Space is constantly expanding and accelerating

Question

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?

In: 1

Answer 1

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.