This is the first student text in the Tracing the Origins of Our Universe activity, a part of the Evolving Universe materials.

The title of this student text is “Tracing the Origins of Quarks”

How far back could you trace the ingredients of your pizza? As far back as the molecules of the three food groups—fats, carbohydrates, and proteins?

Or further back than that—to elements like carbon, oxygen, hydrogen and nitrogen that make up those molecules?

Did you trace the atoms of these elements back to their sources—the water, air, and soil of the Earth?

If so, did you wonder how these atoms happened to be part of the Earth and its environment or how the atoms were formed in the first place?

Since our Sun contains ninety-nine percent of the matter in the Solar System, the Earth and the other planets contain only a very small portion of solar system matter.

Our Earth is a unique planet in the solar system for the following reasons.

First, almost all of the known chemical elements have been found somewhere in the Earth or its immediate environment. Other planets may have the same wide variety of chemical elements but our present scientific investigations have not been able to confirm this.

Second, many large molecules, such as proteins, carbohydrates and fats, that compose living organisms have formed on our planet.

Third, the temperature range of our planet is such that living organisms can exist on the surface in relative comfort.

Fourth, water is found in three phases—solid, liquid and gas—on the earth and in its atmosphere.

For many years, scientists have been asking, “Where did the solar system matter come from?”

Since our solar system itself accounts for a very small portion of the matter in the universe, let’s start with what scientists have observed about the present universe.

Hydrogen, the smallest chemical element, makes up seventy-seven percent of the matter found in the “spongy universe” and the interstellar clouds. Helium makes up another twenty-two percent by mass of the present universe.

Where does all this hydrogen and helium come from? Stars like our Sun are nuclear furnaces where the temperature can be as high as ten million kelvins. At these temperatures, hydrogen nuclei are fused into helium nuclei. These hydrogen and helium nuclei are made of quarks.

The following is a description of figure on Tactile Card A. Have your card available so you can follow the description.

The three arrows in the box on the left represent the two up quarks and one down quark that make up the one proton—a hydrogen nucleus. A proton has a net charge of plus one.

In the box on the right, there are four sets of quarks. Two of them represent protons with two up quarks and one down quark. The other two represent neutrons with one up quark and two down quarks. Neutrons have a net charge of zero. There are two protons and two neutrons in the nucleus of a helium atom.

These quarks are held together in sets by gluons, carriers of the strong nuclear force. The protons and neutrons in the nucleus of atoms are also held very tightly to each other by this strong nuclear force.

We haven’t shown any electrons in Tactile Card One because the temperatures of stars like our Sun are so high that the electrons have enough energy to move freely. So the plasma in the Sun is made of negatively-charged electrons and positively-charged hydrogen and helium nuclei.

This is the end of the description of Tactile Card A.

So where did all the other elements that we find in the Solar System come from? Nitrogen, oxygen, carbon, and heavier elements like iron are formed in stars that are more massive than our Sun.

Any elements heavier than iron can usually form during a huge energy release such as that which occurs in a supernova. A supernova explosion scatters all these elements back into the galaxy, enriching it with heavy elements that then become part of a future generation of stars.

Our Sun will eventually become a red giant star, eventually losing much of its mass in one or more explosions and forming rings or other shapes, called planetary nebulae.

The remaining stellar shell would be classified as a white dwarf, which would eventually cool to a black dwarf. This cooling shell might eventually meet up with a molecular cloud, giving it new fuel for a time. It might also eventually get captured by some other star.

All these components are made of quarks and electrons. When our sun dies, the quarks and electrons in the solar system matter will be recycled to form other suns and planets, just as they have been since the beginning of time.

So, if we trace the origin of Earth’s matter, we find that it consists of atoms of elements that were formed inside stars. The atoms that make up our bodies and our pizzas came from stars, too!

These atoms were distributed through space in supernova explosions that occurred in the early universe. This means that we ourselves are born of the stars. We are made of parts of the millions of supernovas that have exploded in our galaxy during its lifetime.

So, where did the quarks and electrons that form hydrogen and helium come from?

One of the basic precepts of the standard cosmological model states that the early universe was in a state of high density and high energy.

This concept was developed in much the same method you used to trace the ingredients of your pizza to their “origins.” Instead of cheese, meat, tomato sauce and dough flour, cosmogonists started with what scientists have observed in the universe today.

They studied the types of structures that are present in our “spongy universe.”

They observed the rate at which the universe is expanding and the effects of the universal gravitational force that slow that expansion.

They identified the chemical composition of cosmic structures and the energy sources that are present today.

Then they applied descriptive mathematics and fundamental physical laws to develop a model of eight major “epochs” of cosmic evolution.

Just as you focused on one ingredient of your pizza to find its origins, we will focus only on quarks and electrons as we go backward in time to discover what important role they played in those early times when the universe had a high density and high energy.

Keep in mind that the overall picture of the evolving universe is much more complicated than what we will be studying, since we are focusing only on quarks and electrons.