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R. N. Mohapatra
which is the mirror partner of the familiar photon, etc. One word of caution:
the mirror universe is part of our universe and is not necessarily related to
the multiverse picture of string theory, where presumably the universes are
disconnected from one another. There exist different variations of the mirror
universe scenario where the two sets of particles “communicate” with each
other via different kinds of “portals.”
The left–right symmetric theories provide a different way to maintain parity
symmetry without doubling the quarks and leptons and without postulating
another universe of forces. They keep the same number of quarks and leptons
as in the standard model, but only add the right-handed neutrino (three
of them) to the standard model fermions. In such models, the neutrino
automatically has mass by pairing up with the right-handed neutrino via the
seesaw-like mechanism as described. As we saw before, the neutrino mass
requires a right-handed neutrino. Thus this is a picture that fits right into
the modern discussion of neutrino mass and maintains mirror symmetry in
nature.
The left–right symmetric theories for neutrino mass are more economical
compared to the mirror universe model. However, the mirror universe models
do provide an automatic dark matter candidate, which is the lightest baryon
of the mirror sector. It also provides extra sterile neutrino states [56] (or
mirror neutrino states) in the universe. These would be good candidates to
fit some experiments that seem to indicate possible anomalies. The dark or
sterile neutrinos interact only with the mirror W , Z and are thus hidden from
our view, except for their mixing with ordinary neutrinos, which would then
explain the sterile neutrino anomalies.
How can we test for the existence of the mirror universe? Various suggestions
have been made in the literature about it. In the minimal version of the model
where the only force connecting the two universes is gravity (a connection,
which is always there), there can be mirror stars, mirror planets, etc. Since
these objects are transparent to our photon, light from stars behind a mirror
star will pass through it. The rays of light passing on both sides of the mirror
star would get bent due to gravitational force and all light beams can meet
and brighten up the original star. This is called micro-lensing. In 1998, some
micro-lensing events were reported, and they led to speculation that they may
be detecting mirror matter objects in the sky. But those observations soon
found conventional astrophysical explanations. Still, there is no evidence for
mirror universe yet. But astronomers should be alert to these kinds of new
possibilities. Micro-lensing is one of the techniques generally used to search
for extrasolar planets and has been successfully used. This can also be used
for detecting mirror stars and planets. For a picture of how micro-lensing will
work for a mirror star, see Fig. 22.1.
R. N. Mohapatra
which is the mirror partner of the familiar photon, etc. One word of caution:
the mirror universe is part of our universe and is not necessarily related to
the multiverse picture of string theory, where presumably the universes are
disconnected from one another. There exist different variations of the mirror
universe scenario where the two sets of particles “communicate” with each
other via different kinds of “portals.”
The left–right symmetric theories provide a different way to maintain parity
symmetry without doubling the quarks and leptons and without postulating
another universe of forces. They keep the same number of quarks and leptons
as in the standard model, but only add the right-handed neutrino (three
of them) to the standard model fermions. In such models, the neutrino
automatically has mass by pairing up with the right-handed neutrino via the
seesaw-like mechanism as described. As we saw before, the neutrino mass
requires a right-handed neutrino. Thus this is a picture that fits right into
the modern discussion of neutrino mass and maintains mirror symmetry in
nature.
The left–right symmetric theories for neutrino mass are more economical
compared to the mirror universe model. However, the mirror universe models
do provide an automatic dark matter candidate, which is the lightest baryon
of the mirror sector. It also provides extra sterile neutrino states [56] (or
mirror neutrino states) in the universe. These would be good candidates to
fit some experiments that seem to indicate possible anomalies. The dark or
sterile neutrinos interact only with the mirror W , Z and are thus hidden from
our view, except for their mixing with ordinary neutrinos, which would then
explain the sterile neutrino anomalies.
How can we test for the existence of the mirror universe? Various suggestions
have been made in the literature about it. In the minimal version of the model
where the only force connecting the two universes is gravity (a connection,
which is always there), there can be mirror stars, mirror planets, etc. Since
these objects are transparent to our photon, light from stars behind a mirror
star will pass through it. The rays of light passing on both sides of the mirror
star would get bent due to gravitational force and all light beams can meet
and brighten up the original star. This is called micro-lensing. In 1998, some
micro-lensing events were reported, and they led to speculation that they may
be detecting mirror matter objects in the sky. But those observations soon
found conventional astrophysical explanations. Still, there is no evidence for
mirror universe yet. But astronomers should be alert to these kinds of new
possibilities. Micro-lensing is one of the techniques generally used to search
for extrasolar planets and has been successfully used. This can also be used
for detecting mirror stars and planets. For a picture of how micro-lensing will
work for a mirror star, see Fig. 22.1.