What is meant by the word “expanding” when scientists say that the Universe is expanding? Is this about physical matter, gasses, or creation of new galaxies, or something else?
In: Physics
Space itself is expanding, and everything else is being carried along for the ride. The raisin bread and balloon dot analogies have already been brought up.
One of the natural follow-up questions to one of these explanations is “what is it expanding *into*?” Which is usually answered by the incredibly unintuitive, “it expands into itself”. You also encounter claims like “things are moving away from us”, and that these things can be “moving away” faster than light, and yet this isn’t violating anything.
I think it’s a bit more helpful to reframe it as “more space is being created everywhere, all the time”. Two distant objects are not necessarily *moving* in opposite directions, nor are they being *dragged along* by some current of spacetime. They could be perfectly still with respect to one another. But it seems that space itself is *multiplying*. The distance between the two objects is increasing because more space is just *appearing* between them. And distance itself is just a measure of how much space separates two points, so more space appearing results in more distance just *appearing*.
Where is the “extra space” coming from? Why does space seem to do this? We’re not really sure. We have theories, but none are very satisfying. If you’ve heard of “dark energy” (not to be confused with “dark matter”), that’s just one of the theories proposed to explain it. It is called “dark” because we *don’t know* precisely what it could be… we only guess at what combination of properties are needed to explain the phenomena we see, and then we look for anything out there that meets the description. We haven’t yet.
I feel I should also note that the concept of expanding space is itself just a theory. Mind, an incredibly well-tested, successful theory. What we know from experimental evidence is that everything *looks* like it’s moving away from us, with the speed correlating incredibly strongly with distance. It turns out that taking space and pretending it can magically multiply into more space models this behavior almost perfectly. So you could be accurate to say that space itself may not be multiplying after all, but things in it are “moving in such a way that pretending space could do this would approximate it very well”. It *could* be caused by some entirely different process that happens to produce a similar result. But no one has come up with a convincing model for this that can fit all of the data. All signs right now point to the idea that space *can* do this. The next step is to explain why it can, which is unsolved.
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