When we say that the universe is expanding, the space between everything is not. The space between atoms, the space between my house and school, and the space between Earth and Sun remains the same, due to the four fundamental forces of nature.  However, when does the gravitational force becomes comparatively small, to see the expansion in space? Is it between the stars - interstellar? Is it between the galaxies - intergalactic? Is it between the superclusters - intersupercluster? Or something greater that I'm not aware of?
2038
I have looked at other questions on this site (e.g. "why does space expansion affect matter") but can't find the answer I am looking for.  So here is my question: One often hears talk of space expanding when we talk about the speed of galaxies relative to ours. Why, if space is expanding, does matter not also expand? If a circle is drawn on balloon (2d plane), and the balloon expands, then the circle also expands. If matter is an object with 3 spatial dimensions, then when those 3 dimensions expand, so should the object.  If that was the case, we wouldn't see the universe as expanding at all, because we would be expanding (spatially) with it.  I have a few potential answers for this, which raise their own problems:   Fundamental particles are 'point sized' objects. They cannot expand because they do not have spatial dimension to begin with. The problem with this is that while the particles would not expand, the space between them would, leading to a point where the 3 non-gravity forces would no longer hold matter together due to distance Fundamental particles are curled up in additional dimensions a la string theory. These dimensions are not expanding. Same problems as 1, with the added problem of being a bit unsatisfying. The answer seems to be (from Marek in the previous question) that the gravitational force is so much weaker than the other forces that large (macro) objects move apart, but small (micro) objects stay together. However, this simple explanation seems to imply that expansion of space is a 'force' that can be overcome by a greater one. That doesn't sound right to me.
20842
The overly simplified explanation I'm giving myself right now is  causes the opposite of what gravity does, that's why the universe is expanding. Now where gravity is a force, why dark energy is "energy"? Why it's not called "Dark Force" instead? I think I must be missing something here.
3178
How does the  change with the age of the universe?  This question was posted recently, and I had almost finished writing an answer when the question was deleted. Since it's a shame to waste the effort here's the answer anyway.  Maybe this can be one of the canonical answers .
51210
What was the density of the universe when it was only the size of our solar system?  Did it approach neutron star density?  Is it physically correct to even ask such a question?
107185
My question pertains to a possible inconsistency between the first law of thermodynamics and the general expansion of space itself.   First, the knowns:   Space itself has, and continues to expand - the acceleration aspect is not relevant to the discussion Empty space in a vacuum has intrinsic energy - some refer to this as dark energy Energy cannot be created or destroyed as per the first law of thermodynamics   Concisely, what is the source of energy that is imbued into this "new" space?  A few speculations:   The energy for this process is taken from something other source that already exists inside or outside the known universe. The energy for this process is created from nothing, thus violating the first law. If space itself is quantized, perhaps the planck length itself is expanding across the universe, giving the appearance that new space is created, but actually it is just the space "particles" themselves that are expanding.  At some critical point this expanding space would have observable consequences for our own scale of existence.   These ponderings can also be applied to the big bang theory - either energy was created at some point (from nothing) or existence of anything has been an infinite cycle into the past. Either way we are dealing with infinites. Create something from nothing and you will find infinites cropping into your calculations.
77942
I am looking for a story about a tiny universe smaller than our Solar System. It has one star and one planet, and the planet's orbit is on the edge of universe. The planet's single face points to its sun at all times. The world has a very tall wall around its twilight circumference.  A civilization grows on the planet. The ruler of the civilization - curious about what is beyond the wall - commissions for a ladder to be built up the wall. He climbs the ladder and walks towards the darkness beyond the twilight atop the wall. He keeps walking and      comes back to his original location. Anything that goes beyond the wall at the edge of the universe is reflected back on itself.   I think it starts with a quote like this: "Many and strange are the universes that exist like bubbles in the foam along the river of time."
90276
I read in some website that during the period of inflation, the expansion of the universe underwent incredibly fast, and its size increased by a factor of $10 ^{50}$,   In this field, I think, there is nothing for sure, but if there was really inflation, what does it mean that the size increased by a factor of $10 ^{50}$, but from what extent? From 1 mm? Or from the Planck length? Or something else?  Has it been established that (whatever was the original measure) the factor is increased by $10 ^{50}$, and not for example of 40 or $10 ^{50^{60}}$?
43468
If the big bang was the birth of everything, and the big bang was an event in the sense that it had a location and a time (time 0), wouldn't that mean that our universe has a center?  Where was the big bang? What is there now? Are we moving away from the center still? Are these even valid questions?
72173
Some say we live in the golden age of the universe because there exits countless number of stars that shines in the dark universe. As the supply of gas for star formation is steadily being exhausted, it is estimated that star formation will cease in 100 trillion years.  Question: Will there be a day that the universe becomes completly dark when all the stars burn out? Does all the cooled bodies eventually collide due to gravitational forces? Will black holes eventually dominate the universe?
88920
Some scenarios describing the fate of the matter vs  tug of war on the universe involve the acceleration of the universe increasing to the point that it ends up ripping apart even atoms. This is called the .  This makes no sense to me. It looks like all of these   (GR) models of the universe assume it has a uniform isotropic distribution of matter and energy. This works great at long scales, but it is also clearly wrong even at the length scale of the separation of galaxies.  The density of atomic nuclei remains the same even though the universe is expanding even as we speak. I don't think the "scale" in the  can be interpreted so literally. Or maybe said better, it's got to break down when the cosmological horizon distance gets to the scale where the isotropic assumption breaks down, right?  How can scientists be claiming that runaway expansion will rip apart atoms?
26221
Recently I've read that  as proposed by Hubble. Therefore is it possible that instead of space expanding in all directions all matter is falling toward some distant barycentre? Galaxies already closer to it than our own would be accelerating faster and thus we'd observe them redshift. At the same time, our galaxy would be accelerating faster than galaxies which are farther away than us from the "universal centre of mass" which should also result in observing them move away from us. Assuming we have only seen a small part of Universe and in particular only space on one side of the supposed centre so far, all galaxies except the ones on the same sphere around the distant centre as ours would look redshifted to us. For example when looking at  one can see a big purple arc which could be due to galaxies having the same distance as we do from the centre.
95779
After a very long time will we see more stars (due to the fact that more light is get to us) or less stars (as the universe expends and light have to pass larger distance)? In general, can stellar objects go outside of the scope of the observable universe or is it only growing with time? Calculations are always better than just discussion. Does it have any connection to dark matter?
22762
Is new space(time?) being created as the Universe expands, or does the existing spacetime just get stretched?  If it just gets stretched, why do galaxies move along with the expansion instead of just getting smeared (like a drawing on an inflatable balloon)?
90880
By looking at this picture:    The earth is near the center of the universe. I've read that the universe look the same no matter where the observer is located. It is the same distance everywhere.  So I understand that for general relativity the universe need to be homogeneous and isotropic, so it will look the same no matter where I am.  But what if I'm on one of the planets near the right or left of the image, then if I draw the same picture of the universe, but from my perspective, then I would be also located in the center? If that's not the case (I'm actually near an edge), then part of my sky would be completely dark, and all the sky that way won't be isotropic?
52838
I recently read that:     What our current level of thermodynamics does tell us though is that if there is such a thing as a Big Crunch, the universe will look a lot different as it heads towards collapse than it did when expanding – the universe started in a state of low entropy and high order and will end in a state of high entropy and   low order.   But shouldn't the order be in a high state and hence entropy becomes a low state in the Big Crunch (assuming a closed universe)?  The reason I write this is because the gravitational forces in a closed universe are so strong they will cause gravitational collapse so the universe will shrink and go backwards, eventually through all the stages of the big bang and become a singularity once more.   It was my understanding that a singularity has a state of high order and low entropy. Have I got this the wrong way round?   If so; Can someone please explain why?
103640
For instance, for a two-body system, there will always be a point in-between the two in which the forces of gravity completely cancel, and if I were there then I would experience no net force. I believe there is also a point, or a region, for three body systems as well. Assuming we treat the stars or planets as simply point-masses, will there always be, for any number of objects, a point, or a region, of equilibrium? Or no?
94943
I was recently reading some of the nice answers to the (related) question:   From many of the answers their appears to be a symmetry between:   A universe in which all matter-things (plus light) remain the same size, while the space between them expands. (The one we appear to live in) A universe where the space remains the same size and all objects in the universe shrink. Along with an appropriate change in physical length-scale constants. (One equally compatible with our observations?)   Said another way: At one point in time your ruler fit between those two galaxies $10^{50}$ times, now it fits between them $10^{51}$ times. This is usually explained as the space growing, I believe it could equally well be described by the ruler shrinking.  Is this symmetry obeyed? Are their any cosmological models presented in terms of matter shrinking?