Imagine the entire matter content of the universe suddenly becomes antimatter, and vice versa, overnight. Would it change the rate at which space is expanding?
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$\begingroup$ The more antimatter you have in space the more gamma rays you would be observing hence temperature is high in your space thus the temperature in the Friedmann equation would be greater wich causes more acceleration $\endgroup$– BardeenCommented Feb 27, 2018 at 8:48
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2 Answers
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No it would not.
Matter comes into the Friedmann equation as an energy density. Particles and the corresponding antiparticles have the same mass and therefore make an identical contribution to the energy density.
As hinted at in a comment, changing some of the matter to antimatter would make a difference because the resulting annihilation would convert matter to photons. Relativistic particles such as photons have a different equation of state because their energy density scales as $a^{-4}$ while the energy density of matter scales as $a^{-3}$.
answered Feb 27, 2018 at 9:01
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$\begingroup$ You cant have a space with antiparticles only right ? Because the equations we have are only consistent with the existance of both $\endgroup$– BardeenCommented Feb 27, 2018 at 9:13
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$\begingroup$ @001 to a very good approximation space contains only particles. Anti-particles appear only fleetingly and in small quantities in high energy situations. We have no idea why there was an excess of matter over antimatter so we can't comment whether the universe could have been antimatter dominated instead. $\endgroup$ Commented Feb 27, 2018 at 9:20
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Imagine the entire matter content of the universe suddenly becomes antimatter, and vice versa, overnight. Would it change the rate at which space is expanding?
What is called matter and what is called antimatter is a definition. In the universe at this time the bulk of masses are composed of what we call matter, by convention, because what we call antimatter is so rare and dependent on particle interactions.
Matter starts with hydrogen, and protons which by convention we call matter, and the antiparticle of the proton we call antimatter. For the electrons which supply the negative charges to create neutral atoms and molecules the antiparticle is positive, the positron.
"suddenly becomes antimatter, and vice versa, overnight." This would generated an upheaval, and a model is needed on how this "overnight, suddenly" happens so no answer can be given without modeling.
What can be answered is that if there exists a mirror universe where positive charges would describe the electrons and negative the protons ( the constituents of the atoms) there would be no difference in the expansion of the twin universe from ours, because there would complete symmetry in masses and charged behavior.
