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Stupid non-physicist question here: we know the universe is expanding, but where does it expand at? Are there physical areas which we can observe growing? And i

Stupid non-physicist question here: we know the universe is expanding, but where does it expand at? Are there physical areas which we can observe growing? And if so, shouldn't that mean that we observe distant objects as lighter than their interactions with closer objects would imply?

YaxelPerez 8y ago
The explanation I've heard is that it's just space being created everywhere at once. There's no center from which everything expands. This VSauce video explains it better than I can (later in the video): https://youtu.be/3pAnRKD4raY https://youtu.be/3pAnRKD4raY
- andrewshadura 8y ago
  Imagine a flat ‘universe’ on the surface of a balloon which is being inflated. There would be no centre of expansion on the surface of the balloon, the space would appear everywhere equally.
danbruc 8y ago
Space expands everywhere, also right in front of you, it is just a very small effect over short distances. Two points one meter apart expand away from each other with a speed of 0.07 nanometers per year. For short distances electromagnetic and strong forces just pull everything immediately back together as things want do move apart. Over larger distances, think the scale of galaxies, gravitational forces keep the stars, the gas, and all the other stuff together and prevent galaxies from expanding. But once you move beyond the scale of galaxies and galaxy clusters there are no longer any forces strong enough to counter the expansion of all that space between galaxies and so in general the distances between galaxies are increasing. There are still exceptions, in clusters of nearby galaxies gravitational forces can still bind those galaxies together but with increasing distances between galaxies the expansion finally overpowers gravitational attraction. It is important to remember that not only are all forces becoming weaker as distances increase but also that points are expanding faster away from each other as their distance increases. As said, two points one meter apart move away from each other with 0.07 nanometers per year but two points one kilometer apart move away with 70 nanometers per year because there are a thousand meters in between them, all of them simultaneously expanding at 0.07 nanometers per year.
- raattgift 8y ago
  > Space expands everywhere, also right in front of you, it is just a very small effect over short distances ... .07 nanometers per year but two points one kilometer apart move away with 70 nanometers per year because there are a thousand meters in between them No, what you wrote is contradicted by evidence from within the inner solar system (MESSENGER's observations of solar mass loss were highly sensitive; we have excellent VLBI too), from other star systems, especially eclipsing binaries and systems with occluding planetary systems) and from galactic dynamics (LSR tests, peculiar motions, and lots of DM/dynamics evidence for galaxies at different redshifts). It is not how the standard model of cosmology works either. Grossly, 0.07 nanometers per metre per year expansion everywhere would be impossible to hide from solid state physics, and even several everyday polymers and ceramics; an expansion term would have to appear in accurate descriptions. We cannot say there is no expansion at these scales, but the expansion is highly constrained and evidence requires that it effectively vanishes. A fifth-force mechanism like quintessence requires a shutdown mechanism inside galaxies that contain star systems like ours or TRAPPIST-1 or PSR~J0337+1715. Additionally it likely would need a shutdown in small structures (like asteroids) ejected from galaxy clusters by violent events, although we will not spot those in practice any time soon. Where there's a shutdown mechanism there's also a wake-up mechanism that has to be considered too, and high-redshift observations constrain the wakeup of quintessence action to relatively late times. What suppressed quintessence for a bit more than three billion years after the formation of the cosmic microwave background? These type of modellers run into the hard problem that as they work out the parameters of their theory, they (so far) find they all are consistent with the standard cosmology. (That shouldn't be too surprising; LambdaCDM, the standard cosmology, was carefully built to concord with evidence "forward-compatibly".) One way to put it is in the name: cosmic expansion, rather than universal expansion, distinguishing between effects apparent at the largest scales, and effects apparent everywhere in the universe, at all scales. LambdaCDM is a model of an universe well-described by an expanding Robertson-Walker metric and matter in the large obeying the Friedmann equations. The expanding metric includes a term for the cosmological constant. The matter is essentially a space-filling fluid at rest, isotropic, homogeneous, and being diluted away by the expansion of the background Robertson-Walker spacetime. (It is in a special frame of reference in this model in which Dark Energy arises as a non-diluting homogeneous fluid imposing constant isotropic tension on the matter fluids. In general frames Dark Energy is just the cosmological constant.) On the LambdaCDM model we can overlay the 'swiss cheese' model; this is standard too, but we are departing from cosmology and heading toward astrophysics. The motivation of 'swiss cheese' is simple: at the largest scales, the Friedmann Robertson-Walker model sketched above describes all observations between well and extremely well. However, it does not describe gravitationally-bound systems like galaxy clusters, galaxies, or star systems. Those are much much better described with conventional clumping matter (and dark matter) inside a collapsing Tolman spacetime. Notably, none of these systems are homogeneous (they contain lumpy bits like planets and stars, and sparse bits like the interstellar medium) and all of them contain radiating orbiting material; that they are radiating implies gravitational collapse (there's also plenty of other evidence for that). The Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological model does not describe these systems. In 'swiss-cheese', the FLRW cosmological model is the cheese. If we treat the cosmological-scale matter fluid as a dust that is only homogeneous at the largest scales, the dust can be lumpier in some places and sparser in others. The lumpiest bits represent galaxy clusters that remain gravitationally bound over cosmological times. We then "swiss" this lumpy cheese procedurally: we cut out a region of the Friedmann matter on the Robertson-Walker background and replace it with a region of galaxy-cluster matter on a Tolman background, and stitch the two together using junction conditions on the boundary. (Typically we do this to the densest lumps, the galaxy clusters, but we could do the same procedure in deep intergalactic space instead. A spherical region of that will not be exactly void, since it will contain at least photons and neutrinos produced in the early universe, likely a bit of baryonic matter from big bang and supernova nuclear synthesis, and possibly a small amount of dark matter too: however all this stuff is so sparse that it would not generate a collapsing spacetime metric -- calculated out, it would resemble a slightly perturbed expanding Robertson-Walker metric. So we don't put "holes" there; our "holes" contain galaxies.). In a swiss-cheese model there is simply no expansion in the "holes", because the Tolman backgrounds that work do not contain a cosmological constant term. (The typical problem is that something that does contain expansion, like a Kottler vacuole, evolves away from a mass-compensating comoving void, e.g., the hole grows much too fast or in strange ways that do not match any of the tens of millions of galaxy clusters in our sky). Since Tolman/FRW swiss cheese models [a] are tractable in practice and [b] match observations extremely well, it is perfectly reasonable to take the hint that nature doesn't expand at all inside the holes. That is, the non-expansion of the hole background carries down hierarchically; you don't need to introduce an expansion term into a metric describing an individual star or planet. It would be very exciting to find any metric expansion within the solar system, or in another solar system under close scrutiny, or at any scale smaller than that of the swiss-cheese "holes". Since real matter systems like stars and planets can be very well described as a "hole" generating a metric like Schwarzschild out to some boundary, there is no motivation to add in a metric expansion term (it would just vanish within the boundary). Finally, inhomogeneous cosmologies generally import these results, just like they import the local and global measurements of H_0. The viable scale of fluctuations in the Hubble parameter in such approaches is large compared to solar systems. So there's no relief in a "well, your model is wrong" (incidentally, I'd agree, there are some real problems with swiss-cheese). In summary, the evidence is against your claim, and other strands of evidence support theoretical frameworks in which there is no metric expansion around Earth/Earth-Moon. That includes frameworks in which one has explicit terms for such expansion: those terms must vanish around here. - -- See also: the much less wordy way of taking the same position at http://curious.astro.cornell.edu/about-us/97-the-universe/galaxies/cosmology/538-as-the-universe-expands-why-don-t-galaxies-get-stretched-out-intermediate http://curious.astro.cornell.edu/about-us/97-the-universe/ga... Compare: https://medium.com/starts-with-a-bang/ask-ethan-if-the-universe-is-expanding-why-arent-we-71b46b5e9974 https://medium.com/starts-with-a-bang/ask-ethan-if-the-unive...
arduinomancer 8y ago
Think of if space were just two dimensions and it was on the surface of a balloon. Space is expanding in the sense of inflating the balloon. Every point on the balloon is getting farther away from every other point. If you stand on the balloon it doesn't make sense to ask "where is the balloon expanding from?". Its expanding in every direction. Every observer on the balloon sees all points around them getting farther away. Now try to picture that but in 3d. It might help to think of it as like a cake with raisins in it expanding in the oven.
- dragonwriter 8y ago
  > If you stand on the balloon it doesn't make sense to ask "where is the balloon expanding from?". Sure it does, and there is a well-defined answer, it's just the answer does not lie on the surface of the balloon. (And for a perfectly spherical balloon, is equidistant from every point on the surface.)
  - danbruc 8y ago
    That of course only works because the surfaces of balloons we are used to are always embedded in spacetime. There is however no [good][known] reason to assume that spacetime itself is embedded in any space.
    - OscarCunningham 8y ago
      I've always thought this would be an interesting experiment. Assume that spacetime does live in a higher dimensional space, and try to arrange masses so that spacetime is forced to intersect itself. See if anything interesting happens. Of course this probably requires some stellar engineering.

YaxelPerez 8y ago

The explanation I've heard is that it's just space being created everywhere at once. There's no center from which everything expands. This VSauce video explains it better than I can (later in the video): https://youtu.be/3pAnRKD4raY https://youtu.be/3pAnRKD4raY

andrewshadura 8y ago
Imagine a flat ‘universe’ on the surface of a balloon which is being inflated. There would be no centre of expansion on the surface of the balloon, the space would appear everywhere equally.

danbruc 8y ago

Space expands everywhere, also right in front of you, it is just a very small effect over short distances. Two points one meter apart expand away from each other with a speed of 0.07 nanometers per year. For short distances electromagnetic and strong forces just pull everything immediately back together as things want do move apart. Over larger distances, think the scale of galaxies, gravitational forces keep the stars, the gas, and all the other stuff together and prevent galaxies from expanding. But once you move beyond the scale of galaxies and galaxy clusters there are no longer any forces strong enough to counter the expansion of all that space between galaxies and so in general the distances between galaxies are increasing. There are still exceptions, in clusters of nearby galaxies gravitational forces can still bind those galaxies together but with increasing distances between galaxies the expansion finally overpowers gravitational attraction. It is important to remember that not only are all forces becoming weaker as distances increase but also that points are expanding faster away from each other as their distance increases. As said, two points one meter apart move away from each other with 0.07 nanometers per year but two points one kilometer apart move away with 70 nanometers per year because there are a thousand meters in between them, all of them simultaneously expanding at 0.07 nanometers per year.

raattgift 8y ago
> Space expands everywhere, also right in front of you, it is just a very small effect over short distances ... .07 nanometers per year but two points one kilometer apart move away with 70 nanometers per year because there are a thousand meters in between them No, what you wrote is contradicted by evidence from within the inner solar system (MESSENGER's observations of solar mass loss were highly sensitive; we have excellent VLBI too), from other star systems, especially eclipsing binaries and systems with occluding planetary systems) and from galactic dynamics (LSR tests, peculiar motions, and lots of DM/dynamics evidence for galaxies at different redshifts). It is not how the standard model of cosmology works either. Grossly, 0.07 nanometers per metre per year expansion everywhere would be impossible to hide from solid state physics, and even several everyday polymers and ceramics; an expansion term would have to appear in accurate descriptions. We cannot say there is no expansion at these scales, but the expansion is highly constrained and evidence requires that it effectively vanishes. A fifth-force mechanism like quintessence requires a shutdown mechanism inside galaxies that contain star systems like ours or TRAPPIST-1 or PSR~J0337+1715. Additionally it likely would need a shutdown in small structures (like asteroids) ejected from galaxy clusters by violent events, although we will not spot those in practice any time soon. Where there's a shutdown mechanism there's also a wake-up mechanism that has to be considered too, and high-redshift observations constrain the wakeup of quintessence action to relatively late times. What suppressed quintessence for a bit more than three billion years after the formation of the cosmic microwave background? These type of modellers run into the hard problem that as they work out the parameters of their theory, they (so far) find they all are consistent with the standard cosmology. (That shouldn't be too surprising; LambdaCDM, the standard cosmology, was carefully built to concord with evidence "forward-compatibly".) One way to put it is in the name: cosmic expansion, rather than universal expansion, distinguishing between effects apparent at the largest scales, and effects apparent everywhere in the universe, at all scales. LambdaCDM is a model of an universe well-described by an expanding Robertson-Walker metric and matter in the large obeying the Friedmann equations. The expanding metric includes a term for the cosmological constant. The matter is essentially a space-filling fluid at rest, isotropic, homogeneous, and being diluted away by the expansion of the background Robertson-Walker spacetime. (It is in a special frame of reference in this model in which Dark Energy arises as a non-diluting homogeneous fluid imposing constant isotropic tension on the matter fluids. In general frames Dark Energy is just the cosmological constant.) On the LambdaCDM model we can overlay the 'swiss cheese' model; this is standard too, but we are departing from cosmology and heading toward astrophysics. The motivation of 'swiss cheese' is simple: at the largest scales, the Friedmann Robertson-Walker model sketched above describes all observations between well and extremely well. However, it does not describe gravitationally-bound systems like galaxy clusters, galaxies, or star systems. Those are much much better described with conventional clumping matter (and dark matter) inside a collapsing Tolman spacetime. Notably, none of these systems are homogeneous (they contain lumpy bits like planets and stars, and sparse bits like the interstellar medium) and all of them contain radiating orbiting material; that they are radiating implies gravitational collapse (there's also plenty of other evidence for that). The Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological model does not describe these systems. In 'swiss-cheese', the FLRW cosmological model is the cheese. If we treat the cosmological-scale matter fluid as a dust that is only homogeneous at the largest scales, the dust can be lumpier in some places and sparser in others. The lumpiest bits represent galaxy clusters that remain gravitationally bound over cosmological times. We then "swiss" this lumpy cheese procedurally: we cut out a region of the Friedmann matter on the Robertson-Walker background and replace it with a region of galaxy-cluster matter on a Tolman background, and stitch the two together using junction conditions on the boundary. (Typically we do this to the densest lumps, the galaxy clusters, but we could do the same procedure in deep intergalactic space instead. A spherical region of that will not be exactly void, since it will contain at least photons and neutrinos produced in the early universe, likely a bit of baryonic matter from big bang and supernova nuclear synthesis, and possibly a small amount of dark matter too: however all this stuff is so sparse that it would not generate a collapsing spacetime metric -- calculated out, it would resemble a slightly perturbed expanding Robertson-Walker metric. So we don't put "holes" there; our "holes" contain galaxies.). In a swiss-cheese model there is simply no expansion in the "holes", because the Tolman backgrounds that work do not contain a cosmological constant term. (The typical problem is that something that does contain expansion, like a Kottler vacuole, evolves away from a mass-compensating comoving void, e.g., the hole grows much too fast or in strange ways that do not match any of the tens of millions of galaxy clusters in our sky). Since Tolman/FRW swiss cheese models [a] are tractable in practice and [b] match observations extremely well, it is perfectly reasonable to take the hint that nature doesn't expand at all inside the holes. That is, the non-expansion of the hole background carries down hierarchically; you don't need to introduce an expansion term into a metric describing an individual star or planet. It would be very exciting to find any metric expansion within the solar system, or in another solar system under close scrutiny, or at any scale smaller than that of the swiss-cheese "holes". Since real matter systems like stars and planets can be very well described as a "hole" generating a metric like Schwarzschild out to some boundary, there is no motivation to add in a metric expansion term (it would just vanish within the boundary). Finally, inhomogeneous cosmologies generally import these results, just like they import the local and global measurements of H_0. The viable scale of fluctuations in the Hubble parameter in such approaches is large compared to solar systems. So there's no relief in a "well, your model is wrong" (incidentally, I'd agree, there are some real problems with swiss-cheese). In summary, the evidence is against your claim, and other strands of evidence support theoretical frameworks in which there is no metric expansion around Earth/Earth-Moon. That includes frameworks in which one has explicit terms for such expansion: those terms must vanish around here. - -- See also: the much less wordy way of taking the same position at http://curious.astro.cornell.edu/about-us/97-the-universe/galaxies/cosmology/538-as-the-universe-expands-why-don-t-galaxies-get-stretched-out-intermediate http://curious.astro.cornell.edu/about-us/97-the-universe/ga... Compare: https://medium.com/starts-with-a-bang/ask-ethan-if-the-universe-is-expanding-why-arent-we-71b46b5e9974 https://medium.com/starts-with-a-bang/ask-ethan-if-the-unive...

arduinomancer 8y ago

Think of if space were just two dimensions and it was on the surface of a balloon. Space is expanding in the sense of inflating the balloon. Every point on the balloon is getting farther away from every other point. If you stand on the balloon it doesn't make sense to ask "where is the balloon expanding from?". Its expanding in every direction. Every observer on the balloon sees all points around them getting farther away. Now try to picture that but in 3d. It might help to think of it as like a cake with raisins in it expanding in the oven.

dragonwriter 8y ago
> If you stand on the balloon it doesn't make sense to ask "where is the balloon expanding from?". Sure it does, and there is a well-defined answer, it's just the answer does not lie on the surface of the balloon. (And for a perfectly spherical balloon, is equidistant from every point on the surface.)
- danbruc 8y ago
  That of course only works because the surfaces of balloons we are used to are always embedded in spacetime. There is however no [good][known] reason to assume that spacetime itself is embedded in any space.
  - OscarCunningham 8y ago
    I've always thought this would be an interesting experiment. Assume that spacetime does live in a higher dimensional space, and try to arrange masses so that spacetime is forced to intersect itself. See if anything interesting happens. Of course this probably requires some stellar engineering.