Maybe more like a balloon in a straight line tied off several times along the way with infinite length going both ways.
or this poorly drawn crap I took 30 seconds doing -
All coming from the same balloon, but each part tied off where the air can't pass easily inbetween one and the next.
The exiles and others (like Marvel Zombies) can pass inbetween the seperate alternate Earth's so they have to be tied. However the path can only be opened by special means or beings with the power to do so. The 616 Earth has it's own Negative zone, Asgaurd, Hell, Heaven and so on but it's tied to that Earth I believe. The Ultimate universe has it's own versions of that, but unique to it.
Personally I hate alternate realities and time travel as it's a whole can of worms being opened. Like if I walked right at a road there's suddenly a new reality where I walked left and got hit by a car, or was the reality always there and we just chose to do the same things up to that point. It's all a big headache lol.
Either way yes each Marvel Earth should have it's own set of dimensions and Universes, which is why each one has a number. Such as the Marvel Universe and heroes we're all familiar with is 616, where the Ultimate universe has another.