The birth of matter Big Bang 3
There was an important development when the Universe had been around for a tenth of a second. Protons and neutrons started to fuse together to form helium and deuterium (an isotope of hydrogen). The temperature was still very high – five thousand million degrees (5 x 109 oC), but it was cool enough to allow these nuclei to stay bound.
Evolution of particles since the Big Bang
Formative growth
These fusion reactions continued for about 500 seconds. The fusion that took place in this time determined the amount of hydrogen and helium in the Universe today.

Although nuclei had been formed, it was still too hot for atoms to exist. Any electrons that attached themselves to a nucleus were immediately stripped away. The electrons and nuclei (positive ions) formed a gas of oppositely charged, but separate, particles that we call a plasma. The extreme temperature maintained this plasma for the next 300, 000 years.

A plasma absorbs electromagnetic radiation because its particles are charged. So, for all this time, it was impossible to see through the Universe.

A destructive youth
During the first few hundred seconds, particles would frequently meet their anti-particles and annihilate each other. The energy from this destruction would leave as electromagnetic radiation. At the time of the Big Bang, there would have been equal amounts of matter and anti-matter. This would suggest that these two components would wipe each other out and leave a Universe with only radiation but no matter. So how come our Universe is made up of matter?

Matter and antimatter anihilate but leave a little matter to make the Universe
What's the matter?
All particles and their anti-particles should decay in equal amounts into matter and anti-matter. However, luckily for us, there was an exotic high-energy particle whose decay is not symmetrical. In 50.35% of its decays it produces matter whereas 49.65% of the time it produces anti-matter. So there was slightly more matter than anti-matter. Most of the matter was mopped up by the anti-matter but there was a little left over to evolve into our Universe. As we would expect from this theory, there is much more radiation in our Universe than matter. This is radiation left over from all the annihilations that took place in the first few hundred seconds, after which there was no more natural anti-matter.
The story so far
The first nuclei formed after a tenth of a second
Atoms didn't form for a further 300,000 years
There was a plasma of electrons and ions for all this time
The charged plasma absorbed any radiation that came by
Most of the matter of the Universe was annihilated by the anti-matter that was around
Question S3

a) What do we call a stable gas of electrons and nuclei?

b) Why does this absorb electromagnetic waves?

c) Why is there much more radiation than matter in the Universe today?

The story so far
  • The Universe expanded rapidly and cooled
  • Protons and neutrons were formed from quarks after about ten nanoseconds