Where did the matter in the universe come from? (Intermediate)

Okay, if I'm right, acording to the Big Bang theory, the universe started out as a highly concertrated ball of matter. Well, where did that matter come from? I mean, where did the stuff to create everything in the universe come from? Did it just appear or something? (PLEASE answer! This has been bothering me for YEARS!)

In the beginning, there was not yet any matter. However, there was a lot of energy in the form of light, which comes in discrete packets called photons. When photons have enough energy, they can spontaneously decay into a particle and an antiparticle. (An antiparticle is the exact opposite of the corresponding particle--for example, a proton has charge +e, so an antiproton has charge -e.) This is easily observed today, as gamma rays have enough energy to create measurable electron-antielectron pairs (the antielectron is usually called a positron). It turns out that the photon is just one of a class of particles, called the bosons, that decay in this manner. Many of the bosons around just after the big bang were so energetic that they could decay into much more massive particles such as protons (remember, E=mc2, so to make a particle with a large mass m, you need a boson with a high energy E). The mass in the universe came from such decays.

The next question to ask is: where did all the antimatter go? For each particle created in this fashion, there is exactly one antiparticle. In this case, there should have been exactly as much antimatter as there is matter. If that were true, when the universe had cooled somewhat each particle would have found an antiparticle and combined to form a boson (this process is called annihilation of the particles). Actually, this was the fate of most of these pairs--something like 10 billion particles annihilated for every one that survived. The survival of even such a small fraction was enough to form all of the matter in our universe. At some point during this process, something else must have happened to cause the survival of more particles than antiparticles (we call this the particle-antiparticle asymmetry).

There are many theories that try to explain this asymmetry. I will give a very brief description of one of them, called electroweak baryogenesis. (Understanding it requires a lot more background information than I have space for.) Protons and neutrons are particles called baryons, and baryogenesis means the creation of baryons. The current understanding of particle physics, called the standard model, dictates that nowadays the number of baryons is nearly constant, with only a small variation due to quantum mechanical tunneling. In the early universe, however, the temperature was much higher, so that this tunneling was commonplace and a large number of baryons could have been created. Electroweak refers to the time period in question, when the electromagnetic and weak forces were decoupling from a single force into 2 separate forces (between 10-12 and 10-6 seconds after the big bang--the asymmetry probably would have formed towards the end). An additional source of baryons is due to the fact that leptons (another type of particle, including electrons) can be converted into baryons at this epoch.

This page was last updated June 27, 2015.

About the Author

Sara Slater

Sara is a former Cornell undergraduate and now a physics graduate student at Harvard University, where she works on cosmology and particle physics.

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