I believe I understand that space is constantly expanding “outward” in all directions and this is likely due to an event such as the Big Bang. Can someone explain what space is expanding *into* and how exactly the “space between stuff is also expanding” in the sense that A and B are getting further apart but the space between A and B is also expanding.
Is that simply a function of two objects distancing themselves therefore the space must be getting larger, or is the space actually expanding in between two objects regardless of either object being on either side?
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>Can someone explain what space is expanding into
The question has no meaning. The universe contains everything that exists, so either “outside the universe” doesn’t exist, or it exists and is therefore then…contained within the universe.
>how exactly the “space between stuff is also expanding” in the sense that A and B are getting further apart but the space between A and B is also expanding
Whoever can answer “how exactly” has a Nobel Prize waiting for them.
So there isn’t really anything the universe is expanding into. It’s a bit of a head melt, but it’s just kinda getting bigger.
For the second part, it’s not so much that A and B are moving apart via their own motion, the space is expanding between them. Imagine you put a sticker of 2 galaxies on a balloon and then inflate it. The galaxies themselves don’t move, the space between them gets bigger due to the balloon expanding. That’s an eli5 version of universe expansion
I think it is best understood as it being a fundamental property of space that every bit of space is constantly creating a little bit more space. That is, if you have two stationary objects 1 metre apart with no forces at all in between them, that metre of space will very slowly create new space which will make those objects drift apart.
For human-sized amounts of space, the effect is trivial enough to be unnoticeable, and is completely dominated by other more significant forces — even the force of gravity, which is itself usually unnoticeable for human-sized objects completely overcomes the effect of expanding space. But the amount of space created is proportional to the amount of space already present — 10 metres of space creates 10x as much “new space” per second as 1 metre of space does resulting in 10x as much “intrinsic drift”. 100 metres creates 100x as much, &c.
But in our universe, we have things like distant galaxies (i.e. not even the closer-to-us galaxies) that have ridiculous amounts of space between them. For those galaxies there is so much space between us and them, every metre of which is constantly churning out a tiny little bit of new space, but there are so many of those metres that the total effect becomes more significant than even the gravity between galaxies. The intrinsic drift from expanding space is what is mostly responsible for their motion relative to us, with any other motion being just a small correction on that.
It is a little bit off to say that this motion was “due to an event like the big bang”, which gives the impression of some explosion in the past pushing everything away and that distant galaxies are still “riding” the push from that past event. It is more like the big bang was caused by this intrinsic property of space. In earlier times and higher energy densities the “space making more space” effect was stronger and the drift was faster.
Note that this doesn’t assume that “all of space” was at some particular size at the time of the big bang. It could still have had infinite size. It’s just that space, of whatever size, was hotter and more full of stuff than it is now.
I think the problem is infinities are just unintuitive. So I’m going to use Hilberts paradox of the grand hotel. Basically, imagine a hotel with infinite rooms. The rooms are numbered 1,2,3,…and so on to infinity. And not only does this hotel have infinite rooms, it currently has infinite guests too. So every room is occupied.
But then a big convention comes to town, a *really* big convention with another set of infinite guests. And they all want to stay at the Hilbert Hotel, even though the infinite rooms are already occupied by infinite guests. That’s ok though, because infinities are weird and the hotel manager knows it. So he just asks all of the guests to move into the room numbered twice as high as their current room. Room 1 to 2, 2 to 4, 3 to 6, etc. Because it’s infinite, every guest still has a room, but now only the even numbered rooms are filled. So our infinite convention attendees can move into the odd numbered rooms and everyone’s happy.
Those guests are kind of like what’s happening in space. We had infinite guests, and we still have infinite guests even after adding new ones. The old guests didn’t ‘expand’ into anywhere, they all just moved further apart from one another and then new guests could move in in between them. And you can keep doing this every time a new convention with infinite people comes around, and the original guests would just keep moving further and further apart from one another. If you do it again, the guest that was in room 2 went to room 4, and would now be in room 8, which is 4 rooms away from the guest originally in room 1 (who is now in room 4). And they’d be 8 (2^3 ) rooms apart if you did it a third time.
With space, it’s not expanding into anything. Every point in space is just moving away from every other point in space. And things aren’t just moving away from some central point. They’re all moving away from everything else. You could go to any galaxy cluster, and over time you’d see every other galaxy cluster in every direction is getting further and further away. And it happens exponentially faster too. Since they’re not just moving away, and space is actually expanding between them, that means the further apart they are, the more space there is that can expand between them and the faster they’ll continue moving apart.
Here’s my attempt at an ELI5, as a physics student with limited knowledge of cosmology.
Imagine you have a deflated balloon. You draw five dots anywhere on the balloon. Now if you begin inflating it, the dots begin to spread farther and farther away. The space between the dots is expanding. In this instance, the balloon represents space and the dots represent galaxies. As you can see, if you were a 2-dimensional dot (normally a dot is thought of as a 1-dimensional object but in the balloon example you can clearly see it’s 2-dimensional) on the 2-dimensional balloon surface, you don’t necessarily see what it is the balloon is expanding into, since you’re bound to the balloon, but you can clearly see the distance between the dots expanding, and the objects farther from you expand away from you at a greater rate. Therefore, the only reasonable explanation is that the balloon is expanding. What’s it expanding into? To you, nothing. There’s nothing beyond the balloon. You’re bound to balloon’s surface, and yet you can see the space between you and the other dots expanding.
That’s the end of the ELI5. Below is an ELI15ish.
These observations are made by comparing redshift values of galaxies, which is basically seeing how light gets redder for objects moving away from you at very fast rates. The faster it moves, the redder the light from these galaxies appears, and we can clearly see that the objects that are farther away appear the reddest, therefore indicating that the universe is expanding and accelerating.
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