The Standard Model starts out with this simple question: what is stuff made of? Until about 100 years ago the answer would have been “atoms.” The very word atom comes from a Greek word meaning “indivisible.” Scientists thought atoms were the solid building blocks from which everything else was made. Ernest Rutherford and others showed that atoms are mostly empty space. There was a dense nucleus made up of protons (positive electric charge) and neutrons (neutral), surrounded by the cloud of negatively charged electrons. For a time protons, neutrons and electrons were considered fundamental particles, indivisible any further. Then in the 1960s “atom smashers,” machines that accelerate particles to extremely high speeds and smash them into each other, showed that protons and neutrons were composed of partially charged particles called quarks. There was also a weightless particle called a neutrino that could pass through thousands of miles of solid iron without slowing down. But things were still fairly simple. There were quarks, electrons and neutrinos. Quarks are incredibly tiny. If an atom was the size of the Earth, a proton would be a football stadium and a quark would be a tennis ball.
In the next decade, more powerful particle accelerators, capable of accelerating particles to near light speed, demonstrated hundreds of fundamental particles. All seemed to fall into one of 12 categories of fermions, depending on their mass. The lowest level of fermions contains normal matter: up and down quarks, electrons and neutrinos. The next two levels contain fermions that exist only fleetingly in particle accelerators. Each of the 12 fermions has a corresponding antiparticle that can only be created with a sufficiently energetic collision.
As a counterpart to fermions there are four bosons that carry all forms of energy. Photons carry electromagnetic energy (light and all other wavelengths of electromagnetic radiation). W and Z bosons carry the weak force, which gives rise to radioactivity; and the strong force that binds atomic nuclei together. Finally there is the hypothetical graviton that carries the force of gravity. The graviton has never been observed.
The theoretical framework that describes the 12 fermions and four bosons is called the Standard Model. The Standard Model does for particle physics what Newtonian physics first did for the observable world (the world we live in) and General Relativity did for the cosmos. It explains what we see. The Standard Model predicts that the four fundamental forces, which vary greatly in their strength, will be unified at sufficiently high energy. This prediction was partially proven in the 1970s when the electromagnetic and weak forces were unified. Like Newtonian physics and General Relativity, the Standard Model is incomplete. It fails to explain why there are three levels of quarks and light particles. It leaves open the possibility that quarks may be further divisible. For a time scientists theorized another boson called the Higgs boson that gives all other particles their mass in order to preserve the Standard Model. In July 2012 scientists using the Large Hadron Collider at CERN discovered the Higgs boson. This gave a huge boost to the Standard Model, which some felt had outlived its usefulness.
Why is this important to us? Let’s revisit the Big Bang. One millionth of a second after the explosion the universe is about the size of our solar system and is as dense as air. It’s 10,000 times hotter than the core of the sun. Particles and antiparticles appear, collide and annihilate each other at a staggering rate. By ten millionths of a second after the Big Bang expansion has cooled the universe to the point that particles and antiparticles can no longer form from the radiation. The number of particles and antiparticles is fixed and they begin a war of mutual extermination. By 100 microseconds, a twentieth of the time it takes a bee to flap its wings, it’s all over. All the particles and antiparticles have paired up like guys and gals at a dance. There are no wallflowers. All matter is extinguished and the universe contains only radiation.
But wait, you say. That would be the “nothing” universe and we already know there is “something” because we’re here to wonder how it all began. And you would be right. For some unknown reason the symmetry between matter and antimatter wasn’t perfect. It was off by one part in a billion. For every billion antiparticles there were a billion and one particles. When the last particle of antimatter paired with a particle of matter there were a billion photons for every particle left, and no antiparticles. The photons continued to spread out to be discovered billions of years later as cosmic microwave background radiation and the remaining particles combined under the force of gravity to become 100 billion galaxies, one of which contains a very ordinary star around which eight planets revolve, one of which we call home.
How big is one part in a billion? Imagine someone has laid out pennies over an area three miles on a side. All pennies show heads except one. That one penny showing tails is the one particle of matter left after the billion pennies showing heads were annihilated by another billion pennies showing tails. It’s from those leftover tails that the universe is made.