File this one under the "bold predictions" section.
A new theory says there might be a bizarro universe out there, or an "anti-universe", that has been running backward in time since before the Big Bang.
The wild theory suggests that, as the early universe was small, hot, and dense, it was uniform enough that time looked symmetric going backward and forwards, a report from LiveScience explains.
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A universe running backward
If the new theory were true, it would suggest that dark matter is simply a new type of neutrino particle and that there was no period of "inflation" rapidly expanding the very early universe. The team behind the theory says that future experiments investigating gravitational waves or neutrinos could help to uncover the existence of this mirror anti-universe.
The theory is based on the law of CPT symmetry, which says that charge, parity, and time are symmetrical and you will see the same reaction if they are flipped. Sometimes, there are violations of this rule, but physicists have never observed a violation of charge, parity, and time at the same time.
In their new paper, published in the journal Annals of Physics, the team behind the new theory proposed extending this combined symmetry beyond the Big Bang. If we apply the law of CPT symmetry to the entire universe, there must be a mirror-image cosmos that balance out our entire existence. This anti-universe would have all opposite charges to our own and run backward in time, the researchers theorize.
Can we observe the anti-universe?
The researchers say a CPT-symmetry universe would naturally fill out with particles due to its opposing symmetrical partner, meaning there would be no need for the theory of inflation to explain the rapid expansion of the very early universe.
The theory might also provide an explanation as to why physicists have never observed "right-handed" neutrinos — they have only ever observed three neutrino types, the electron-neutrino, muon-neutrino, and tau-neutrino, all of which spin to the left relative to their motion.
A CPT-respecting universe would require the existence of at least one right-handed neutrino type that would be largely invisible to physics experiments, and would only influence the rest of the universe through gravity. The researchers also note that such a particle provides an explanation for dark matter, as the conditions of an anti-universe would fill our universe with enough right-handed neutrinos to account for the mysterious force.
All of this might sound like it was inspired by the Marvel cinematic universe's recent forays into multiverse-focused narratives, but the researchers say there are several factors that could lend credence to their theory. We would never be able to access this so-called anti-universe as it technically occurs before our own Big Bang. Their theory, for example, suggests that inflation never occurred. So if upcoming experiments searching for gravitational waves from the early period of the universe — that were caused by that inflation — come up empty, that would be a strong indication that the CPT anti-universe theory might be correct.