HUGE globs of exotic hydrogen from a mirror universe are the latest quarry in
the hunt for the mysterious dark matter that fills the Universe. They could also
explain why far fewer neutrinos than expected emerge from the Sun.
Rabi Mohapatra and his colleagues at the University of Maryland in College
Park followed up an idea that dates back to the 1980s. This was that every
elementary particle has a partner in a mirror world, an idea that would help
tidy up theories of how the four forces of nature acted as one just after the
Universe was born.
The two components, each made up of its own set of fundamental
particles—such as the electron and the three normal types of
neutrino—would be mixed up. The force of gravity could easily clump all
the mirror matter into mirror stars and mirror planets. But that would be the
only force of our world that they would feel. Mirror matter would not feel the
electromagnetic force, so mirror stars would be invisible.
Mohapatra realised that if one of the three neutrinos that would exist in the
mirror world was much heavier than its everyday counterpart, this could explain
why the Sun’s nuclear reactions produce only a third of the neutrinos that
Physicists have suggested that the reason for this is that the type of
neutrino expected from nuclear reactions is turning into the two other normal
types—which are as yet undetectable—en route to the Earth. This
happens through a so-called oscillation—two or three neutrinos are
effectively “mixed up” and flip from one type to another over a regular
distance. Some inconclusive evidence for this comes from detectors at Los Alamos
National Laboratory in New Mexico (see “Cosmic changelings”, New
Scientist, 16 March 1996, p 28).
The oscillation distance of the neutrino mixture depends on the masses of the
neutrinos involved. The masses of everyday neutrinos seem too small to allow
oscillations over the distances that would explain the solar neutrino problem.
But a fourth, heavier neutrino from the mirror universe could interact with the
normal neutrinos through gravity and make the oscillation distance just right,
Now, working with his colleague Vigdor Teplitz of the University of Maryland,
Mohapatra has looked in detail at the implications of this theory for the mirror
universe (Astrophysical Journal, vol 478, p 29). If the mirror world
contains a heavy neutrino, it turns out that beta-decay—which involves
neutrinos—is much faster there. In that case, mirror neutrons would decay
so quickly that the only element that could exist is mirror hydrogen.
Without neutrons, nuclear fusion would be impossible, so there would be no
stars, says Mohapatra. As the Universe expands, the mirror hydrogen would clump
into “globs” the size of dense star clusters. The globs would form
part—perhaps all—of the invisible “dark matter” that exerts a pull
on stars and galaxies. Mohapatra adds that the idea can be tested, because the
globs would bend the light from background galaxies and focus it just like a
Rocky Kolb of Fermilab, near Chicago, calls Mohapatra’s work “a really nice
study”, and adds that it could help to extend the current Standard Model of
particle physics. “There seem to be several loose threads fraying at the edges
of the Standard Model, and nobody is sure which one to pull to make the model
unravel,” he says. “Perhaps this is it.”