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EDIT: My first answer seemed to imply that radiation is at rest in the Cosmic Rest Frame. Radiation is not in rest in any frame. See below. The sentence shouldn't be read as "[velocity of energy] forms", but "velocity of [energy forms]"$^\dagger$. The sentence refers to "energy forms", i.e. the different forms in which energy can manifest itself. These ...

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The observable universe is defined by how far light has been able to travel since the Universe began. If a distant star were to be turned into a mirror, we would only be able to see that portion of the universe that has had time for its light to reach the star, reflect off the mirror, then travel to us. Take an imaginary star right on the edge of the ...

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