How do we know the universe isn't "collapsing"? Main questions: -Is the conventional view of two galaxies moving away from each other in a uniform space correct? -Or is it conceivable that two galaxies may be more or less stationary as the density of space increases? -Does it matter? Background: I've been thinking of the balloon analogy of the expansion of the universe, whereby two dots of ink drawn on a deflated balloon will appear to move further apart as the balloon is inflated. I am somewhat confused by the coordinate system in use here: -The two dots do indeed become more distant if you use a coordinate system not bound to the surface of the balloon. For example, a scientist in a lab observes the inflation of the balloon and uses a ruler or arc length tool to measure the distance. but... -The two dots remain the same distance apart if you apply a coordinate system to the balloon itself. An example here would be measuring distance based upon the amount of latex between the two points (assuming the line between them has tangible width). In other words, the balloon's coordinate system expands along with the balloon. The latter is more analogous to our perspective of the universe. We have only our own conceptions of a coordinate system, lacking the outside view held by an observer looking at a balloon. Applying the balloon analogy to "our" universe, two galaxies that are on a common "shell" with negligible curvature (our equivalent to a balloon surface), with spacing of 10^20 km, will remain at that distance as the shell's coordinate system expands with them. The shell may be growing larger, but it still has the same amount of "latex." Obviously, within our galaxy we can define distance by observing atomic phenomena and the speed of light in a vacuum, and then apply that to the distance between galaxies to show they are receding. However, in the balloon model, even these units would expand at the same rate as the distance between galaxies. Thought experiment: This FAQ site states that, according to the current models, "[The Big Bang] was an explosion of space, not an explosion in space." Going off of this I made some assumptions: 1. Two points exist that are equidistant from a hypothetical "center" of the Big Bang, and are not constrained to a given "shell" (made only to make comparison to the balloon analogy easier). 2. Space can flow around mass in much the same way gases from an explosion flow around a massive object (the converse of mass moving through space). 3. Space has finished expanding, and is now returning to its origin (i.e. the universe is shrinking) 4. There is a finite amount of space. To illustrate this, assume the scientist I mentioned earlier replaces the balloon with a large sphere of latex (which can in this example be thought of as many concentric latex balloons) and has somehow stretched the sphere in all radial directions. If the scientist designates two different points within the sphere (which will remain stationary from his perspective) with a common radius and releases the forces keeping the sphere stretched, the sphere will shrink, placing more latex between the two points (again assuming the path between them has tangible width). A hypothetical observer on one of the points will have just observed the other point "accellerating away" as the sphere transitions from a stretched object with low density to a smaller object with a higher density. This thought experiment, as it pertains to our universe, can be thought of as space contracting toward a hypothetical center as two galaxies remain relatively stationary. In our balloon model, this would equate to two points maintaining the same separation (in the lab frame) as the balloon is deflated. An observer on one of the points would measure an increase in the amount of latex between his dot and the other.