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).
In a new paper recently accepted for publication in
the journal Annals of Physics, scientists propose extending this combined
symmetry. Usually this symmetry only applies to interactions — the forces and
fields that make up the physics of the cosmos. But perhaps, if this is such an
incredibly important symmetry, it applies to the whole entire universe itself.
In other words, this idea extends this symmetry from applying to just the
"actors" of the universe (forces and fields) to the "stage"
itself, the entire physical object of the universe.
Creating dark matter
We live in an expanding universe. This universe is
filled with lots of particles doing lots of interesting things, and the
evolution of the universe moves forward in time. If we extend the concept of
CPT symmetry to our entire cosmos, then our view of the universe can't be the
entire picture.
Instead, there must be more. To preserve the CPT
symmetry throughout the cosmos, there must be a mirror-image cosmos that
balances out our own. This cosmos would have all opposite charges than we have,
be flipped in the mirror, and run backward in time. Our universe is just one of
a twin. Taken together, the two universes obey CPT symmetry.
The study researchers next asked what the
consequences of such a universe would be.
They found many wonderful things.
For one, a CPT-respecting universe naturally expands
and fills itself with particles, without the need for a long-theorized period
of rapid expansion known as inflation. While there's a lot of evidence that an
event like inflation occurred, the theoretical picture of that event is
incredibly fuzzy. It's so fuzzy that there is plenty of room for proposals of
viable alternatives.
Second, a CPT-respecting universe would add some
additional neutrinos to the mix. There are three known neutrino flavors: the electron-neutrino,
muon-neutrino and tau-neutrino. Strangely, all three of these neutrino flavors
are left-handed (referring to the direction of its spin relative to its
motion). All other particles known to physics have both left- and right-handed
varieties, so physicists have long wondered if there are additional
right-handed neutrinos.
A CPT-respecting universe would demand the existence
of at least one right-handed neutrino species. This species would be largely
invisible to physics experiments, only ever influencing the rest of the
universe through gravity.
But an invisible particle that floods the universe
and only interacts via gravity sounds a lot like dark matter.
The researchers found that the conditions imposed by
obeying CPT symmetry would fill our universe with right-handed neutrinos,
enough to account for the dark matter.
Predictions in the mirror
We would never have access to our twin, the
CPT-mirror universe, because it exists "behind" our Big Bang, before
the beginning of our cosmos. But that doesn't mean we can't test this idea.
The researchers found a few observational
consequences of this idea. For one, they predict that the three known
left-handed neutrino species should all be Majorana particles, which means that
they are their own antiparticles (in contrast to normal particles like the
electron, which have antimatter counterparts called the positrons). As of now,
physicists aren't sure if neutrinos have this property or not.
Additionally, they predict that one of the neutrino
species should be massless. Currently, physicists can only place upper limits
on the neutrino masses. If physicists can ever conclusively measure the
neutrino masses, and one of them is indeed massless, that would greatly bolster
the idea of a CPT-symmetric universe.
Lastly, in this model the event of inflation never
occurred. Instead, the universe filled with particles naturally on its own.
Physicists believe that inflation shook space-time to such a tremendous degree
that it flooded the cosmos with gravitational waves. Many experiments are on
the hunt for these primordial gravitational waves. But in a CPT-symmetric
universe, no such waves should exist. So if those searches for primordial
gravitational waves turn up empty that might be a clue that this CPT-mirror
universe model is correct.
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