This student text describes Epoch Four in our Evolving Universe. It is the sixth student text of the Tracing the Origins of Our Universe activity.

The Fourth Epoch lasted only a fraction of a second—from ten to the negative three to ten to the negative six second.

Feel the number Four in the upper left corner of the card. Place your Epoch Four Tactile Card to the right of the Epoch Five Tactile card on the table.

The left box of the card has representations of the protons, neutrons, and electrons present at the end of the epoch. quarks are usually about ten to the negative twelfth centimeters apart at the present temperature of the universe.

The distance between the quarks in these protons and neutrons is about ten to the negative ninth centimeter. Remember that the

The right-hand box represents the composition of matter at the beginning of the epoch.

The universe is a very dense plasma that contains about ten to the twenty-seventh protons and the same number of neutrons per cubic centimeter.

The temperature is about one million billion kelvins. It is high enough to overcome the strong nuclear force between the quarks in protons and neutrons.

In addition, there are ten to the thirty-sixth electrons in one cubic centimeter, some photons and neutrinos.

So quarks and gluons, as well as electrons, are free particles during the first part of the epoch.