History of the Universe

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Quark Epoch 10-36 to 10-6 seconds

Next we will watch as the inflaton field cooled and see the different sorts of particles which were produced. At different times, some were more common than others.

The earliest common particles were the quarks. The short period of time when they were commonest is called the Quark Epoch. The Quark Epoch probably started about 10-36 seconds came to an end about 10-6 second (one microsecond) after the creation of the Universe.

So what is a quark?

Quarks

A quark is a particle which today is only ever found inside other particles. No solitary quark has ever been found, but there are good theoretical reasons for believing that they existed as independent particles in the very young Universe.

There are 6 types of quark: the up, down, top, bottom, strange and charm quark.

Lightest Generation

Up quark

Down quark

Middle Generation

Charm quark

Strange quark

Heaviest Generation

Top quark

Bottom quark

How the quarks are arranged according to the Standard Model

A quark’s electric charge is a fraction of that carried by the electron (which is the common standard of charge). They have mass, the different types having different masses, and they also carry a type of charge called "color". This is very different from our normal idea of color. The reason this word is used will be seen when we meet protons.

Quarks also have properties called flavour, mass and spin. Other particles would appear later in history but, because of all their properties, quarks alone are able to feel all four types of fundamental force.

Gluons

As we have seen, the strongest force, and the most important for quarks, was the strong nuclear force. This force was carried by gluons, which are really particles, but it is easier to think of them as tiny elastic bands.

When quarks are very close together and the gluon elastic is not stretched, the strong nuclear force is actually very weak. This was the case soon after the Big Bang. Quarks were so close that the gluons had almost no effect and quarks were free to move anywhere.

As quarks separate, the strong nuclear force between them gets stronger. It is as if the gluon elastic is being stretched and so pulls them together more forcefully. We will see the consequences of this later, during the Hadron Epoch.

Quark-gluon plasma

During the Quark Epoch, all the quarks constituted an ocean of free quarks filling the whole of space, held together loosely by the constant exchange of gluons. Such a state is called a quark-gluon plasma.

Antimatter

Quarks often collided as they shot around, driven by the heat of the Big Bang, and usually these collisions resulted in them bouncing away in new directions. But occasionally when two quarks met they would annihilate each other, fusing their energy and converting it into some other type of particle. This occurred but rarely, only when the two quarks were identical in every way except charge, which was equal but opposite. Two such quarks are said to be "antiparticles" of each other.

There are two kinds of particles. The types we find in the world today (which I will describe in the following pages) known as matter, and others which are similar in every way except one: they normally have the opposite kind of electrical charge. For example the antiproton has the same mass as the proton but a negative charge.

Antimatter is opposite to ordinary matter

The antineutron has the opposite magnetic moment to a neutron. These opposite kinds of particle are called antimatter.

We never meet these antimatter particles in normal life, and that is a good thing, because when matter meets antimatter they annihilate each other and their matter energy is changed into radiation. Antiparticles are real, however, and can be made in high energy particle accelerators.

If a space ship came to Earth from some distant galaxy we would not be able to tell whether it was made of antimatter just by looking at it since matter and antimatter look the same. But if it was antimatter it would explode when it tried to land!

By adding a positron to an antiproton it is possible to make an anti-atom of hydrogen. This was first achieved in 1995 at the European Laboratory for Particle Physics (CERN) by firing antiprotons through a xenon gas jet. Some of the antiprotons hit protons in the xenon nuclei, creating pairs of electrons and positrons. A few of these positrons then stuck to the antiprotons to form anti-hydrogen.

Anti-atoms do not last very long on Earth. Each anti-atom produced at CERN survived for only about forty-billionths of a second before it met ordinary matter and changed into gamma radiation.

For the rest of this story we will not be much concerned with antiparticles. Now I introduce the particles which were found in the young Universe.

Missing antimatter

Particles and antiparticles should have been created in equal numbers in the Big Bang. But we never normally find antimatter in the world today.

Some people think that distant regions of the Universe might be made of antimatter. Perhaps whole galaxies might be made of it, protected from annihilation because they are separated from normal matter by vast oceans of empty space? Based on our current understanding of the early events in the Universe, this seems very unlikely. Particles and antiparticles were created close together in the young Universe. Nobody has yet thought of a way they could have been separated into different galaxies.

So where did the missing antimatter go? Nobody knows. It is one of the unsolved mysteries of the history of the Universe.

One theory is that more matter was produced than antimatter, perhaps due to the strange decay of some heavy particles.

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