Backwards in Time

If the universe has a twin and on that twin time runs backward, then scientists could explain dark matter. (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY/Getty Images)

A wild new theory suggests there may be another "anti-universe," running backward in time prior to the Big Bang.

The idea assumes that the early universe was small, hot and dense — and so uniform that time looks symmetric going backward and forward.

If true, the new theory means that dark matter isn't so mysterious; it's just a new flavor of a ghostly particle called a neutrino that can only exist in this kind of universe. And the theory implies there would be no need for a period of "inflation" that rapidly expanded the size of the young cosmos soon after the Big Bang.

If true, then future experiments to hunt for gravitational waves, or to pin down the mass of neutrinos, could answer once and for all whether this mirror anti-universe exists.

Preserving symmetry
Physicists have identified a set of fundamental symmetries in nature. The three most important symmetries are: charge (if you flip the charges of all the particles involved in an interaction to their opposite charge, you'll get the same interaction); parity (if you look at the mirror image of an interaction, you get the same result); and time (if you run an interaction backward in time, it looks the same).

Physical interactions obey most of these symmetries most of the time, which means that there are sometimes violations. But physicists have never observed a violation of a combination of all three symmetries at the same time. If you take every single interaction observed in nature and flip the charges, take the mirror image, and run it backward in time, those interactions behave exactly the same.

This fundamental symmetry is given a name: CPT symmetry, for charge (C), parity (P) and time (T).  READ MORE...