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

The Seventh Epoch began when the universe was thirty minutes old and lasted almost three hundred thousand years.

During this time the universe expanded and cooled down from about one hundred million kelvins to three thousand kelvins.

This is a description of figure on Tactile Card Seven. Have your card available so you can follow the description. Feel the number Seven in the upper left corner of the card.

The Epoch Seven tactile card illustrates where quarks and electrons were found during this epoch. The card has two different sections. Boxes A and C on the left side of the card show quarks and electrons during the later part of the epoch. Boxes B and D on the right side of the card show quarks and electrons during the early part of the epoch.

The difference between the two sections of the cards is temperature. This means that there is also a difference in the kinetic energy of the protons, neutrons, and electrons. As the temperature increases, the kinetic energy of these particles increases.

The temperature in boxes on the left side of the Epoch Seven tactile card is three thousand kelvins, the average temperature at the end of the epoch. At this temperature, quarks and electrons are found in neutral hydrogen and helium atoms, just as they are in Epoch Eight.

The boxes on the right side of the Epoch Seven tactile card represents the beginning of the epoch, when the average temperature was one hundred million kelvins.

At these high temperatures, however, there were no atoms, so there are no circles that represent the outer limits of the atoms.

Quarks were still tightly held together by gluons in hydrogen and helium nuclei but electrons had enough energy to overcome the attractive positive charges of the protons so they existed as separate particles.

In Box B you will find a positively-charged hydrogen nucleus, made of two up quarks and one down quark, and a negatively-charged free electron.

In Box D you will find a positively-charged helium nucleus, made of two protons and two neutrons, and two negatively-charged free electrons.

Hydrogen and helium were the only elements present in the Seventh Epoch. Early in the epoch, the cosmic gases were made of hydrogen and helium nuclei and the negatively-charged electrons liberated as the positively-charged nuclei were formed. We call gases made of charged particles plasma.

As the universe cooled late in the epoch, kinetic energy of the electrons decreased and they became part of the neutral hydrogen and helium atoms that are found everywhere in the Eighth Epoch.

The percentage of hydrogen found in both epochs remained the same—seventy-seven percent. The percentage of helium was twenty-three percent of the matter at the beginning of the Seventh Epoch and decreased during the epoch to twenty-two percent.

When you have finished examining Epoch Seven tactile card, place it to the right of the Epoch Eight tactile card.