What Does An Expanding Universe Really Mean?

Credit: Reid Wiseman/NASA

Marcelo Gleiser: How can something infinitely-large be expanding? The usual follow-up question (inspired by the expanding balloon image) is: “Expanding into what?” Good questions. The straight answer is that the universe doesn’t expand into anything. There is no space “out there” for it to expand into. What the cosmic expansion does is stretch space itself, as if space were made of some kind of stretchy rubber material. There is no physical border out here, only stretching space. To picture this, we need to build a “view from the inside,” as human observers inside the cosmos.

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