History of the Universe

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Hadron Epoch 10-6 to 1 second

Space was still expanding rapidly after the creation. The Universe was about the size of our Solar System today. The quark-gluon plasma was getting larger and its quarks were being pulled further apart. Now the strong force carried by gluons revealed an exotic and important property.

As quarks moved further apart, the gluon elastic force began to increase, and increase very rapidly. When the distance between any two quarks was more than 10-15 meters, the gluon elastic broke and produced two new quarks, one at each end of the break.

Not only was the space between quarks increasing as space expanded; they were also moving more slowly. The temperature dropped to around 10 thousand million degrees Celsius. Soon they did not have enough energy to break the gluon elastic, and each quark found itself permanently tied to one or more of its neighbours.

The Hadron Epoch started about 10-6 seconds after the creation and ended about 1 second later.


Quarks combined with each other to produce other particles which we find in the world around us today. Up and down quarks grouped together in threes to produce protons and neutrons. A quark combined with an anti-quark to produce a meson. To summarise:


    Three quarks make a proton or a neutron. Particles of this type are called baryons.

    A quark and antiquark make a meson.

Both of these types, baryons and mesons, are grouped together under the name of hadrons. All hadrons are therefore made of quarks.


Of all the particles we have met in the young Universe so far, the proton was one of the first which would last down to the present day.

Protons are made of three quarks. There must be two up quarks and one down quark to make a proton.

A proton consists of three quarks

As I mentioned before, each quark has a property called color charge and now we will see why this property is given this name. Although the property is nothing like the colors we see with our eyes, but the name was given in analogy with color mixing.

To make white light you mix red, green and blue light. In a similar way, to make a proton you need three quarks each with a different sort of color charge, as shown in the diagram. In fact the naming of the colors is arbitrary.

Protons carry something called a positive electric charge. This will be very important later in our story because, as we will see, it will enable a proton to capture another particle (called an electron) and so make a very interesting object called an atom.

The antiproton is an antimatter particle which has the same mass as a proton but carries a negative electric charge.

Protons on Soccearth

A proton is extremely small. One proton is only about 2*10-15 meters across (2 femtometers) and it weighs 1.673 *10-27 kilograms.

What would a proton look like on Soccearth, a soccer ball blown up to the size of the Earth? It would still be much too small to see, even under a microscope! You could only tell that it is there because it would attract an electron to make an atom, as I will describe later.


Another particle created during the Hadron Epoch was the neutron.

The neutron was discovered in 1932 by the English physicist James Chadwick.

Like the proton, the neutron consisted of three quarks, one of each color charge. But, unlike the proton, the neutron contained two down quarks and one up.

Quarks inside a neutron

The neutron is a little heavier than a proton. Its mass is nearly 1,840 times that of the electron. But, unlike the proton, the neutron has no electric charge. The neutron's name means neutral or without charge.

Free neutrons easily pass through atoms, because they have no electrical charge, and so they form highly penetrating radiation, interacting with matter only through collisions with atomic nuclei.

The neutron is important in forming a chain reaction in nuclear fission, as used in nuclear reactors and atom bombs. The absorption of neutrons by other nuclei, exposed to the high neutron densities in nuclear reactors, generates radioactive isotopes useful for a wide variety of purposes.

Notice that the neutron has one property which the proton lacks. A free neutron (one not contained in an atomic nucleus) is radioactive. I will explain this next.

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