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Every time we look at a star we are looking to the past. That's because the light of that star needed to travel long distances at the speed of light. When the light reaches our sight maybe the star no longer exists, since it had to come a long way to arrive here. That led me to a thought experiment: we now can look, not sure about the exact number, but 11 or 12 billion years ago, it is said that the universe has 13,7 billion or so.

What we would see, if we look at the light from 13,7 billion years? The universe being formed?

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    $\begingroup$ An interesting addition to the below answer, light is a great source for gaining info on the early universe. But if one could see the C$\nu$B, the cosmic neutrino background, one could peek back to a mere three minutes after the big bang as neutrinos decoupled from the early universe very soon after it's formation unlike photons. $\endgroup$
    – Triatticus
    Commented Mar 3, 2022 at 12:22

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This is not a thought experiment, we can, in fact, see the oldest light in the universe:

The oldest radiation is the cosmic microwave background, dating from the recombination period when the universe after about 400000 years after the Big Bang had cooled down enough for atoms to form. There is no earlier radiation for us to see because the hot universe prior to recombination was not transparent for electromagnetic radiation due to scattering by free electrons and protons.

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