Page 106 - BBC Sky at Night - September 2017 UK

106 EXPERT INTERVIEW SEPTEMBER

WHAT I REALLY WANT TO KNOW IS…

Where does antimatter come from?

Dr Roland Crocker has been on a quest to discover what

produces anitmatter particles called positrons in our Galaxy

INTERVIEWED BY PAUL SUTHERLAND

t has been known since the 1930s that the best map of the distribution of gamma rays
antimatter exists alongside ordinary across the Galaxy.
matter in the Universe. It was first About 10 years ago, astronomers
Idiscovered in cosmic rays from realised that positrons were being
deep space – but where does it come created at low energies, supporting
from? I’ve been trying to find out. the idea that they come from the
For every elementary radioactive decay of unstable
particle, there’s an antimatter isotopes. One isotope, nickel-56,
particle with an opposite is produced copiously in
quantum state. So for the Type Ia supernovae, where
electron, which has a a white dwarf star becomes
negative charge, there’s too massive and destroys
an antimatter particle, itself. But that explanation
called a positron, with a doesn’t solve our problem.
positive charge. Positrons Those supernovae don’t
are the dominant occur in stars as old as
component of antimatter those in the Galaxy’s bulge.
in our Galaxy. We use We also ruled out the
them medically in PET supermassive black hole at
scanning – positron the heart of the Galaxy and
emission tomography. dark matter as possible causes.
When an antimatter particle My team looked for a
encounters its ordinary-matter weaker type of supernova that
partner, they annihilate each other. can occur in older stellar populations
However, ordinary matter greatly approaching the age of the bulge. Our
outweighs antimatter in the Universe, by many calculations showed that a good candidate
orders of magnitude. We also can’t detect positrons would be a binary system with two fairly low-mass
in the Galaxy directly; we infer their existence from An unusual type of ‘dim’ stars that are exchanging material. Each star isn’t
supernova seen in old
the gamma-ray signal emitted when they collide that massive – more massive than the Sun but less
stellar populations may
with electrons in the interstellar medium. than two times its mass. Because they’re fairly
explain the origin of
most of the antimatter low-mass stars, they have long lifetimes. You end up
Something strange is going on detected in our Galaxy with two white dwarfs. One will be a helium white
The first instruments capable of detecting gamma dwarf, because it’s not massive enough to undergo
rays were attached to high-altitude balloons in the helium burning in its core, and the other is a
early 1970s and later launched on satellites. Even carbon-oxygen white dwarf.
those early experiments indicated that the centre Eventually, the helium white dwarf gets so close
of the Galaxy was quite bright in gamma rays to its more massive companion that it’s disrupted
produced by annihilating electrons and positrons. by its tidal field. The two stars merge into a single
Over subsequent decades, the quality of object with carbon and oxygen at its centre and
ABOUT ROLAND
instrumentation improved. It became clear an unstable shell of helium, ready to explode. It
CROCKER
that the central bulge of the Galaxy is producing Dr Roland Crocker produces an unusual type of supernova, a much
a lot of positrons. This seemed odd because the studies high-energy fainter subclass of a Type Ia supernova. The
bulge is made up of ancient stars that are about astrophysics, cosmic rays explosion produces another type of radioisotope,
10 billion years old. You wouldn’t expect much and galactic astronomy called titanium-44, which is also a positron source.
to be happening there. In all other wavelengths, at the Australian This looks to be the answer because the process
National University’s
the bulge is quite dim, but in gamma rays it’s College of Physical and can explain about 90 per cent of the antimatter
MARK GARLICK about as bright as the rest of the Galaxy. We’ve Mathematical Sciences, in the Galaxy. It neatly explains both where the
been investigating using data gathered by ESA’s
positrons are being created and the number that
in Canberra.
Integral space telescope, which has produced
are being created too. S
skyatnightmagazine.com 2017