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.
Originally published on
Live Science.
Reference:
The Big Bang, CPT, and neutrino dark matter by Latham Boyle, Kieran Finn,
Neil Turok arxiv.org/abs/1803.08930