Ask Ethan: How Do We Know Space Is Expanding?
“How do we know space is expanding? In relation to what? Red shifting galaxies growing farther apart could be doing so in infinite space as opposed to expanding space.”
The potential for confusion surrounding the expanding Universe is tremendous. Why is it expanding? How do we know? Why couldn’t we just be at the center of an explosion? What is the Universe expanding into? And, if the Universe is already infinite, how could it still be expanding? The answer to these questions relies on a combination of our observations of the Universe, near-and-far, as well our greatest theory of the Universe itself: Einstein’s General Relativity. When you put all of these pieces together, you figure out how to tell the expanding Universe possibility apart from all the others, and when you make the critical measurements, you come to the inescapable conclusion that the Universe is expanding.