What happened in the universe after the Big Bang?
When the universe was first born, it was dense and hot.
In these cramped and heated conditions,
particles of light and matter were
rushing around continuously running into each other.
No atoms could form because matter particles were traveling
too fast to stay together.
But as the universe expanded, it became less dense
and it cooled.
The particles slowed down.
Soon it was cool enough for protons and neutrons
to glue themselves together and combine into these small units
called nuclei.
But electrons couldn't gather onto those nuclei,
because they were relentlessly smashed aside
by powerful photons.
This made the nuclei ionized.
They had protons, which are positively charged,
but no negatively charged electrons
to balance out their charge and make them neutral.
Meanwhile, the photons couldn't go very far
without crashing into electrons and newly formed nuclei.
The photons bounced around like sunlight does in a dense fog.
Because all the light was scattered around,
the entire universe was foggy.
But as time passed, the universe continued to expand and cool
and it became much less dense.
With more space between them, photons and electrons
collided less often.
Electrons began joining with nuclei to form neutral atoms.
And now with the electrons out of the photons' way,
the path was clear for light's great escape.
378,000 years after the Big Bang, the fog finally lifted.
No matter how big or powerful our telescopes become,
the cosmic microwave background is the oldest light
we will ever be able to see.