I was wondering how the universe would look like if it would have been perfectly symmetrical in terms of matter and antimatter. If I understand correctly, there would be no "particle" but the energy released by matter-antimatter annihilation shouldn't just disappear, thus such universe probably wouldn't be an empty void. How does it differ from the current universe, apart from being deprived of persistent matter and related phenomena?
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1$\begingroup$ It's unclear why the question was closed, as it can be straightforwardly answered in its original form. But basically there wouldn't be anything new replacing the matter. The energy released by annihilation is already there in the form of the cosmic microwave and neutrino backgrounds. Keep in mind matter-antimatter pairs vastly outnumbered asymmetric matter in the early universe. Even if they didn't, adding more matter-antimatter pairs is equivalent to just shifting the cosmic time parameter. $\endgroup$– StenCommented Feb 3, 2023 at 18:31
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2$\begingroup$ To be clear we don't know that the universe wasn't initially perfectly symmetrical with respect to particles and anti-particles and that some unknown mechanism produces what we see. See Baryon asymmetry. $\endgroup$– StephenG - Help UkraineCommented Feb 3, 2023 at 18:47
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$\begingroup$ I am not sure either why this question was closed, a clarification would be nice! $\endgroup$– RedirectkCommented Feb 3, 2023 at 19:53
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1$\begingroup$ This video From Dr. Don Lincoln at FermiLab may help: Can leptogenesis explain why there's something instead of nothing? $\endgroup$– mmesser314Commented Feb 4, 2023 at 3:55
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1 Answer
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Nothing new would replace the matter. Asymmetric matter was a tiny contribution to the energy density of the early universe, of order one part in a billion. Moreover, if we were to add the energy of the asymmetric matter to the primordial radiation bath, that would essentially just shift the exact same cosmic evolution to a slightly later time. An observer like us who arises from that cosmic evolution would arise at a correspondingly later time, so the added energy would make no difference to what they find.
So, would the universe be empty? Not quite. It would still contain the cosmic microwave and neutrino backgrounds, as well as a (potentially minuscule) primordial gravitational wave background. To the best of our knowledge, it would also still contain dark energy and dark matter. And indeed it would still contain essentially the same structure that we find today at scales larger than galaxies, except that structure would be composed of (almost) entirely dark matter.
Also, there would be some ordinary matter/antimatter in this universe. Its abundance would simply be low enough that a given particle is not expected to meet another particle to annihilate, even given the entire age of the universe. (In the field, we would say that these are thermal relics with an abundance fixed by freeze-out.) Specifically, the ordinary matter would be about a billion times less abundant than it is in our universe.
