Slower change Big Bang 4
The Universe was 300, 000 years old before atoms began to form. By then the temperature had dropped to about 5000 oC – cool enough to allow electrons to stay bound to a nucleus. However, there were still atoms of only two elements – hydrogen and helium.
Evolution of particles since the Big Bang
A distant echo
Up until that time, the charged particles of the plasma had absorbed any electromagnetic radiation. However, by then they were mainly neutral particles which did not interact with electromagnetic waves. These waves could therefore travel further across the Universe. We can see faint traces of this radiation as cosmic microwave background radiation. The map of this radiation shows that the early Universe was not uniform. There were clusters of matter which are marked by changes in the intensity of this distant echo.
Starry night
Between the ages of a hundred million years and a thousand million years, the Universe began to get its first stars. Stars are still being born today (about 14 thousand million years after the Big Bang). However, they do not last for ever and millions of stars have died since the Big Bang. It was in these dying stars that heavier elements, such as carbon and iron, were first formed. And these elements make up the contents of any planets that orbit newer stars, like our Sun.
The life cycle of stars
A long journey
The atoms in your body were made through fusion reactions in the nuclear furnace of a dying star about 8 billion years ago. They may well have been part of more than one star before that. But at some time they were all atoms of either hydrogen or helium. And for a split second after the Big Bang they were a seething micro-dot of quarks and leptons - the fundamental particles of the Standard Model.
The story so far
When the Universe was 300,000 years old, the first atoms started to form
The neutral atoms did not absorb electromagnetic radiation
Stars and galaxies started to form after about 100 million years
Heavier elements were formed in the cores of dying stars
Question S4

a) Only two elements existed for the first billion years of the Universe. What were they?

b) There have been many stars which were born and have dies. How are these stars important for you?

The story so far
  • The Big Bang made all the matter in the Universe
  • Quarks joined into protons and neutrons after about 10 nanoseconds
  • Hydrogen nuclei fused into helium nuclei up until about 500 seconds
  • There were still no atoms as it was too hot for electrons to stay bound to a nucleus