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Dark matter and cosmic inflation might be explained by an anti-universe that runs backwards in time.

According to a bizarre new notion, there may be another “anti-universe” that exists prior to the Big Bang.

The theory holds that the early cosmos was tiny, hot, and dense — and so uniform that time seems to be symmetric moving backward and forward.

If the new hypothesis is correct, dark matter isn’t that mysterious after all; it’s merely a new flavor of a ghostly particle known as a neutrino that can only exist in this kind of world. And, according to the hypothesis, there would be no need for a phase of “inflation” that dramatically enlarged the size of the nascent universe just after the Big Bang.

If this is true, then future tests looking for gravitational waves or determining the mass of neutrinos might finally determine if this mirror anti-universe exists.

Maintaining symmetry

Nature’s basic symmetries have been recognized by physicists. The three most essential symmetries are charge (if you flip the charges of all the particles participating in an interaction to their opposite charge, you’ll get the same interaction); parity (if you look at an interaction’s mirror image, you’ll get the same outcome); and time (if you run an interaction backward in time, it looks the same).

Most of these symmetries are obeyed by physical interactions most of the time, which implies that there are occasional violations. However, physicists have never witnessed a violation of all three symmetries at the same time. Every single interaction seen in nature behaves precisely the same if you flip the charges, take the mirror image, and run it backward in time.

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This basic symmetry is known as CPT symmetry, which stands for charge (C), parity (P), and time (T).

Scientists suggest expanding this combined symmetry in a new work approved for publication in the journal Annals of Physics. This symmetry usually only applies to interactions – the forces and fields that comprise the physics of the universe. But, if this is such an essential symmetry, it may apply to the whole cosmos itself. In other words, this theory extends the symmetry of the universe’s “actors” (forces and fields) to the “stage” itself, the whole physical object of the cosmos.

Producing dark matter

We live in a cosmos that is expanding. This cosmos is packed with many particles performing many intriguing activities, and the universe’s development continues ahead through time. If we apply the notion of CPT symmetry to our whole universe, our vision of the world cannot be complete.

Rather, there must be more. To maintain CPT symmetry across the universe, a mirror-image cosmos must exist to balance out our own. This universe would have all opposite charges to ours, be reversed in the mirror, and run backward in time. Our universe is just one of two. When the two worlds are combined, they follow CPT symmetry.

The study’s researchers then speculated on the implications of such a universe.

They discovered many lovely things

For one thing, a CPT-respecting universe automatically expands and fills itself with particles, eliminating the requirement for inflation, a long-theorized phase of fast expansion. While there is substantial evidence that an event such as inflation happened, the theoretical image of that event is quite hazy. It’s so hazy that there’s plenty of opportunity for credible alternatives to be proposed.

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Second, a CPT-respecting universe would introduce some more neutrinos into the mix. There are three types of neutrinos: electron-neutrinos, muon-neutrinos, and tau-neutrinos. Surprisingly, all three of these neutrinos are left-handed (referring to the direction of its spin relative to its motion). Because all other particles known to physics have both left- and right-handed variants, scientists have long speculated about the existence of extra right-handed neutrinos.

A world worthy of the CPT would need the presence of at least one right-handed neutrino species. This species would be mostly undetectable to physics experiments, impacting the rest of the cosmos primarily via gravity.

However, an unseen particle that floods the cosmos and interacts exclusively via gravity sounds a lot like dark matter.

The researchers discovered that the constraints required by CPT symmetry would fill our universe with enough right-handed neutrinos to account for dark matter.

Predictions made in the mirror

We’d never be able to visit our twin, the CPT-mirror world, since it resides “behind” our Big Bang, before the birth of our universe. But it doesn’t mean we can’t put this theory to the test.

The researchers discovered a few observable repercussions of this hypothesis. For one thing, they anticipate that the three known left-handed neutrino types are all Majorana particles, which implies they are antiparticles to one another (in contrast to normal particles like the electron, which have antimatter counterparts called the positrons). Physicists aren’t clear if neutrinos have this feature or not.


They also expect that one of the neutrino types will be massless. At the moment, scientists can only set upper bounds on neutrino masses. If physicists can ever definitively measure neutrino masses and discover that one of them is massless, it would strongly support the hypothesis of a CPT-symmetric universe.

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Finally, the occurrence of inflation never happened in this paradigm. Instead, the cosmos organically filled with particles. Physicists think that inflation rocked space-time so violently that it inundated the universe with gravitational waves. Many experiments are looking for these enigmatic gravitational waves. Such waves, however, should not exist in a CPT-symmetric world. If such searches for primordial gravitational waves come up empty, it might be a sign that the CPT-mirror universe hypothesis is right.

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