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May I ask a dumb question? What is meant by the phrase "expansion of space"? Is new space being created? Are the physical distances between things changing?

drdeca 5y ago
> Are the physical distances between things changing? Yes. > Is new space being created? I’m not sure what you mean by this question. Imagine ants living on the surface of a balloon which is inflating, and, I think that’s not that far off? (Except, the balloon is compact and wraps around, and the universe likely doesn’t.)
phs318u 5y ago
Here's a rabbit hole you may wish to go down: https://www.physicsforums.com/threads/what-is-the-fabric-of-space-made-of.282203/ https://www.physicsforums.com/threads/what-is-the-fabric-of-...
iszomer 5y ago
My dumb answer: the universe is an expanding balloon being inflated by The Big Bang. If you were a standing on one point on the surface of the balloon, objects relative to your observable distance would be moving away from you. This observation can be measured by a phenomenon known as the redshift. I'm lost from here..
- awb 5y ago
  I think more like you’re in the middle of a balloon being blown up and all points are accelerating away from you. Except every point in the universe is like the middle of the balloon with space accelerating away from it in all directions.
  - galaxyLogic 5y ago
    I've read about the fruitcake metaphor. When you bake a fruitcake in the oven it expands and thus all the little pieces of fruit in it move away from each other. But if that is the proper metaphor then my question is: Is there any difference between everything "moving away from everything else" (like pieces of fruit in expanding fruitcake) vs. "space expanding"? Why do we say "space is expanding" rather than saying "all stars are moving away from each other"? What is the difference between "space expanding" and "things moving away from each other"?
    - awb 5y ago
      > What is the difference between "space expanding" and "things moving away from each other"? Not an astrophysicist, but I think both are happening. Space is the medium we live in. It’s the cake in the fruit cake. The fruit wants to stay in the same place, but the cake is expanding which is causing the fruit to appear to move away from each other even though the fruit wants to stay in one place. Same with the universe, space is expanding which is causing objects to become farther apart, even if they could somehow have 0 speed. But in our universe the planets, stars, galaxies, etc. (the fruit) do have speed. Some are getting closer to us, like the Andromeda galaxy, while many others are moving away from us. However, given the vast distances and how fast space is expanding (how fast the cake is rising), even those distant objects that want to move towards us are actually moving away from us. Imagine a swimmer swimming against a tsunami. You can be swimming any direction you want but the power and speed of the tsunami will offset that and dictate where you end up. Space is expanding at incredible speeds, so it’s quite the tsunami: https://www.forbes.com/sites/startswithabang/2020/06/12/ask-ethan-how-does-the-fabric-of-spacetime-expand-faster-than-the-speed-of-light/ https://www.forbes.com/sites/startswithabang/2020/06/12/ask-...
      - galaxyLogic 5y ago
        Let's assume that at Bing Bang all the matter was in a very small area. Then it exploded, in other words all the pieces of matter started moving away from the starting area, presumably at an accelerating speed. That would simply mean that everything started moving away from everything else and that movement seems to be even accelerating. Fine. That is a good and easy to understand description. Why then do I have to add "BTW. space is expanding"? Things are moving away from each other I get it. Why do they need to add to that "Space itself is expanding"? I don't doubt there is a reason since best scientist are thinking about it. I just haven't seen a good explanation for it, yet.
        drdeca 5y ago
        AIUI, “Started moving away from the starting area” is wrong. The starting area is everywhere. It isn’t that there is some point where the center of the Big Bang was. The idea is that, stuff was basically evenly distributed everywhere , and very dense, and then later, without needing the amount of stuff to have decreased, stuff is nevertheless more spread out. Now, you might just explain this with like, “well, space is infinite in extent, so you can just spread it all out, without running out of room, by moving things out towards infinity” but you can describe the same sort of situation even if space is compact, turning back around in itself (as in the analogy of “imagine space is the surface of a balloon”)
maskedinvader 5y ago
tldr, energy density of the universe remains same while the universe is expanding, so yes space is being created, and yes physical distances between things are changing (outside of the local influence of gravity that is, so no distance between earth and moon is not changing for example). Now a longish answer, space is like a stage or a backdrop, it's not made of anything, yet we can model the behavior of radiation and matter within space, also we cannot talk only about space, we have to talk about both space and time since they are intertwined. So spacetime has geometry, ie it can be curved by presence of matter, and space tells matter how it can move (geodesics), As for the expansion of space itself, Hubble observations found that every star outside the local group is moving away from earth and not only are they moving away from earth but they are speeding up as they move away from us, also we observe that the acceleration is proportional to the distance to those stars, for example a star 100 light years away is racing away 2x faster than a star 50 light years away (we inferred this based on the redshifting of the light coming from these stars that are moving away from us), based on these observations, we can conclude that the space between stars itself is expanding at rate proportional to the distance between stars, ie something like the opposite of positive pressure is pushing stars away and the further they are from each other the stronger the push away, we have some sort of always accelerating negative pressure affecting everything. This negative pressure is referred to as dark energy and it is responsible for this expansion of space. To put it another way outside of local cluster of galaxies, gravity is losing the battle against this negative pressure expanding force we call dark energy and eventually everything would be so far away that light from these distinct racing away stars won't ever make it to earth no matter how long we wait, which is why the author ends with future astronomers would see a barren universe. disclaimer: I am no physicist, just another nerd who's watched a lot of documentaries and thinks he understands a thing or two about this stuff but could totally be drawing the wrong analogy or incorrect in some technical way for the above explanation, so I am happy to be corrected and learn.
isaacimagine 5y ago
From what I understand: all points in space are being pushed away from each other; from two atoms in your hand to two atoms across the universe. The reason objects aren't getting bigger is because atomic/nuclear/gravitational forces hold nearby objects together: if you stretched the earth to be a millimeter wider (massive amount), the earth would just collapse down to the same size it was before. Objects are held together locally, but pushed apart globally.
- mr_mitm 5y ago
  Not quite, expansion doesn't happen on such small scales. Expansion is a property of the Friedman-Lemaitre-Robertson-Walker metric, which is a solution to the Einstein equations if you assume everything is isotropic and homogeneous. Spacetime around a star or a galaxy is obviously not homogeneous. It looks much more like a Schwarzschild metric, which features no expansion. It may help to think of the universe as a cake with raisins in it. When you bake the cake, the dough expands uniformly, but the raisins do not. Small caveat: If dark energy has a certain property, it changes things and a "big rip" will occur, tearing apart everything but elementary particles.
  - skulk 5y ago
    If expansion is indeed happening, wouldn't it play a role everywhere, even if it's eclipsed by whatever else happens to be going on?
    - Akronymus 5y ago
      from my understanding, it happens at ever scale. But the other forces (weak, strong, gravity, electromagnetism) are MUCH more powerful than that at small scales, and thus not observable at small scales.
- lordnacho 5y ago
  Don't take my word for it, but I think of it as magnets on a stretchy scarf. Three magnets that are already clumped together stay together but other clumps become further and further removed.
_Nat_ 5y ago
From a basic perspective: Yes, new space is being created. And, yes, the physical distance between things are changing. The [Hubble constant](https://www.wolframalpha.com/input/?i=hubble+constant https://www.wolframalpha.com/input/?i=hubble+constant ) is roughly (2.2 * 10^-18)/s -- often written as, e.g., 68 km/s/Mpc (same thing). Which basically asserts that, every second, each unit of space is increasing by 2.2*10^-18. For example, if we suppose that there's a ball 1 meter away from you, then in a second, it'ld be ~1.0000000000000000022 meters away from you. Not that it moved away from you, but rather that ~2.2*10^-18 meters of space was created between you and it. At least, in basic theory. There're a lot of reasons to not take it too literally. For example, at local-scales, forces may resist expansion, instead pulling things back together. For another, the idea that the value is constant seems dubious. But, it's really simplified/approximate/speculative/etc.. Note that this does imply that things can, in a sense, move faster than the speed of light without violating Einstein's Relativity. Because Einstein's Relativity asserts that things can't move faster than light locally; however, the expansion of space isn't considered to be a factor in that. Which is relevant to the main-article above: when something's so far that space is being created between you and it faster than the speed-of-light, it's theoretically outside of the universe that's relevant to you, and thus outside of one (disfavored) notion of "the observable universe". For example, say that two aliens are separated by such a distance that the expansion-of-space between them is greater than the speed-of-light. Then, even if both aliens blasted off at nearly the speed-of-light toward each other, they'd still approach each other more slowly than the distance is expanding, such that they wouldn't reach each other. (At least, under a lot of assumptions.) That distance-of-separation is currently [estimated at about 14-billion light-years](https://www.wolframalpha.com/input/?i=%28speed+of+light%29+%2F+%28hubble+constant%29 https://www.wolframalpha.com/input/?i=%28speed+of+light%29+%... ), called the [Hubble length](https://en.wikipedia.org/wiki/Hubble%27s_law#Hubble_length https://en.wikipedia.org/wiki/Hubble%27s_law#Hubble_length ).
- oceanghost 5y ago
  I think i get it. Thank you for the great reply.

drdeca 5y ago

> Are the physical distances between things changing? Yes. > Is new space being created? I’m not sure what you mean by this question. Imagine ants living on the surface of a balloon which is inflating, and, I think that’s not that far off? (Except, the balloon is compact and wraps around, and the universe likely doesn’t.)

phs318u 5y ago

Here's a rabbit hole you may wish to go down: https://www.physicsforums.com/threads/what-is-the-fabric-of-space-made-of.282203/ https://www.physicsforums.com/threads/what-is-the-fabric-of-...

iszomer 5y ago

My dumb answer: the universe is an expanding balloon being inflated by The Big Bang. If you were a standing on one point on the surface of the balloon, objects relative to your observable distance would be moving away from you. This observation can be measured by a phenomenon known as the redshift. I'm lost from here..

awb 5y ago
I think more like you’re in the middle of a balloon being blown up and all points are accelerating away from you. Except every point in the universe is like the middle of the balloon with space accelerating away from it in all directions.
- galaxyLogic 5y ago
  I've read about the fruitcake metaphor. When you bake a fruitcake in the oven it expands and thus all the little pieces of fruit in it move away from each other. But if that is the proper metaphor then my question is: Is there any difference between everything "moving away from everything else" (like pieces of fruit in expanding fruitcake) vs. "space expanding"? Why do we say "space is expanding" rather than saying "all stars are moving away from each other"? What is the difference between "space expanding" and "things moving away from each other"?
  - awb 5y ago
    > What is the difference between "space expanding" and "things moving away from each other"? Not an astrophysicist, but I think both are happening. Space is the medium we live in. It’s the cake in the fruit cake. The fruit wants to stay in the same place, but the cake is expanding which is causing the fruit to appear to move away from each other even though the fruit wants to stay in one place. Same with the universe, space is expanding which is causing objects to become farther apart, even if they could somehow have 0 speed. But in our universe the planets, stars, galaxies, etc. (the fruit) do have speed. Some are getting closer to us, like the Andromeda galaxy, while many others are moving away from us. However, given the vast distances and how fast space is expanding (how fast the cake is rising), even those distant objects that want to move towards us are actually moving away from us. Imagine a swimmer swimming against a tsunami. You can be swimming any direction you want but the power and speed of the tsunami will offset that and dictate where you end up. Space is expanding at incredible speeds, so it’s quite the tsunami: https://www.forbes.com/sites/startswithabang/2020/06/12/ask-ethan-how-does-the-fabric-of-spacetime-expand-faster-than-the-speed-of-light/ https://www.forbes.com/sites/startswithabang/2020/06/12/ask-...
    - galaxyLogic 5y ago
      Let's assume that at Bing Bang all the matter was in a very small area. Then it exploded, in other words all the pieces of matter started moving away from the starting area, presumably at an accelerating speed. That would simply mean that everything started moving away from everything else and that movement seems to be even accelerating. Fine. That is a good and easy to understand description. Why then do I have to add "BTW. space is expanding"? Things are moving away from each other I get it. Why do they need to add to that "Space itself is expanding"? I don't doubt there is a reason since best scientist are thinking about it. I just haven't seen a good explanation for it, yet.
      - drdeca 5y ago
        AIUI, “Started moving away from the starting area” is wrong. The starting area is everywhere. It isn’t that there is some point where the center of the Big Bang was. The idea is that, stuff was basically evenly distributed everywhere , and very dense, and then later, without needing the amount of stuff to have decreased, stuff is nevertheless more spread out. Now, you might just explain this with like, “well, space is infinite in extent, so you can just spread it all out, without running out of room, by moving things out towards infinity” but you can describe the same sort of situation even if space is compact, turning back around in itself (as in the analogy of “imagine space is the surface of a balloon”)

maskedinvader 5y ago

tldr, energy density of the universe remains same while the universe is expanding, so yes space is being created, and yes physical distances between things are changing (outside of the local influence of gravity that is, so no distance between earth and moon is not changing for example). Now a longish answer, space is like a stage or a backdrop, it's not made of anything, yet we can model the behavior of radiation and matter within space, also we cannot talk only about space, we have to talk about both space and time since they are intertwined. So spacetime has geometry, ie it can be curved by presence of matter, and space tells matter how it can move (geodesics), As for the expansion of space itself, Hubble observations found that every star outside the local group is moving away from earth and not only are they moving away from earth but they are speeding up as they move away from us, also we observe that the acceleration is proportional to the distance to those stars, for example a star 100 light years away is racing away 2x faster than a star 50 light years away (we inferred this based on the redshifting of the light coming from these stars that are moving away from us), based on these observations, we can conclude that the space between stars itself is expanding at rate proportional to the distance between stars, ie something like the opposite of positive pressure is pushing stars away and the further they are from each other the stronger the push away, we have some sort of always accelerating negative pressure affecting everything. This negative pressure is referred to as dark energy and it is responsible for this expansion of space. To put it another way outside of local cluster of galaxies, gravity is losing the battle against this negative pressure expanding force we call dark energy and eventually everything would be so far away that light from these distinct racing away stars won't ever make it to earth no matter how long we wait, which is why the author ends with future astronomers would see a barren universe. disclaimer: I am no physicist, just another nerd who's watched a lot of documentaries and thinks he understands a thing or two about this stuff but could totally be drawing the wrong analogy or incorrect in some technical way for the above explanation, so I am happy to be corrected and learn.

isaacimagine 5y ago

From what I understand: all points in space are being pushed away from each other; from two atoms in your hand to two atoms across the universe. The reason objects aren't getting bigger is because atomic/nuclear/gravitational forces hold nearby objects together: if you stretched the earth to be a millimeter wider (massive amount), the earth would just collapse down to the same size it was before. Objects are held together locally, but pushed apart globally.

mr_mitm 5y ago
Not quite, expansion doesn't happen on such small scales. Expansion is a property of the Friedman-Lemaitre-Robertson-Walker metric, which is a solution to the Einstein equations if you assume everything is isotropic and homogeneous. Spacetime around a star or a galaxy is obviously not homogeneous. It looks much more like a Schwarzschild metric, which features no expansion. It may help to think of the universe as a cake with raisins in it. When you bake the cake, the dough expands uniformly, but the raisins do not. Small caveat: If dark energy has a certain property, it changes things and a "big rip" will occur, tearing apart everything but elementary particles.
- skulk 5y ago
  If expansion is indeed happening, wouldn't it play a role everywhere, even if it's eclipsed by whatever else happens to be going on?
  - Akronymus 5y ago
    from my understanding, it happens at ever scale. But the other forces (weak, strong, gravity, electromagnetism) are MUCH more powerful than that at small scales, and thus not observable at small scales.
lordnacho 5y ago
Don't take my word for it, but I think of it as magnets on a stretchy scarf. Three magnets that are already clumped together stay together but other clumps become further and further removed.

_Nat_ 5y ago

From a basic perspective: Yes, new space is being created. And, yes, the physical distance between things are changing. The [Hubble constant](https://www.wolframalpha.com/input/?i=hubble+constant https://www.wolframalpha.com/input/?i=hubble+constant ) is roughly (2.2 * 10^-18)/s -- often written as, e.g., 68 km/s/Mpc (same thing). Which basically asserts that, every second, each unit of space is increasing by 2.2*10^-18. For example, if we suppose that there's a ball 1 meter away from you, then in a second, it'ld be ~1.0000000000000000022 meters away from you. Not that it moved away from you, but rather that ~2.2*10^-18 meters of space was created between you and it. At least, in basic theory. There're a lot of reasons to not take it too literally. For example, at local-scales, forces may resist expansion, instead pulling things back together. For another, the idea that the value is constant seems dubious. But, it's really simplified/approximate/speculative/etc.. Note that this does imply that things can, in a sense, move faster than the speed of light without violating Einstein's Relativity. Because Einstein's Relativity asserts that things can't move faster than light locally; however, the expansion of space isn't considered to be a factor in that. Which is relevant to the main-article above: when something's so far that space is being created between you and it faster than the speed-of-light, it's theoretically outside of the universe that's relevant to you, and thus outside of one (disfavored) notion of "the observable universe". For example, say that two aliens are separated by such a distance that the expansion-of-space between them is greater than the speed-of-light. Then, even if both aliens blasted off at nearly the speed-of-light toward each other, they'd still approach each other more slowly than the distance is expanding, such that they wouldn't reach each other. (At least, under a lot of assumptions.) That distance-of-separation is currently [estimated at about 14-billion light-years](https://www.wolframalpha.com/input/?i=%28speed+of+light%29+%2F+%28hubble+constant%29 https://www.wolframalpha.com/input/?i=%28speed+of+light%29+%... ), called the [Hubble length](https://en.wikipedia.org/wiki/Hubble%27s_law#Hubble_length https://en.wikipedia.org/wiki/Hubble%27s_law#Hubble_length ).

oceanghost 5y ago
I think i get it. Thank you for the great reply.