Scientists have put forward the theory that outside of our own universe there may be an “anti-universe”, where time runs backwards. The theory is based on the fact that nature has a set of fundamental symmetries, the most important of which are charge, parity and time.
In particular, if you flip the charges of particles in interaction with their opposite charge, the same interaction (charge) will occur. If you look at the mirror image of the interaction, you get the same result (parity), and if you run the interaction back in time, it looks the same (time).
Sometimes there are violations of these symmetries, but physicists have never observed a violation of the combination of all three symmetries at the same time. This fundamental symmetry has been called CPT regarding charge, parity and time.
In a new paper published in Annals of Physics, a team of scientists proposes extending this concept of combined symmetry to the entire universe, suggesting that the early universe was small, hot and dense, and so homogeneous that time was both backward and forward symmetrical.
Based on this concept, the idea of our universe, where evolution moves forward in time, should be expanded to include a mirror image that moves backward in time.
The theory can explain the existence of dark matter. Scientists speculate that it could be an invisible particle that interacts only through gravity and that offers a twin to the three known varieties of neutrinos in our universe: the electron neutrino, the muon neutrino, and the tau neutrino.
All three of these varieties of neutrinos are described as left-handed due to the direction of spin relative to motion, and all other known particles in physics have both left-handed and right-handed versions. If the universe obeyed the CPT rules, it would require the existence of at least one right kind of neutrino.
The researchers found that the conditions created with such a universe meant it was full of right-handed neutrinos, enough for dark matter to exist.
Apart from the fact that the theory offers a potential explanation for dark matter. The theory implies that there would be no need for the expansion of the universe after the Big Bang, because an “observing” CPT universe would naturally expand and fill with particles.
We would never be able to access this alternate universe if it existed, but future experiments to look for gravitational waves or confirm the neutrino mass could prove the theory correct.
