12 ms·

this is because things aren’t ‘moving apart’— space itself is expanding (into what? nobody knows). it’s not that the galaxies are being propelled apart from a f

this is because things aren’t ‘moving apart’— space itself is expanding (into what? nobody knows). it’s not that the galaxies are being propelled apart from a force in a particular place either; everything is expanding away from everything else. On the traditional balloon model, the surface of the balloon represents space itself. Those dots on the surface of an expanding balloon are not moving relative to space (the surface of the balloon), and so despite the distance increasing between us, the rate of increase of that distance is not bound by the speed of light, which only limits passage through spacetime. The wikipedia page on redshift describes well how redshift applies to objects which are in space that is expanding: https://en.wikipedia.org/wiki/Redshift#Expansion_of_space https://en.wikipedia.org/wiki/Redshift#Expansion_of_space

dumpsterdiver 6y ago
>"space itself is expanding (into what? nobody knows)" I have a terrifying theory on this (granted, the Earth will be long gone). What if the universe is not actually expanding in the sense that it is being propelled by an initial explosion, but instead falling towards a much greater mass - the end of time so to speak, where all mass ends. Considering that gravity is a "weak force", the scale of such a mass makes me uncomfortable to think about. Perhaps what we observe as the fabric of space-time itself "stretching" is actually a similar process to what happens to matter as it enters a black hole?
- YorickPeterse 6y ago
  Isn't this basically what the Big Rip hypothesis is about?
- bena 6y ago
  It's possible. The issue is that the we really don't have any information beyond what we have. We have an idea of the big bang and the expansion of the universe. We have knowledge of certain factors etc. But really, we're not really equipped to handle the scale of the cosmos.
- DeRock 6y ago
  If you want to delve further into this idea, check out http://www.flatuniversesociety.com http://www.flatuniversesociety.com
- meowface 6y ago
  From my layman's perspective, I think this is plausible, and many physicists seem to agree it's plausible. Maybe another plausible explanation could be the reverse: maybe before the Big Bang, our/the universe was in the state you describe, and the Big Bang is our perception of it "snapping back" and exploding violently outward (aka a white hole). Carlo Rovelli and others propose that black holes actually only appear as they do to us due to the extreme time dilation, and similar to how a photon's "perspective" is that it's experiencing all of time at once, a black hole's perspective is that it's basically a massive explosion that occurs at essentially the same moment it forms. So under this hypothesis, a black hole never reaches a true singularity, but as close to a singularity as physically possible, and then it almost instantaneously bounces back like a taut rubber band that's stretched back as far as it'll go. From our perspective on the outside, it's an intense explosion occurring in very slow motion, and from the perspective on the inside, it's an intense explosion happening all at once. Rovelli posits a Planck star, not a singularity, exists past the black hole's event horizon, and the Planck star is the state of maximal compression between the black hole and white hole phases. And somewhat related to this, if I understand the theories correctly, our universe might be a supermassive white hole, and we might hold the perspective of being inside of a super(super)massive black hole that's turned into a white hole and is exploding and jettisoning everything inside of it. Or some stage after that. The ultra-compressed energy before the Big Bang would be an (extremely massive and dense) Planck star. I have no idea how likely the Planck star theory or white hole universe theory might be, but my blind speculation is that if it's true, maybe dark energy is whatever's fundamentally responsible for gravity/curvature causing our white hole explosion to bend some outer medium - the hypothetical potential medium which our black hole/universe originally formed in - and interact with other extreme contractions and explosions also happening in that medium, in complex and multivariate ways. Maybe a white hole being pushed into or prodded or yanked by many other black holes and white holes from different directions and angles could be perceived from the inside as accelerating expansion of space at certain scales. Maybe this is happening recursively, even; maybe that outer medium is itself within another even greater medium, which also is within another greater medium, etc. Maybe the nesting depth is small, or near-infinite, or infinite. Or maybe there is no outer medium to our universe, in which case that whole idea's shot. And I'm not sure if this could be related, but Rovelli suggests the hypothetical Planck star cycle of black hole to white hole applied to primordial black holes that formed in our early universe could explain dark matter, so even without the outer medium idea there could be something here that helps us understand our large-scale observations. Maybe there could be some relation to the "dark fluid" and negative energy idea, too: https://en.m.wikipedia.org/wiki/Dark_fluid https://en.m.wikipedia.org/wiki/Dark_fluid I have absolutely no clue if your or my speculation is reasonable or not (there's currently zero evidence of any sort of outer medium, as far as I know), but the general concept of our universe being the result of a black and/or white hole does definitely seem to interest a lot of physicists.
- Swizec 6y ago
  My high school physics professor had a great take on this: The universe is defined as “everything”. If we ever find anything outside the universe, we’ll call that the universe too.
  - whatshisface 6y ago
    That doesn't really answer the question, what the parent is really asking about is whether we are living on the surface of a manifold that's due for some kind of a collision with something else.
  - GlennS 6y ago
    The atom used to be defined as the smallest indivisible thing, but we didn't rename it when we learned that you could split them into smaller things.
- throwaway568 6y ago
  Interesting, this adds to a thought I had a couple of weeks ago. We can see the past, but we can’t go there, like an observer inside event horizon looking out, they can see light coming in, but can’t leave. We can’t see the future, but when we are going there, like an observer falling into the event horizon. They can’t see what’s inside, but they can fall inside. As we experience time, it’s like we are falling into a black hole made of time.
ddevault 6y ago
In case anyone is confused, I prefer to explain the expansion of space without metaphors. Space itself is getting larger, everywhere, all at once. The scale factor of the universe is getting larger. New space is constantly being "made". Every second, every kilometer in the universe grows by 2.2×10^-18 centimeters. If no other forces were in play, it'd carry everything along with it and pull everything a bit further apart. However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed. That's a small number, but the universe is very big. The hubble constant is usually represented in km/s/Mpsc (kilometers per second per megaparsec), and in those units it comes out to 68. So, every second, every Megaparsec in the universe grows by 68 kilometers. The more parsecs between us and some distant thing, the more space is growing between us and them every second. For reference, our nearest galactic neighbor is Andromeda, which is 67 kpc away, so about 100 meters of new space is created between our galaxies every second. That being said, Andromeda is heading towards the Milky Way at 110 km/s, so it easily overcomes this and will crash into us any day now^W^W^Win 4.5 billion years.
- joshspankit 6y ago
  It would be interesting to hear your answer to mekkkkkk‘s question
  - maxekman 6y ago
    It was answered, as far as I can tell: > However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed.
    - thaumasiotes 6y ago
      OK, we have a meter stick. It's made of pure iron and, for whatever physical reasons, it is 1 meter long exactly. After some time, the scale factor of the universe grows, so that 1 new meter is 1.05 old meters. But, by hypothesis, as this happens, our meter stick contracts, so that it ends up being just 0.952 new meters (= 1 old meter) long. Why? Did the physical laws governing iron atoms change so that they pack more densely than before? And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule?
mekkkkkk 6y ago
Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would seem that way. In your balloon example, the dots would get bigger.
- ddevault 6y ago
  Local factors (gravity, strong & weak nuclear force) easily overcome the expansion of space. So things stick together.
- chowells 6y ago
  Not in practice. In practice, this provides a very tiny force pulling subatomic particles apart in an atom's nucleus, or pulling atoms away from each other in a molecule, or pulling a celestial body's molecules away from each other. It makes no difference at all, because that force is absolutely dwarfed by the strong nuclear force, electromagnetism, or gravity, respectively. Systems that are coupled together by those forces, up to and including galaxies, are impacted only very minutely by this expansion. For the expansion to have a significant impact on relative distances, the distances involved have to be so extreme that there's no real other interaction between the objects you're measuring from.
  - hashmymustache 6y ago
    Does that expansion provide potential energy by creating space between attractive bodies? If so how do you account for conservation of energy?
    - chowells 6y ago
      Yes, it will create some additional potential energy. That doesn't contradict strict conservation of energy - it just adds another energy source that has to be accounted for when balancing everything. It might contradict the aphorism "energy is never created or destroyed", but I'm not sure that was ever science. It's more fun to think about it in terms of thermodynamics. Can that extra energy be used to reverse the second law of thermodynamics? I'm guessing not - it's so diffuse that it's probably impossible to use it to reduce entropy. The math for actually doing those calculations is well beyond what I know, though, so I'll just say that one's no more than a guess.
iwonderwhy 6y ago
Interesting analogy. So in effect to someone “watching” from outside our universe anything moving at the speed of light in our context, is moving either faster or slower than the speed of light in their context. Since the ballon and everything inside is expanding it means that either things are moving at the speed of light + speed of expansion, or speed of light - speed of expansion, when watched from outside. Assuming that everything expands along with the balloon. If I understand this correctly, and according to: “Due to the expansion increasing as distances increase, the distance between two remote galaxies can increase at more than 3×108 m/s, but this does not imply that the galaxies move faster than the speed of light at their present location (which is forbidden by Lorentz covariance).” But from outside we can move faster than the speed of light? Sorry having trouble getting my head around this.
- pas 6y ago
  kind of yes, relatively from an "outside observer" that would be the case. but if the usual laws of physics apply to that outside "observer" then it will not be able to observe photons from those objects. the usual assumption is that space is basically infinite but we can only observe our own "observable universe" sphere of it. every point in space has its own universe basically. see also cosmic light horizon: https://www.thedallasgeek.com/single-post/2018/06/11/Our-Cosmic-Light-Horizon https://www.thedallasgeek.com/single-post/2018/06/11/Our-Cos... (the difference between regions we can communicate with and regions from where a photon coming toward us will eventually reach us)
- timcederman 6y ago
  Since there's no frame of reference outside the universe, you can't make that analogy. I think you're getting tripped up on a variation of this idea, e.g. moving a laser pointer across the moon's surface faster than light. https://www.universetoday.com/109147/how-a-laser-appears-to-move-faster-than-light-and-why-it-really-isnt/ https://www.universetoday.com/109147/how-a-laser-appears-to-...
  - Supermancho 6y ago
    > Since there's no frame of reference outside the universe That's unknown. The observable universe is all we know for sure and there's evidence that there is both something beyond what we can observe, which conforms to what we know as space and matter, and something beyond that (whatever existed around the big bang locality).
cgriswald 6y ago
> On the traditional balloon model, the surface of the balloon represents space itself... For any still confused, there is also a 'raisin bread' model (which I prefer because it's 3D, unlike the surface of a balloon). The raisins are galaxies, and the bread is the space between them. As it bakes, the raisins get further apart, even though the raisins aren't moving through the dough. (Although galaxies also have movement through space, this isn't what is causing them to get further apart.)
- nelsonenzo 6y ago
  Omg, thank you for this! The balloon model always confused me because I could never understand what was in the middle of the balloon, and why we were on top of it. It's an odd 2d example using an object that is 3d, the balloon. Raisin bread, oh I love thee even more today.
  - jecel 6y ago
    The raisin bread is a 3D expansion in 3D space. So from the viewpoint of a specific raisin the others are moving differently depending on what direction you look and you can use that to determine where the center of the bread is. The balloon is a 2D expansion in 3D space, which is a better analogy for our 3D universe expanding in 4D space. From the viewpoint of a particular dot you see the exact same thing no matter what 2D direction you look. There is no center. I agree that the balloon is more confusing, but it is more correct in that it doesn't lead to ideas like "the point where the big bang happened".
    - anigbrowl 6y ago
      Is the a term for the 4-dimensional coordinate of the 'inflationary center', like there a 3d coordinate for the center of the balloon (inaccessible from the flat universe on its surface)?
      - svachalek 6y ago
        Imagine that the balloon is actually completely flat and (to all intents and purposes) stretches infinitely in X and Y directions without wrapping around. As I understand it, we currently believe we're in a 3D version of that: https://en.wikipedia.org/wiki/Shape_of_the_universe https://en.wikipedia.org/wiki/Shape_of_the_universe
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Enginerrrd 6y ago
>space itself is expanding (into what? nobody knows) This is a conflation of two different things. Most of the current evidence points to a universe which, when it began, was already infinite in size, but infinitely dense. In that model, it remains infinite in size, but is also expanding.
vl 6y ago
Is there red shift between Sun and Milky Way? Sun and Earth? Earth and Moon? If so, have we detected it?

dumpsterdiver 6y ago

>"space itself is expanding (into what? nobody knows)" I have a terrifying theory on this (granted, the Earth will be long gone). What if the universe is not actually expanding in the sense that it is being propelled by an initial explosion, but instead falling towards a much greater mass - the end of time so to speak, where all mass ends. Considering that gravity is a "weak force", the scale of such a mass makes me uncomfortable to think about. Perhaps what we observe as the fabric of space-time itself "stretching" is actually a similar process to what happens to matter as it enters a black hole?

YorickPeterse 6y ago
Isn't this basically what the Big Rip hypothesis is about?
bena 6y ago
It's possible. The issue is that the we really don't have any information beyond what we have. We have an idea of the big bang and the expansion of the universe. We have knowledge of certain factors etc. But really, we're not really equipped to handle the scale of the cosmos.
DeRock 6y ago
If you want to delve further into this idea, check out http://www.flatuniversesociety.com http://www.flatuniversesociety.com
meowface 6y ago
From my layman's perspective, I think this is plausible, and many physicists seem to agree it's plausible. Maybe another plausible explanation could be the reverse: maybe before the Big Bang, our/the universe was in the state you describe, and the Big Bang is our perception of it "snapping back" and exploding violently outward (aka a white hole). Carlo Rovelli and others propose that black holes actually only appear as they do to us due to the extreme time dilation, and similar to how a photon's "perspective" is that it's experiencing all of time at once, a black hole's perspective is that it's basically a massive explosion that occurs at essentially the same moment it forms. So under this hypothesis, a black hole never reaches a true singularity, but as close to a singularity as physically possible, and then it almost instantaneously bounces back like a taut rubber band that's stretched back as far as it'll go. From our perspective on the outside, it's an intense explosion occurring in very slow motion, and from the perspective on the inside, it's an intense explosion happening all at once. Rovelli posits a Planck star, not a singularity, exists past the black hole's event horizon, and the Planck star is the state of maximal compression between the black hole and white hole phases. And somewhat related to this, if I understand the theories correctly, our universe might be a supermassive white hole, and we might hold the perspective of being inside of a super(super)massive black hole that's turned into a white hole and is exploding and jettisoning everything inside of it. Or some stage after that. The ultra-compressed energy before the Big Bang would be an (extremely massive and dense) Planck star. I have no idea how likely the Planck star theory or white hole universe theory might be, but my blind speculation is that if it's true, maybe dark energy is whatever's fundamentally responsible for gravity/curvature causing our white hole explosion to bend some outer medium - the hypothetical potential medium which our black hole/universe originally formed in - and interact with other extreme contractions and explosions also happening in that medium, in complex and multivariate ways. Maybe a white hole being pushed into or prodded or yanked by many other black holes and white holes from different directions and angles could be perceived from the inside as accelerating expansion of space at certain scales. Maybe this is happening recursively, even; maybe that outer medium is itself within another even greater medium, which also is within another greater medium, etc. Maybe the nesting depth is small, or near-infinite, or infinite. Or maybe there is no outer medium to our universe, in which case that whole idea's shot. And I'm not sure if this could be related, but Rovelli suggests the hypothetical Planck star cycle of black hole to white hole applied to primordial black holes that formed in our early universe could explain dark matter, so even without the outer medium idea there could be something here that helps us understand our large-scale observations. Maybe there could be some relation to the "dark fluid" and negative energy idea, too: https://en.m.wikipedia.org/wiki/Dark_fluid https://en.m.wikipedia.org/wiki/Dark_fluid I have absolutely no clue if your or my speculation is reasonable or not (there's currently zero evidence of any sort of outer medium, as far as I know), but the general concept of our universe being the result of a black and/or white hole does definitely seem to interest a lot of physicists.
Swizec 6y ago
My high school physics professor had a great take on this: The universe is defined as “everything”. If we ever find anything outside the universe, we’ll call that the universe too.
- whatshisface 6y ago
  That doesn't really answer the question, what the parent is really asking about is whether we are living on the surface of a manifold that's due for some kind of a collision with something else.
- GlennS 6y ago
  The atom used to be defined as the smallest indivisible thing, but we didn't rename it when we learned that you could split them into smaller things.
throwaway568 6y ago
Interesting, this adds to a thought I had a couple of weeks ago. We can see the past, but we can’t go there, like an observer inside event horizon looking out, they can see light coming in, but can’t leave. We can’t see the future, but when we are going there, like an observer falling into the event horizon. They can’t see what’s inside, but they can fall inside. As we experience time, it’s like we are falling into a black hole made of time.

ddevault 6y ago

In case anyone is confused, I prefer to explain the expansion of space without metaphors. Space itself is getting larger, everywhere, all at once. The scale factor of the universe is getting larger. New space is constantly being "made". Every second, every kilometer in the universe grows by 2.2×10^-18 centimeters. If no other forces were in play, it'd carry everything along with it and pull everything a bit further apart. However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed. That's a small number, but the universe is very big. The hubble constant is usually represented in km/s/Mpsc (kilometers per second per megaparsec), and in those units it comes out to 68. So, every second, every Megaparsec in the universe grows by 68 kilometers. The more parsecs between us and some distant thing, the more space is growing between us and them every second. For reference, our nearest galactic neighbor is Andromeda, which is 67 kpc away, so about 100 meters of new space is created between our galaxies every second. That being said, Andromeda is heading towards the Milky Way at 110 km/s, so it easily overcomes this and will crash into us any day now^W^W^Win 4.5 billion years.

joshspankit 6y ago
It would be interesting to hear your answer to mekkkkkk‘s question
- maxekman 6y ago
  It was answered, as far as I can tell: > However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed.
  - thaumasiotes 6y ago
    OK, we have a meter stick. It's made of pure iron and, for whatever physical reasons, it is 1 meter long exactly. After some time, the scale factor of the universe grows, so that 1 new meter is 1.05 old meters. But, by hypothesis, as this happens, our meter stick contracts, so that it ends up being just 0.952 new meters (= 1 old meter) long. Why? Did the physical laws governing iron atoms change so that they pack more densely than before? And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule?

mekkkkkk 6y ago

Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would seem that way. In your balloon example, the dots would get bigger.

ddevault 6y ago
Local factors (gravity, strong & weak nuclear force) easily overcome the expansion of space. So things stick together.
chowells 6y ago
Not in practice. In practice, this provides a very tiny force pulling subatomic particles apart in an atom's nucleus, or pulling atoms away from each other in a molecule, or pulling a celestial body's molecules away from each other. It makes no difference at all, because that force is absolutely dwarfed by the strong nuclear force, electromagnetism, or gravity, respectively. Systems that are coupled together by those forces, up to and including galaxies, are impacted only very minutely by this expansion. For the expansion to have a significant impact on relative distances, the distances involved have to be so extreme that there's no real other interaction between the objects you're measuring from.
- hashmymustache 6y ago
  Does that expansion provide potential energy by creating space between attractive bodies? If so how do you account for conservation of energy?
  - chowells 6y ago
    Yes, it will create some additional potential energy. That doesn't contradict strict conservation of energy - it just adds another energy source that has to be accounted for when balancing everything. It might contradict the aphorism "energy is never created or destroyed", but I'm not sure that was ever science. It's more fun to think about it in terms of thermodynamics. Can that extra energy be used to reverse the second law of thermodynamics? I'm guessing not - it's so diffuse that it's probably impossible to use it to reduce entropy. The math for actually doing those calculations is well beyond what I know, though, so I'll just say that one's no more than a guess.

iwonderwhy 6y ago

Interesting analogy. So in effect to someone “watching” from outside our universe anything moving at the speed of light in our context, is moving either faster or slower than the speed of light in their context. Since the ballon and everything inside is expanding it means that either things are moving at the speed of light + speed of expansion, or speed of light - speed of expansion, when watched from outside. Assuming that everything expands along with the balloon. If I understand this correctly, and according to: “Due to the expansion increasing as distances increase, the distance between two remote galaxies can increase at more than 3×108 m/s, but this does not imply that the galaxies move faster than the speed of light at their present location (which is forbidden by Lorentz covariance).” But from outside we can move faster than the speed of light? Sorry having trouble getting my head around this.

pas 6y ago
kind of yes, relatively from an "outside observer" that would be the case. but if the usual laws of physics apply to that outside "observer" then it will not be able to observe photons from those objects. the usual assumption is that space is basically infinite but we can only observe our own "observable universe" sphere of it. every point in space has its own universe basically. see also cosmic light horizon: https://www.thedallasgeek.com/single-post/2018/06/11/Our-Cosmic-Light-Horizon https://www.thedallasgeek.com/single-post/2018/06/11/Our-Cos... (the difference between regions we can communicate with and regions from where a photon coming toward us will eventually reach us)
timcederman 6y ago
Since there's no frame of reference outside the universe, you can't make that analogy. I think you're getting tripped up on a variation of this idea, e.g. moving a laser pointer across the moon's surface faster than light. https://www.universetoday.com/109147/how-a-laser-appears-to-move-faster-than-light-and-why-it-really-isnt/ https://www.universetoday.com/109147/how-a-laser-appears-to-...
- Supermancho 6y ago
  > Since there's no frame of reference outside the universe That's unknown. The observable universe is all we know for sure and there's evidence that there is both something beyond what we can observe, which conforms to what we know as space and matter, and something beyond that (whatever existed around the big bang locality).

cgriswald 6y ago

> On the traditional balloon model, the surface of the balloon represents space itself... For any still confused, there is also a 'raisin bread' model (which I prefer because it's 3D, unlike the surface of a balloon). The raisins are galaxies, and the bread is the space between them. As it bakes, the raisins get further apart, even though the raisins aren't moving through the dough. (Although galaxies also have movement through space, this isn't what is causing them to get further apart.)

nelsonenzo 6y ago
Omg, thank you for this! The balloon model always confused me because I could never understand what was in the middle of the balloon, and why we were on top of it. It's an odd 2d example using an object that is 3d, the balloon. Raisin bread, oh I love thee even more today.
- jecel 6y ago
  The raisin bread is a 3D expansion in 3D space. So from the viewpoint of a specific raisin the others are moving differently depending on what direction you look and you can use that to determine where the center of the bread is. The balloon is a 2D expansion in 3D space, which is a better analogy for our 3D universe expanding in 4D space. From the viewpoint of a particular dot you see the exact same thing no matter what 2D direction you look. There is no center. I agree that the balloon is more confusing, but it is more correct in that it doesn't lead to ideas like "the point where the big bang happened".
  - anigbrowl 6y ago
    Is the a term for the 4-dimensional coordinate of the 'inflationary center', like there a 3d coordinate for the center of the balloon (inaccessible from the flat universe on its surface)?
    - svachalek 6y ago
      Imagine that the balloon is actually completely flat and (to all intents and purposes) stretches infinitely in X and Y directions without wrapping around. As I understand it, we currently believe we're in a 3D version of that: https://en.wikipedia.org/wiki/Shape_of_the_universe https://en.wikipedia.org/wiki/Shape_of_the_universe

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Enginerrrd 6y ago

>space itself is expanding (into what? nobody knows) This is a conflation of two different things. Most of the current evidence points to a universe which, when it began, was already infinite in size, but infinitely dense. In that model, it remains infinite in size, but is also expanding.

vl 6y ago

Is there red shift between Sun and Milky Way? Sun and Earth? Earth and Moon? If so, have we detected it?