History of the Universe

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Lepton Epoch 1 to 10 seconds

The Hadron Epoch ended when the Universe had expanded so much, and the energy density had fallen to such a level, that no more hadrons could be created. There was still enough to create lighter particles, however. These are known collectively as leptons.

The Lepton Epoch started about 1 second after the creation and ended about 10 seonds later.


According to the Standard Model, the six leptons are arranged in three generations – the ‘electron’ and the ‘electron-neutrino’, the ‘muon’ and the ‘muon-neutrino’, and the ‘tau’ and the ‘tau-neutrino’. The electron, the muon and the tau all have an electric charge and a mass, whereas the neutrinos are electrically neutral with very little mass.











An electron is like a cloud of probability

The young Universe contained tiny particles called electrons which behaved like clouds. We think of them as clouds of probability. The thickness of the cloud decided how likely it was that you would find the electron at that point.

Electrons carried something called a negative electric charge.

Electrons are important today because, together with protons and neutrons, they make the atoms from which you and I are made.

Electrons feel the electromagnetic, weak, and gravitational forces but not the strong nuclear force.

The electron has an antimatter particle called the positron. This has the same mass but carries a positive charge. If it meets an electron, both are annihilated in a burst of radiation.

Electrons are much lighter than protons. It would need 1,836 electrons to equal the mass of a single proton.

Size of electron

If a soccer ball were inflated to the size of the Earth (which I call Soccearth), electrons would be fuzzy clouds which can change in size and shape. You could squash them down so they were too small to see, or they could grow to be bigger than you are.

Why did the balloon stick to the ceiling?

We can now explain why the balloon sticks to the ceiling.

As you rub, you pull electrons off the balloon onto the wool. This gives the balloon a positive charge (because it is short of electrons). When put near the ceiling it attracts electrons in the ceiling.

Electrons in the ceiling are attracted slightly downwards towards the balloon, so giving the surface of the ceiling a slight negative charge. The negative of the ceiling and the positive of the balloon attract and hold the balloon up.


As we mentioned before, the young Universe also contained positrons, the antimatter equivalent of the electron. It was the same as an electron except it had a positive charge

We do not find positrons on Earth today, although they are released during the nuclear fusion in stars.


A neutrino is a particle which has very little mass (nobody is quite sure how much) and no electric charge, so it hardly seems like a real particle at all. There are three types of neutrino: the electron neutrino, muon neutrino and tau neutrino. Each of these also has its anti-particle, called an antineutrino. Electron-antineutrinos for example are created during the decay of neutrons via the weak interaction. They are also created in large quantities in the stars.

 A key feature of neutrinos is that they hardly interact with other particles at all. Huge numbers of them are created by the Sun. Every second 60 billion neutrinos (mostly from the Sun) pass through every square centimeter on Earth. Yet we do not even notice them. These elusive particles can only be detected in lakes of liquid sunk deep in coal mines

If neutrinos have mass, even if only a little, then because there are so many of them they would weigh more than all the stars in the Universe.

But the mass of the neutrino seems to be too small to account for dark matter by a factor of 20.

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