Reporting by XinZhiyuan
Edit: David
Our universe is actually just one of the "twins"? A new theory holds that there exists an "anti-universe" that is perfectly symmetrical to the present universe, where the charge is reversed and time is reversed.
Our universe may have a "twin" that flows backwards in time.
If the universe had a twin and time ran backwards, scientists could explain the existence of dark matter.
A crazy new theory suggests that before the Big Bang, there could be another "anti-universe" that would have gone backwards in time
This idea assumes that the early universe was small, hot, and dense. And it's uniform, and the direction of time seems to be symmetrical back and forth.
If so, this new theory means that dark matter is not as mysterious as we previously thought; it is just a new flavor of ghost particles called neutrinos that can only exist in this universe. The theory implies that there is no need for a "inflationary" period shortly after the Big Bang to rapidly expand the size of the young universe.
If true, then future experiments looking for gravitational waves or determining the mass of neutrinos could answer once and for all whether this mirror anticosmic exists.
The Three "Fundamental Symmetries" of the Universe
Physicists have identified a set of fundamental symmetries in nature.
The three most important symmetries are: charge symmetry (if you flip the charges of all particles involved in an interaction to the opposite charge, the interaction does not change); parity symmetry (if you mirror the interaction, you get the same result); and temporal symmetry (if you run an interaction backward in time, it looks the same).
Physical interactions adhere to these symmetries most of the time. That is, these symmetries are sometimes not followed. But physicists have never observed a violation of all three symmetries at the same time.
If every interaction observed in nature were to flip the charge, take a mirror image, and run backward in time, the interactions would still behave exactly the same.
This fundamental symmetry is called CPT symmetry, which is charge (C), parity (P), and time (T).
Thesis link: https://arxiv.org/pdf/1803.08930.pdf
In a new paper recently accepted for publication in the journal Annals of Physics, scientists propose to extend this combinatorial symmetry. Usually this symmetry applies only to interactions, the forces and fields that make up the physics of the universe.
But perhaps, if this is an incredibly important symmetry, it applies to the entire universe itself. In other words, the idea extends this symmetry from the "actors" (forces and fields) that apply only to the universe to the "stage" itself, the entire physical object of the universe.
Create dark matter
We live in an expanding universe. This universe is full of many particles, doing a lot of interesting things, and the evolution of the universe moves forward on the timeline. If we extend the concept of CPT symmetry to the entire universe, then our current view of the universe cannot reflect the full picture of the universe.
If this CPT view is accepted, in order to maintain the CPT symmetry of the entire universe, there must be a "mirror universe" to balance our current universe.
The charge in this mirror universe is the exact opposite of our current universe and flows backwards on the timeline of the current universe. Our universe is just one of the twin universes. In general, both universes adhere to CPT symmetry.
So, what are the consequences of the existence of such a universe?
First, a universe that follows CPT would naturally expand and fill itself with particles without going through a period of rapid expansion that has been theorized for a long time. While there is plenty of evidence of this bloat, the theoretical picture that supports this view is incredibly blurry, blurred enough to have enough room to come up with a viable alternative.
Second, a universe that follows the CPT would contain some additional neutrinos. There are three known neutrinos: electron neutrinos, μ meson neutrinos, and Dow neutrinos. Curiously, all three neutrinos are left-handed (referring to the direction in which their spins move relative to their motion).
All other particles known in physics have left-handed and right-handed nature, so physicists have long wondered if there are additional right-handed neutrinos.
A universe that follows the CPT would require the presence of at least one right-handed neutrino. Such particles are largely invisible to physics experiments, only influencing other parts of the universe through gravity.
This invisible invisible particle, which interacts only through gravity, sounds a lot like dark matter.
The "Mirror Universe" big prediction
We will never have access to our twin, the CPT mirror universe, because it existed "behind" our Big Bang, before our universe began. But that doesn't mean we can't test the idea.
The researchers found some observations of this idea.
First, they predicted that the three known left-handed neutrino species should all be Majorana particles, meaning they are their own antiparticles. As of now, physicists aren't sure if neutrinos have this property.
In addition, they predicted that one of the neutrino species should be massless. Currently, physicists can only make an upper limit judgment on the mass of neutrinos. If physicists can finally measure the mass of neutrinos, and one of them is indeed massless, this would greatly support the idea of a CPT symmetric universe.
Finally, in this model, inflation events never occur. Instead, the universe itself is naturally filled with particles. Physicists believe that inflation vibrates space-time so much that gravitational waves fill the entire universe. A large number of current experiments are looking for these primordial gravitational waves.
But in a CPT symmetrical universe, such waves should not exist. So if those search results for the original gravitational waves are empty, this could be a clue that this CPT-mirrored model of the universe is correct.
Resources:
https://www.livescience.com/mirror-universe-explains-dark-matter
https://wordpress.futurism.com/anti-universe-time-backwards/amp