes (grand unified theories mostly), but experimentally, no decay has ever been observed. You can also add that quarks are stable but they are confined inside baryons and mesons. Note that even stable particles might be affected by the universe's ultimate fate (in a big crunch, for example, electrons and protons should combine to make neutrons like in a neutron star, I guess) and, in general, by unknown physics beyond our current understanding.
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1$\begingroup$ One could argue that neutrino oscillation means they wouldn't count here. $\endgroup$– HearthCommented 16 hours ago
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1$\begingroup$ @Hearth good point! I thought about it, but didn't include it in my answer because neutrino oscillations just change the flavor of the neutrino, but the neutrino number persists. They don't get destroyed as when, say, a neutron decays into a proton (plus an electron and an electron antineutrino), so the neutron number goes from 1 to 0. In neutrino oscillations, you don't get a decay. $\endgroup$– cconsta1Commented 15 hours ago
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1$\begingroup$ That's just the wrong basis for this question. There are three mass eigenstates of neutrinos, and the heavier two decay via known SM physics to the lightest one. See 10.1103/PhysRevD.25.766 for an early calculation. $\endgroup$– SethKCommented 9 hours ago
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Any particle has a chance to collide with its antiparticle and ceasing to exist in the resulting annihilation. Depending on the particle that chance may be super tiny, in particular if chance is required to form the antiparticle in the first place, but you want to wait an eternity, so it will happen eventually.
As a bonus, the less likely a given antiparticle is to form by chance, the more likely –?very roughly?– is the respective particle to decay on its own.
