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Why does looking at distant galaxies reveal the universe’s past?

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Looking at distant galaxies reveals the universe’s past because of the finite speed at which light travels through space.1

Because light has a constant, finite velocity, it takes time for the light emitted by distant objects to reach observers on Earth. When astronomers observe a galaxy that is millions or billions of light-years away, they are not seeing the galaxy as it exists today; rather, they are detecting light that was emitted millions or billions of years ago.1

Consequently, these observations provide a direct window into earlier stages of the universe. By studying these distant, "young" galaxies—such as those captured in deep surveys by the Hubble Space Telescope—astronomers can observe how galaxies formed, evolved, and interacted when the universe was much younger and smaller than it is at present. This allows for the study of cosmic history, including the building blocks that eventually merged to form larger structures like the Milky Way Galaxy.12