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The way I am thinking about this is thinking of a bomb floating in the middle of a room until it is set off. Once it explodes, the room doesn't expand. Instead the contents of the bomb does.

Is that what universal expansion is? The matter, particles, energy, etc simply moving away from the starting point in all directions. Or can the universe be thought of as a bubble that contains all those things, but is itself expanding and taking along its contents with it?

Patrick
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    Please see https://physics.stackexchange.com/q/136860/123208 – PM 2Ring Jan 19 '20 at 07:48
  • That isn't what I am asking though @PM2Ring. I am asking more about the fundamental nature of the universe. In essence, is the Universe an object that is expanding (and thus pulling everything within it outward) or is the Universe simply a void that contained matter in a tight bundle that finally expanded outward at some (possibly unknown) impetus with such force that all that matter is still in the acceleration phase of the expansion? – Patrick Jan 22 '20 at 18:04
  • I linked that question because it explains that there wasn't a starting point: the Big Bang happened everywhere. Your 2nd option "a bubble that contains all those things, but is itself expanding" is kind of what the Big Bang model says, as long as you remember that the bubble isn't in some kind of void: everything that exists is inside the bubble. So describing the expansion as an outward motion doesn't really make sense in that model. The distance between the galaxy clusters is increasing in all directions, no matter where you're located in the universe. – PM 2Ring Jan 22 '20 at 18:52
  • This article might be helpful: https://people.smp.uq.edu.au/TamaraDavis/papers/SciAm_BigBang.pdf Davis & Lineweaver also published a famous paper on this topic, but it is a bit technical, since it was written for astrophysicists, not the lay reader. Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe – PM 2Ring Jan 22 '20 at 18:55

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There is a difference though. With a bomb, there would be a shock wave. In the universe, we don't observe such a shock wave. With a bomb, the velocities of objects would diminish as one goes further away from the point where the bomb exploded due to the decrease in energy further away from the explosion. In the universe by contrast, we see that the velocities increase with distance. As a result of that, one cannot identify a point where such an explosion would have occurred. One would observe the same increase from every point in the universe. Therefore, it is concluded that the universe itself is expanding.

flippiefanus
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What it is increasing is the distances between the galaxies measured in the comoving rest space (the 3-space orthogonal to the world lines of the galaxies). This phenomenon at large scale is homogeneous and isotropic in this sense it is an universal expansion at large scale. It does not make sense instead to think of an universal expansion of everything at every scale: local rulers cannot expand otherwise we could not measure the expansion.

Valter Moretti
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