Ingredients of the Universe
Ordinary MatterThe Standard ModelThe Standard Cosmology

The Standard Model

So, we used to think that ordinary, atomic matter dominated the mass inventory of the universe. Even then, we knew that other forms of matter were possible. A zoo of other particles have been created in the lab and have been produced by cosmic rays. You may have heard of some of them: mesons, positrons, muons...

Ordinary matter is almost the only stable known form of matter.

So, how do those other particles relate to ordinary atomic matter, and why don't they contribute to the mass of the universe?

First, the easy answers: Of all the possible types of particles from the zoo (the zoo is made of members and combinations of members of the "Standard Model"), the only stable particles (stable meaning they don't decay very rapidly) are:

The components of ordinary atomic matter: protons: They have never been observed to decay neutrons: They are stable only when bound in a nucleus. A free neutron will decay with a half-life of about 10 minutes. electrons: They have never been observed to decay. Neutrinos: The cosmos teems with neutrinos, which are exceedingly light and very difficult to detect. They don't seem to contribute much to the mass inventory of the universe, but they are important in stellar nuclear reactions. So, to summarize: The combinations of particles which form ordinary atomic matter happen to be the only particles (except for the ghostly neutrinos) which are long-lived enough to be important in considerations of the cosmological mass scales.

But, what of the others and what are their relationships to ordinary atomic matter?

The Standard Model

Let's see if we can cover the entire Standard model as simply and succintly as possible. According to the Standard Model, there are two types of things (besides empty space, which may not be really 'empty'):

  • fundamental fermions
  • force carriers

Let's first talk about the force carriers. An example of a force carrier is the photon, which is a packet of light. It actually carries the electromagnetic force. There are others which correspond to the other forces:

  • gluons: carry the strong force which holds protons and neutrons together and keeps them bound to each other in the nucleus
  • 'W' and 'Z' bosons: involved in the 'weak force' which is associated with neutrino scattering and forcing some particles to decay and change form (e.g. in radioactive beta decay). The Standard Model also predicts a yet-undiscovered Higgs boson, which relates the W and Z bosons to the more familiar photon.
  • gravitons: the hypothetical carriers of the gravitational force

These particles mediate interactions between the fundamental fermions, the particles we usually think of as "matter". These come in two basic types:

  • quarks: The components of the proton and neutron (among other things), bound together by the strong force. Six kinds of quarks are known to exist, known as 'up', 'down', 'charmed', 'strange', 'top', and 'bottom'. Quarks are not found free in nature, but always bound up inside composite particles: protons, neutrons, pions, etc.
  • leptons: These generally-lighter particles do not feel the strong force. They include the three charged leptons (the electron, muon, and tau) and the three corresponding kinds of neutral neutrinos.

Many different kinds of particles may be constructed from the quarks, though all except the proton and neutron are extremely unstable. The strong force holds quarks together into nucleons and nucleons together into nuclei. The electrons which orbit the nucleus are leptons, held in place by the electromagnetic force. The weak force allows some types of radioactive decay, as well as providing neutrinos with their very feeble interactions with the visible world.

For vastly more detail on this subject, check out The Particle Adventure.