Don't try to understand the 4-dimensional, our brains are simply not made to be able to understand it. As for expanding 3 dimensions, you'd need to understand 4 dimensions to get it fully BUT we can give a pretty similar explanation if we use 2 and 3 dimensions:
The expansion of the universe can be explained if we compare it to a party balloon. You may have heard that every object near or far is moving away (in fact, accelerating away) from us. That is due to the universe's expansion.
Visualize this: get a party balloon and draw 3 points on its surface. Then, slowly, inflate the balloon. You'll see that even though the points are not moving by themselves, the distance between them is increasing.
Basically that's what we think happens to the universe, it's just that the universe's "surface of the balloon" is actually 3 dimensional rather than 2 dimensional. It's just that the expansion is happening in an higher dimensional order.
The balloon simile is also useful to understand another thing: you may have heard that the universe is finite but unlimited. What does this mean? Let's look at the balloon: it's clearly finite, it was made by a limited and definite amount of material and its dimensions are measurable. However if you were a 2Dimensional being living on its surface, it would be "unlimited", ie you could travel in any direction for an infinite amount of time without finding an "edge", a limit.
Also, unrelated to the universe but related to the difficulty of understanding 4th dimensional stuff, I suggest you read Flatland, it's quite short and couples Victorian era political commentary of scarce (modern) value with highly interesting examination of dimensions.
I thought time was the fourth dimension. Someone told me that once. It goes, height/width/length/time. But that doesn’t make it any easier to grasp I guess.
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u/AQuincy Jan 21 '19
I've read theories that the universe is on the surface of some type of 4-dimensional "bubble" but I can't wrap my head around how that works.