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?



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

They aren’t talking about matter, they’re talking about space itself. A common analogy is when you bake a loaf of bread that has raisins or something in it; the raisins all move apart when the bread bakes because the bread itself expanded. If you were tiny and on one of the raisins, it would appear that all of the other raisins were moving away from you. That would be true no matter which raisin you were on. That’s essentially what you see in real life when you look at other galaxies, they’re all moving away from us because space is expanding.

The universe is taking up a bigger area. Everything is moving further away from eatchother and has been since the big bang. All of the matter has been here.

It literally means growing larger in three dimensions from the “inside-out.” Simply that existing space creates more space, literally from nothing (well, nothing except itself). Space itself contains a certain amount of energy (the vacuum energy) and that energy is repulsive – it forces space apart, and as it is pushed apart, more space spontaneously pops into existence. This new space also has vacuum energy, so even more space is created. Rinse and repeat and space expands and expands. Note that while space everywhere expands in this way, objects in space that are gravitationally bound will (at least for some time) remain close together as the gravitational force overcomes the repulsive force of the vacuum energy. Some physicists posit that eventually even local gravitational attraction will be overcome and all macroscopic objects will move apart. Repeat this on ever smaller scales, until all gravitational attraction is overcome, leaving only atoms and other subatomic particles. Even these will be torn apart eventually in what is known as the Big Rip (according to some) as the strong nuclear force is overwhelmed by expansion.

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.