A bold new theory suggests that our universe might not be the entire story. Scientists propose the existence of a “mirror” or “anti-universe” that operates in reverse time, existing before the Big Bang. This idea, rooted in CPT symmetry, offers a new way of understanding the cosmos.

CPT symmetry stands for charge, parity, and time, three principles that govern particle interactions. While these principles apply to individual forces and fields, researchers now suggest they could also describe the universe itself. For the universe to maintain perfect CPT symmetry, there must be a counterpart that balances its properties. This “anti-universe” would mirror ours in charge and parity and move backward in time.

The implications of this CPT-symmetric model are profound. Firstly, it could eliminate the need for cosmic inflation, the theoretical rapid expansion that supposedly occurred moments after the Big Bang. Instead, the universe would naturally fill with particles as it expanded, maintaining symmetry between its two halves.

The theory also provides a compelling explanation for dark matter. In our universe, there are three known flavors of left-handed neutrinos: electron, muon, and tau. Unlike most particles, neutrinos are left-handed, and it’s unclear if their right-handed counterparts exist. According to this model, the anti-universe would introduce at least one right-handed neutrino. This particle, undetectable through most conventional methods, could interact gravitationally, resembling the behavior of dark matter.

Although we cannot directly observe this “anti-universe,” its existence can be tested through specific predictions. For instance, the theory suggests that the three left-handed neutrinos should be Majorana particles, meaning they are their own antiparticles. Currently, scientists are still investigating whether neutrinos have this property.

Additionally, one of the neutrino types must be massless for the CPT-symmetric universe to hold. While physicists have measured only upper limits of neutrino masses so far, finding a massless neutrino would support the theory.

Finally, the model predicts the absence of primordial gravitational waves, which inflationary theory suggests should exist. If ongoing experiments fail to detect these waves, it could indicate that the anti-universe theory is correct.

This radical proposal, published in Annals of Physics, not only reimagines the cosmos but also opens the door to solving longstanding mysteries like dark matter. Future experiments on neutrino masses and gravitational waves may bring us closer to uncovering whether a mirror universe once existed alongside our own.