Author: Cendes Yvette
Date: January 2007
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At its core, physics is a field devoted to solving fundamental questions. These puzzles have inspired and tormented countless physicists throughout the ages, and the best minds are still busy at work pinning down the details through both active research and philosophizing beside the department coffee machine. What was the universe like in the beginning? What makes up all the stuff we see around us? Why is there something instead of nothing, and why is so much of it crammed onto my desk?
During the last few decades, physicists have worked out general answers to the questions listed above, with the notable exception of figuring out what to do about desk clutter. (They prefer to leave that solution as an exercise for the reader.) By observing radiation left over from the Big Bang, known as the Cosmic Microwave Background, we know that within the first few microseconds the early universe was about the size of a baseball and consisted of a dense, superheated soup of fundamental particles. We also know a lot about the particles and how their composition and collisions were similar to the ones observed in modern particle accelerators.
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When it comes to the question of how the matter today came to be, part of a topic known as baryogenesis, methods of observation fall short. The reason for this lies in a basic tenet of particle physics whereby there are not one but two types of particles that need to be accounted for. The first particle type consists of "normal" baryons, which encompass all particles we are familiar with down to negatively charged electrons surrounding positively charged protons in an atomic nucleus. The second group of particles falls into the category of antibaryons, which have the same amount of mass as their baryon counterparts but have the opposite charge. In a symmetrical Yin Yang of particle physics, creating a baryonic particle also requires creating an antiparticle.
When a particle and an antiparticle come into contact with each other, as science fiction fans know, the two annihilate and are converted into energy. Judging by this guideline alone, all the matter in the early universe should have been annihilated quickly after it was created because the particles were so tightly packed together.
While the idea of particle-antiparticle annihilation has been proven by countless experiments, there is a problem: we see a lot of matter in the universe! Moreover, everything we see in the natural world is comprised of baryonic matter, from rocks on the ground to stars in the sky, which should have been annihilated by their antibaryon equivalents long ago. The fact that this lack of symmetry between particles and antiparticles occurred in the early universe is known as baryon asymmetry.
So why are baryons asymmetric? "There are a lot of theories, but there are three conditions that have to be met in order to explain the asymmetry," says Mark Trodden, a physicist from Syracuse University. These conditions were outlined by Soviet scientist Andrei Sakharov in 1967 and are as follows:
- There must be a violation in baryon number, made up of both the number of particles in a system and the particle type, where every baryon in the system is counted as a +1 and every antibaryon is counted as a -1. The baryon number going into a reaction must remain the same after the reaction, similar to balancing the equation for a chemical reaction. A violation would happen if this condition is not met, but such a violation has never been observed in physics.
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- There must be a violation of CP symmetry. For most particles, the laws of physics stay the same when you flip the particle's direction (such as adding a minus sign in front of your coordinates), so the particle would look the same on both sides of a mirror. When there is a conservation of this operation, called the parity, and a conservation of charge, you have CP symmetry. In the past few decades, several particles have been observed to violate this symmetry while undergoing radioactive decay: that is to say, it is possible to observe a difference between a "right-handed" and a "left-handed" particle. This is called CP violation.
- Interactions outside of thermal equilibrium must occur. Thermal equilibrium refers to a state where the temperature is the same throughout a system, such as our early universe. Because the universe was so hot and small, all the particles were closely packed together and undergoing collisions at a fast rate. As the universe expanded in size and cooled down, the temperature was no longer the same everywhere in the universe. Furthermore, the rate of interactions between the particles decreased and fewer collisions occurred over time. In this way, this condition would be met over time by the expansion of the universe.
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Looking at these three conditions, one can have a general idea of what happened in the early universe to cause asymmetry. For the first part of the universe, the era whose signature is the Cosmic Microwave Background, baryons and antibaryons particles alike were extremely close together and undergoing collisions at a fantastic speed. As time wore on, however, the universe expanded and cooled which caused a slight fluctuation between the density of matter and antimatter: a fluctuation so small that only one in a billion particles was affected by it. By sheer chance, these few particles favored baryons instead of antibaryons, and they eventually seeded our universe to be one of baryonic matter.
Had it been the other way around, of course, our universe would be filled with antimatter. Physicists believe this "anti-universe" would be nearly indistinguishable from the one we have today except particle charges would be switched- electrons would have positive charge, for example, and would orbit negatively charged protons in atomic nuclei. While these particles do not occur naturally they have been made in the lab: in 1995, a team of scientists at the CERN laboratory in Switzerland announced the creation of the first antihydrogen atom consisting of an antiproton orbited by a positron. Millions of antihydrogen atoms have been created to date, but so far all the atoms were immediately annihilated upon hitting the experimental apparatus.
But what caused the small glitch which came to favor our version of matter? Physicists studying baryogenesis have proposed several theories to explain the asymmetry. "Baryon asymmetry could be due to grand unification physics," says Lawrence Krauss, a Case Western Reserve University physicist, referring to the grand unification theory (GUT) which unites the electromagnetic and weak forces in physics and is heavily relied on in particle physics theory. CP violation can work with GUT easily, which makes the theory very appealing on paper.
The problem, however, is testing this theory is difficult due to the immensely large energy scales involved, which are too high for particle accelerators in the near or distant future. "GUT is still plausible and it may still be right, but it's a strained idea," Krauss explains.
Another possibility is that baryon asymmetry occurs only at very high energies akin to those seen in the early universe. "All the ingredients are there," says Trodden, "and the threshold is just above that of what modern particle accelerators can observe."
This piece of information is exciting for physicists because the next generation supercollider, the Large Hadron Collider (LHC), is currently being constructed in Europe and will begin operation in November 2007. In what is perhaps the largest scientific undertaking of all time, the collider consists of a 27 kilometer underground ring and is being built by a collaboration of 2,000 physicists in 34 countries. Its large size will allow particles to be smashed into each other at speeds approaching the speed of light and with enough energy to detect many theorized effects in particle physics, which potentially includes baryon number violation.
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Will the question about baryogenesis be answered by the LHC? It may take a few years to find out: though operations will begin in 2007, the data analysis will be a long and time-consuming process. Cracking the secrets of asymmetry will have to wait.
But until then, the physicists will still have an age-old question to ponder during coffee breaks.
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Comments:
The author has tackled a difficult topic and makes a strong attempt at explaining it to a lay audience. With this in mind there are still some areas of the article that are confusing and unclear to a non-physics major. In general the writing is engaging, especially the introductory paragraph.
Maybe antibaryons were just 'sequestered' (as entropically impossible as that may be) and now compose an alternate universe."
What I really enjoy about this article is the word usage. The tone of this article presents the topic in a very inviting way that draws the reader in--everyone would rather read an article that sounds like they're talking to a friend rather than reading a textbook. Also, the title is so mysterious, I immediately was pulled in. Much kudos to the author.