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Expanding Universe: The idea of the Universe expanding is often described using the analogy of an inflating balloon. It is tempting as a new physics student to imagine the expansion as galaxies whizzing away from each other through some 'medium', however in actuality it is spacetime itself that is expanding. If we glue some pieces of confetti (representing ...


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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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