Scientists proposed a wild new theory , according to which there is another “anti-universe” that moves back in time before the Big Bang. The idea suggests that the early universe was small, hot and dense – and so uniform that time appears symmetrical in both directions.

If this is true, then the new theory means that dark matter is not so mysterious; it’s just a new kind of ghost particle called a neutrino that can only exist in this kind of universe. And the theory implies that there is no need for a period of “inflation” that quickly increased the size of the young cosmos shortly after the Big Bang.

If this is true, then future experiments to look for gravitational waves or to determine the mass of the neutrino could answer once and for all whether this mirror anti-universe exists.

The theory is based on an extension of the concept of fundamental symmetry in nature, which combines charge ©, parity (P) and time (T). Usually this symmetry applies only to the interactions – the forces and fields that make up the physics of the cosmos. But perhaps, if it’s such an important symmetry, it applies to the entire universe.

In other words, this idea extends this symmetry from applying only to the “actors” of the universe (forces and fields) to the “stage” itself – the entire physical object of the universe.

We live in an expanding universe. This universe is filled with many particles that do many interesting things, and the evolution of the universe is moving forward in time. If we extend the concept of CPT symmetry to our entire cosmos, then our view of the universe cannot be complete.

There must be another anti-universe – a mirror image of our own – that is moving back in time before the Big Bang. In this anti-universe, all particles have the opposite charge and spin to our universe. And this anti-universe is also expanding – not from the Big Bang, but towards it.

This means that the Big Bang was not the beginning of time and space, but a point of symmetry where two universes met and separated.

Such a theory could explain some of the mysteries of cosmology, such as dark matter and cosmic inflation. Dark matter is an unknown form of matter that makes up about 85% of all matter in the universe but does not interact with ordinary matter except through gravity. Cosmic inflation is a hypothetical period when the universe expanded rapidly by many orders of magnitude shortly after the Big Bang.

According to the authors of the theory, dark matter may simply be a new kind of neutrino that can cross between the two universes. These neutrinos can only exist in a symmetrical universe where time runs backwards. And cosmic inflation may not be necessary, because the early universe was so homogeneous and isotropic that it did not require an additional mechanism to even out its structure.

This theory has not yet been experimentally tested, but scientists offer two possible ways to test it. One of them is to measure the mass of neutrinos using the Euclid space observatory, which will launch in July 2023. The other is to detect gravitational waves from the Big Bang with the BICEP3 project, which is already in operation at the South Pole.

If these experiments confirm the predictions of the theory, then this would mean that our universe has a mirror anti-universe where time runs backwards.