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What happened in the
universe after the Big Bang?

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When the universe was first
born, it was dense and hot.

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In these cramped and
heated conditions,

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particles of light
and matter were

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rushing around continuously
running into each other.

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No atoms could form because
matter particles were traveling

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too fast to stay together.

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But as the universe expanded,
it became less dense

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and it cooled.

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The particles slowed down.

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Soon it was cool enough
for protons and neutrons

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to glue themselves together and
combine into these small units

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called nuclei.

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But electrons couldn't
gather onto those nuclei,

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because they were
relentlessly smashed aside

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by powerful photons.

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This made the nuclei ionized.

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They had protons, which
are positively charged,

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but no negatively
charged electrons

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to balance out their charge
and make them neutral.

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Meanwhile, the photons
couldn't go very far

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without crashing into electrons
and newly formed nuclei.

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The photons bounced around like
sunlight does in a dense fog.

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Because all the light
was scattered around,

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the entire universe was foggy.

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But as time passed, the universe
continued to expand and cool

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and it became much less dense.

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With more space between
them, photons and electrons

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collided less often.

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Electrons began joining with
nuclei to form neutral atoms.

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And now with the electrons
out of the photons' way,

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the path was clear for
light's great escape.

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378,000 years after the Big
Bang, the fog finally lifted.

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No matter how big or powerful
our telescopes become,

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the cosmic microwave
background is the oldest light

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we will ever be able to see.

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