I have to say, I get very confused by terminology used in physics discussions. I feel like words are borrowed from normal English usage that imply things about the models that are not being implied, and this causes me to conclude that there are illogical aspects to physics that probably aren't there.
In this case, the word is "expand" when applied to the universe. The concept of an "expanding" universe seems nonsensical to me, when expand is considered in its normal English meaning.
"Expand" means to grow larger in size over time. But the concept of "size" implies a relationship between two entities, that being existing win the same physical reality, both being measurable via the same means, and those measurements then being directly comparable, the one producing the larger measurement being called "larger".
The concept of "size", in other words, implies an unchanging, fixed coordinate system of measurement, applied equally to two objects whose size is to be compared.
And thus something can only "expand" relative to something else; that is to say, can only change its size relative to something else that can be measured and likewise has a size to be measured against.
Now of course the universe, by definition being "the entirety of measurable reality", cannot be measured. It *is* the coordinate system, in effect, and you can't measure the coordinate system. So the "size" of the universe cannot be measured, and even if it could, saying that it were "expanding" in the English sense of the word would imply that there was something *else* that it was expanding within; but that seems like nonsense.
And so I have to conclude that the concept of an "expanding universe" is just a very bad way to describe what's actually going on.
I have, by thinking about what kinds of meanings physicists may be trying to impart by using the term "expanding universe", and with my very rudimentary knowledge of the experimental evidence by which this idea of an "expanding universe" is based (i.e. red shift in all directions implying that everything is moving away from everything else, with things that are 'further away' redshifting more), concluded that the important fact is that "measured distances are becoming longer over time".
I realize that this last sentence itself is fraught with loaded terms; but I think that this way of describing things at least avoids the unfortunate implication that the word "expanding" has of there being things outside of the universe into which the universe is "expanding".
I feel like there's some kind of intrinsic connection between what we call 'distance' and what we call 'time', and that the speed of light is a fundamental connection between the two; like the speed of light is the constant that comes out of the aspects of reality at the boundaries where time and spatial dimensions are equivalent somehow. Almost kind of like how the number "Pi" is an artifact of the relationship between two dimensions (in which a curve can be described) and one dimension (in which a line can be described) when is expressed in the realm of symbolic thought (i.e. mathematics). So we have the speed of light which is in the same way the symbolic expression of a relationship between the dimension of time and the dimensions of space.
Anyway I'm getting off course here. The point is that physicists often use terms that make it difficult for me to ascertain the aspects of the model for which the term was really implied, and those aspects which are just baggage carried along by natural interpretations of the meaning of the term but that aren't actually mean to be part of the model.
I'll give another example: we've all heard about this concept of "curved space" that is used as a component of a theory of gravity. The term "curved space" to me seems to me to be an oxymoron: space cannot curve because space is the coordinate system; and the coordinate system does not curve; curves are things that can exist within coordinate systems, not features of the coordinate system itself.
Thus, space cannot curve. For those who propose that space does curve, the question becomes "into what does space curve? What is the reality in which one could ascertain that there is curvature to some region of space; being able to identify a curve implies that one could also identify a straight line and differentiate that straight line from a curve, which thus implies some kind of coordinate system in which these lines and curves are being described. But space *is* the coordinate system, and therefore it is nonsensical to say that it is curved.
Therefore physicists *must* mean to be saying something else when they say space is curved, but I can't really figure out what. I suspect that the term is useful where it matches the same aspects of what we call curvature as measured in a regular cartesian coordinate system; that somehow the measurements and math work out *as if* they were describing the curvature of 3d space in some higher order space, but that the reality is that to accept the model you have to accept that what it's really saying is that there are rules that apply *as if* space is curving, but that space actually isn't curving.
An example of where the concept of curved space and gravity falls down again is in the classic visualization of gravity as "a bowling ball on a rubber sheet"; in such a configuration, we would expect other balls to rotate around the bowling ball as they slide along the curved surface of the rubber. But that visualization of gravity *already assumes the concept of gravity, and that the rubber sheet would be positioned in a gravitational field such that the bowling ball and all other balls would exert a force down into the rubber sheet*. So this example completely falls flat for me for that reason. If your model already incorporates the thing you are modelling well ... I just can't imagine how that's supposed to be a useful model!
And so time and again, the way that physicists try to explain things leaves me feeling like there is some serious hand waving going on, and also that there is probably significant subtlety in the math and measurement that is run over roughshod by the language and models that are used to describe these things to the masses, myself included.