6 ms·

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 fa

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?
    - ddevault 6y ago
      >And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule? The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.
      - thaumasiotes 6y ago
        But the scale of quarks increases just as much as every other scale, doesn't it? Why did the iron rod shrink while space grew? Why are the iron atoms closer together now than they were before? Why are the protons in each iron atom closer together now than they were before?
        ddevault 6y ago
        The iron rod didn't shrink, it's more like it just held together. If I have a rubber sheet with a ball on it, and I pull at the ends of the rubber sheet, it's not going to pull the ball apart. The binding force of the ball's constituent atoms is sufficient to overcome the pulling force. And in the case of dark energy, the pulling force is absoultely miniscule at those scales.
        thaumasiotes 6y ago
        The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?
        ddevault 6y ago
        >The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink? In the sense that it did not become 0.952 meters long. It didn't "become" anything - it did not change at all. The universe grew around it, or perhaps "underneath" it. More space appeared for it to exist in. It's like putting an iron rod in a 10 meter square house, then moving the walls out another meter. Edit: let me phrase this another way. Let's imagine that you and I were pulling on the bar at either end. We could pull it forever and ever and it would never come apart (unless it rusted and became brittle or something, but let's imagine it doesn't). That's because we aren't putting in enough force to overcome the forces which are holding it together. Well, the expansion of the universe is putting like 10^-20 times less force into it than our arms would be.
- ddevault 6y ago
  I didn't really understand mekkkkkk's question, but I wrote up a longer reply here at your prompting: https://news.ycombinator.com/item?id=24424626 https://news.ycombinator.com/item?id=24424626
  - joshspankit 6y ago
    Thank you. If I understand what you’ve written already, the essence of the answer in terms of mekkkkkk’s question (for reference: “ 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.”), I think the answer would be: Anything held together (for example by gravity, or the force that keeps out atoms together) is not growing along with the universe simply because those forces are (for now) stronger than the expansion. This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us?
    - ddevault 6y ago
      >This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us? Yep.
      - joshspankit 6y ago
        (thinking emoji) (thinking emoji) (thinking emoji) (mindblown emoji)
- noizejoy 6y ago
  I thought it was explained. Local gravity is moving things together much faster than general expansion is moving them apart. At super super long distances, gravity is weaker than expansion, so something very far is disappearing while something nearby may be collapsing. However, that questions a different future I’ve seen in popular science talks: That the future becomes dark because of expansion. That makes less sense to me now, since it would seem that a merged set of galaxies would keep plenty of local stars in the skies. Assuming that stars are born as fast as they die (but I don’t recall seeing an answer to the ratio of star births vs star deaths at a local level).
  - ddevault 6y ago
    The rate of expansion is speeding up. If it continues to increase, eventually it'll have observable effects at interstellar scales, and even interplanetary scales, and ultimately tear everything apart in the "big rip". Note that I'm omitting a lot of context here, there's a lot of other factors which make the big rip scenario much less likely. Also, the age of star formation will eventually end. Stars will stop forming somewhere between 1 trillion and 100 trillion years in the future.
iwonderwhy 6y ago
Very interesting. So the I wonder if everything expands along with the universe, does it mean that from outside, things moving at the speed of light in our universe, are potentially moving at speeds greater than the speed of light within our universe? If i was watching from outside, a rectangle moving at the speed of light in our universe, would one of its edges be moving away from me at the speed of light + the speed of expansion? If it makes sense.
- ddevault 6y ago
  That question is too abstract for me to reason about. A "rectangle" couldn't move at the speed of light; light is point-like (oh dear, saying that could get me in trouble). Anything with mass cannot move at the speed of light, and anything without mass must move at the speed of light, and all known objects without mass are point-like. And then there's the question of observation in the first place - observation itself is governed by fundamental particles, and observing a fundamental particle at its scale is theoretically impossible. I know you want me to cheat and set aside all of these complications and just imagine a rectangle moving at the speed of light, but such an answer would be nonsensical. But the answer to the more specific question, "does the expansion of space make something move faster than the speed of light?", is "no", but it does cause the distance between objects to increase faster than the speed of light. To an external observer (which is inherently difficult to reason about, for the record), these objects would appear to "move" away from one another faster than lightspeed. However, the very idea of "movement" is rooted in the context of spacetime, and is bounded by the speed of light, so it's a confusing/nonsensical question.
  - saagarjha 6y ago
    See also the observable universe: https://en.wikipedia.org/wiki/Observable_universe https://en.wikipedia.org/wiki/Observable_universe. As the universe's expansion speeds up, we will never receive further information from some parts of the universe as it "moves away from us" faster than the speed of light.
mensetmanusman 6y ago
Do particles in this model re-adjust their spacing to the new space being created? e.g. after 10^18 seconds, would the diameter of a hydrogen atom still be the same? or is it scaled to be huge (relative to now) ?
- ddevault 6y ago
  Space is growing, not the matter within it. The diameter of a hydrogen atom is dominated by the fundamental forces, and the expansion of space is insufficient to overcome this force.
  - mensetmanusman 6y ago
    so shouldn’t this fine-scale readjustment of the electron/proton distance be measurable? Maybe this is the ‘wind’ preventing the electron from reaching the nucleus :)
perl4ever 6y ago
>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 This sounds like a measurable (in principle) pressure. But you say it cannot be locally observed. So how do we actually know it is happening? I have heard for many, many years that "space itself" is expanding, but never got a sense of what that meant from all the explanations. It's always sounded to me like doubletalk. Probably because I'm dumb. What I have heard about observations of the expansion of the universe is that objects appear to be moving away, and I've never been able to understand how we know it is not "ordinary" motion.
- ddevault 6y ago
  The motion is consistent with an expanding universe. It gets exponentially greater the further away you go, and everything is moving away from everything else - not away from some "center" position. Also, some of this movement is faster than the speed of light, and the expanding of space explanation is consistent with our understanding of physics whereas proper motion is not consistent with our understanding of physics.
  - perl4ever 6y ago
    Ok, so given a bunch of objects, there are some configurations of velocities that prove different physics are at work, than other configurations. This just sounds weird, like I thought that Newtonian or Einsteinian physics explained objects with velocities regardless of what those velocities are. I thought there were somehow complete, you know? I don't really understand any of it, but I read a popular account of relativity by Einstein, and you know, the examples were very 100 years ago, trains, light, geometry and stuff. No magic. Space literally expanding seems like this star trek thing jumping out of nowhere, and it's confusing to me where it comes from, even though I appreciate that it's orthodoxy nowadays. Maybe the point is the magic has something to do with general relativity?

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?
  - ddevault 6y ago
    >And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule? The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.
    - thaumasiotes 6y ago
      But the scale of quarks increases just as much as every other scale, doesn't it? Why did the iron rod shrink while space grew? Why are the iron atoms closer together now than they were before? Why are the protons in each iron atom closer together now than they were before?
      - ddevault 6y ago
        The iron rod didn't shrink, it's more like it just held together. If I have a rubber sheet with a ball on it, and I pull at the ends of the rubber sheet, it's not going to pull the ball apart. The binding force of the ball's constituent atoms is sufficient to overcome the pulling force. And in the case of dark energy, the pulling force is absoultely miniscule at those scales.
        thaumasiotes 6y ago
        The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?
        ddevault 6y ago
        >The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink? In the sense that it did not become 0.952 meters long. It didn't "become" anything - it did not change at all. The universe grew around it, or perhaps "underneath" it. More space appeared for it to exist in. It's like putting an iron rod in a 10 meter square house, then moving the walls out another meter. Edit: let me phrase this another way. Let's imagine that you and I were pulling on the bar at either end. We could pull it forever and ever and it would never come apart (unless it rusted and became brittle or something, but let's imagine it doesn't). That's because we aren't putting in enough force to overcome the forces which are holding it together. Well, the expansion of the universe is putting like 10^-20 times less force into it than our arms would be.
ddevault 6y ago
I didn't really understand mekkkkkk's question, but I wrote up a longer reply here at your prompting: https://news.ycombinator.com/item?id=24424626 https://news.ycombinator.com/item?id=24424626
- joshspankit 6y ago
  Thank you. If I understand what you’ve written already, the essence of the answer in terms of mekkkkkk’s question (for reference: “ 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.”), I think the answer would be: Anything held together (for example by gravity, or the force that keeps out atoms together) is not growing along with the universe simply because those forces are (for now) stronger than the expansion. This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us?
  - ddevault 6y ago
    >This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us? Yep.
    - joshspankit 6y ago
      (thinking emoji) (thinking emoji) (thinking emoji) (mindblown emoji)
noizejoy 6y ago
I thought it was explained. Local gravity is moving things together much faster than general expansion is moving them apart. At super super long distances, gravity is weaker than expansion, so something very far is disappearing while something nearby may be collapsing. However, that questions a different future I’ve seen in popular science talks: That the future becomes dark because of expansion. That makes less sense to me now, since it would seem that a merged set of galaxies would keep plenty of local stars in the skies. Assuming that stars are born as fast as they die (but I don’t recall seeing an answer to the ratio of star births vs star deaths at a local level).
- ddevault 6y ago
  The rate of expansion is speeding up. If it continues to increase, eventually it'll have observable effects at interstellar scales, and even interplanetary scales, and ultimately tear everything apart in the "big rip". Note that I'm omitting a lot of context here, there's a lot of other factors which make the big rip scenario much less likely. Also, the age of star formation will eventually end. Stars will stop forming somewhere between 1 trillion and 100 trillion years in the future.

iwonderwhy 6y ago

Very interesting. So the I wonder if everything expands along with the universe, does it mean that from outside, things moving at the speed of light in our universe, are potentially moving at speeds greater than the speed of light within our universe? If i was watching from outside, a rectangle moving at the speed of light in our universe, would one of its edges be moving away from me at the speed of light + the speed of expansion? If it makes sense.

ddevault 6y ago
That question is too abstract for me to reason about. A "rectangle" couldn't move at the speed of light; light is point-like (oh dear, saying that could get me in trouble). Anything with mass cannot move at the speed of light, and anything without mass must move at the speed of light, and all known objects without mass are point-like. And then there's the question of observation in the first place - observation itself is governed by fundamental particles, and observing a fundamental particle at its scale is theoretically impossible. I know you want me to cheat and set aside all of these complications and just imagine a rectangle moving at the speed of light, but such an answer would be nonsensical. But the answer to the more specific question, "does the expansion of space make something move faster than the speed of light?", is "no", but it does cause the distance between objects to increase faster than the speed of light. To an external observer (which is inherently difficult to reason about, for the record), these objects would appear to "move" away from one another faster than lightspeed. However, the very idea of "movement" is rooted in the context of spacetime, and is bounded by the speed of light, so it's a confusing/nonsensical question.
- saagarjha 6y ago
  See also the observable universe: https://en.wikipedia.org/wiki/Observable_universe https://en.wikipedia.org/wiki/Observable_universe. As the universe's expansion speeds up, we will never receive further information from some parts of the universe as it "moves away from us" faster than the speed of light.

mensetmanusman 6y ago

Do particles in this model re-adjust their spacing to the new space being created? e.g. after 10^18 seconds, would the diameter of a hydrogen atom still be the same? or is it scaled to be huge (relative to now) ?

ddevault 6y ago
Space is growing, not the matter within it. The diameter of a hydrogen atom is dominated by the fundamental forces, and the expansion of space is insufficient to overcome this force.
- mensetmanusman 6y ago
  so shouldn’t this fine-scale readjustment of the electron/proton distance be measurable? Maybe this is the ‘wind’ preventing the electron from reaching the nucleus :)

perl4ever 6y ago

>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 This sounds like a measurable (in principle) pressure. But you say it cannot be locally observed. So how do we actually know it is happening? I have heard for many, many years that "space itself" is expanding, but never got a sense of what that meant from all the explanations. It's always sounded to me like doubletalk. Probably because I'm dumb. What I have heard about observations of the expansion of the universe is that objects appear to be moving away, and I've never been able to understand how we know it is not "ordinary" motion.

ddevault 6y ago
The motion is consistent with an expanding universe. It gets exponentially greater the further away you go, and everything is moving away from everything else - not away from some "center" position. Also, some of this movement is faster than the speed of light, and the expanding of space explanation is consistent with our understanding of physics whereas proper motion is not consistent with our understanding of physics.
- perl4ever 6y ago
  Ok, so given a bunch of objects, there are some configurations of velocities that prove different physics are at work, than other configurations. This just sounds weird, like I thought that Newtonian or Einsteinian physics explained objects with velocities regardless of what those velocities are. I thought there were somehow complete, you know? I don't really understand any of it, but I read a popular account of relativity by Einstein, and you know, the examples were very 100 years ago, trains, light, geometry and stuff. No magic. Space literally expanding seems like this star trek thing jumping out of nowhere, and it's confusing to me where it comes from, even though I appreciate that it's orthodoxy nowadays. Maybe the point is the magic has something to do with general relativity?