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Where is all the antimatter? Finding out what caused the matter-antimatter asymmetry
This article is an introduction to the unsolved problem of matter-antimatter asymmetry in particle physics and cosmology. Why is the amount of matter in the universe so much greater than the amount of antimatter? This article answers the question "what is antimatter?" and reviews the scientific theories that have been suggested to explain the asymmetry.
What is antimatter?
To answer this, we first need to take a step back and ask: what is ordinary matter? In simple terms, it is the stuff that we see around us everyday. Speaking more scientifically, it consists of vanishingly small particles that join together to form atoms, which in turn stick together in various configurations to make up every object on Earth. The very smallest building blocks – such as quarks and electrons – are known as elementary particles.
For each type of elementary particle, there also exists a type of antiparticle which has the same mass and spin properties as its particle sibling, but opposite charge. The existence of antiparticles was proposed by Paul Dirac in 1928, and the first one was observed four years later by Carl D. Anderson, who was looking at tracks left by particles in a cloud chamber. Anderson saw a track that looked exactly like the path of an electron except for one detail: it curved anticlockwise instead of clockwise in the magnetic field of the cloud chamber, which indicated that the particle responsible for the track was positively rather than negatively charged. The object responsible for the track had to be a new kind of particle - a positive electron, or "positron". Anderson was awarded the Nobel Prize for Physics in 1936 for this discovery.
Antiparticles such as positrons are extremely rare in our universe. They are found in cosmic rays – streams of particles from space which continually stream through our atmosphere – but in numbers which are vanishingly small compared to the billions of electrons that are present in a single grain of sand. And this is where the mystery arises. Even the seemingly chaotic interactions between elementary particles follow rules: in all processes that have been observed to date the “baryon number”, defined as the number of particles minus the number of antiparticles in the interaction, is conserved. This means that if the universe started with equal numbers of particles and antiparticles, their numbers should always stay equal.
So how is it possible that we have ended up with a universe in which the amount of antimatter is almost insignificant compared to the amount of normal matter?
Explanation 1: That's just the way it is
We could suppose that the Universe simply came into existence with a large preponderance of matter over antimatter, and that this initial imbalance has simply perpetuated. This is a deeply unsatisfying explanation. Why should our universe – which otherwise displays a remarkable degree of symmetry – have been born with such a blatant asymmetry built in?
Explanation 2: Conservation laws aren't always obeyed
An alternative explanation is to suppose that the extremes of temperature and pressure that existed in the first few seconds after the Big Bang allowed processes that violate the conservation of baryon number to take place. We do not see these processes taking place today because the energy density of the Universe is simply too low to allow them. Theoretical physicists across the globe are currently working on extensions to the standard model of particle physics that describe such a mechanism for the creation of particles.
Explanation 3: The antimatter isn't missing – just hidden
Another suggestion is that the missing antimatter is in fact not missing after all, but rather that it is hidden, perhaps in regions of the Universe that are too far away for us to see. Some scientists point to the heavy clouds of dark matter which lurk within galaxies, undetectable except by the gravitational effects of its mass on other astronomical objects, and suggest that this dark matter may in fact be made of antiparticles.
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topquark works as a researcher in theoretical particle physics and blogs about research at The Particle Pen.