History of the Universe

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Radioactivity has nothing to do with active radios! It is the breaking up of particles or nuclei into smaller ones. These go shooting off at high speed, hitting other nuclei and making them hot. All the smaller particles which are produced are called radiation, hence the name.

There are several types of radioactivity.

Alpha decay

Beta decay

Gamma Radioactivity

Nuclear Fission

The first type of radioactivity seen during history was beta decay.

Beta decay Radioactivity

A neutron contains up-down-down quarks while the proton has up-up-down quarks. So you could imagine changing a neutron into a proton by simply turning a down quark into an up quark.

Because the neutron contains a little more matter energy than the proton, so the change of a down quark into an up quark would release energy. Therefore a neutron is less stable than a proton, so this change tends to happen spontaneously.

Feynman diagram of beta decay of neutron

This diagram shows how a neutron n changes into a proton p+ over time t.

The neutron contains an up quark u and two down quarks dd. To begin with, one of the down quarks emits a negatively charged W- boson as it changes into an up quark. This leaves two up quarks and a down quark, so the neutron has become a proton.

Meanwhile the W- boson soon decays into two other particles: an electron e- and an electron anti-neutrino. These are particles we will meet a little later in the story.

The down quark was negatively charged and the up quark is positively charged, so the neutron has lost some negative electric charge and gained a positive charge.

This is an example of beta radioactivity. Today, something similar happens in some large nuclei such as thorium.

Unpredictable decay

An extraordinary fact about the decay of neutrons, and all other radioactive decay, was that it was impossible to predict when any individual neutron would decay. It appeared to be completely random. And yet, given a collection of several neutrons, it was always possible to predict how long it would take for half of them to decay. This is called the half-life of the material. In the case of neutrons, for example, the half-life is about 10 minutes and 11 seconds. This fact will be important later in our story.

So, 10 minutes and 11 seconds after the Big Bang, half the neutrons in the Universe had changed into protons. 10 minutes and 11 seconds later, only a quarter of them were left, and so on.

But this takes us far beyond the Hadron Epoch, which lasted only about 1 second. Before we leave, we need to see what else was created during this first second of history.


As well as protons and neutrons, made from three quarks each, an entirely different type of particle was created during the Hadron Epoch. These were the mesons, which consisted of one quark and one antiquark, stuck together by the strong interaction.

Mesons are about half the size of protons and neutrons, about 10-15 meter (1 femtometer) across. All mesons are radioactive with half-lives of only about 10−8 second (about 10 nanoseconds).

Their decay products include electrons, neutrinos and photons.

Mesons are important in conveying the strong nuclear force which holds the parts of an atomic nucleus together. Without them, matter as we know it would not be possible.

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