12 ms·

> The Universe is expanding at > every known point at the same > time at the same rate This part I don't get (I am in no way versed in astrophysics). If I am

by dieppe 7y ago

> The Universe is expanding at > every known point at the same > time at the same rate This part I don't get (I am in no way versed in astrophysics). If I am in a box, and that the box grows 10%, but that at the same time I grow 10%, what has changed? I mean, if every point in the universe expands at the same rate at the same time, what does it change? Relatively speaking, everything is the same size before and after expanding right? It feels a bit like expanding a 1 meter line and expanding the meter standard the same rate as the line. If anyone has an explanation and is willing to spend some time writing it down, I would be most grateful :) This itches my brain ^^

cjonas 7y ago
My (limited) understanding is that only space is expanding. Iow there is more space but the space that matter takes up stays constant. Maybe I'm also confused though
- simcop2387 7y ago
  That's basically my lay-person's understanding also. It's because the expansion isn't significant at small scales, so gravity and electromagnetism overpower it. This causes existing objects and clouds to effectively remain the same size [I have to assume that there's an effect, stealing potential energy at very small scales].
  - shpeak 7y ago
    Gravity and EM are holding our galaxy, our Laniakea, our Universe. See https://www.youtube.com/watch?v=rENyyRwxpHo https://www.youtube.com/watch?v=rENyyRwxpHo .
- adrianN 7y ago
  As I understand it, the forces that keep matter together are strong enough to counteract any expansion.
  - Koshkin 7y ago
    Unlikely, because expansion of space is equivalent to the slowing of light, as other commenters have mentioned.
    - cygx 7y ago
      Such heuristic explanations are distractions: Locally, gravitational effects (including spatial expansion) manifest as pseudo-forces and hence can be counteracted.
Hendrikto 7y ago
Your interpretation of the expansion is incorrect. Basically, it looks like all points are moving away from each other over large distances. It is easier to understand in 2D: Imagine a deflated balloon with two dots drawn on it. When you inflate the balloon, its 2D surface increases, which results in the two points "moving away" from each other, without either of them actually moving. It‘s just that the surface on which they lie grows. This is also the reason why distant galaxies are getting more distant faster than the speed of light: They are not actually moving, but the space in between us and them grows. Notice that the farther two points on the surface of the balloon are apart, the faster they will appear to be moving away from each other, which is why the effect is unnoticeable over small (in astronomical terms) distances. Gravity pulls things together much faster than space expansion pulls them apart. Bringing this back to your example: Imagine "blowing up" the box. Things inside it are growing a little bit (but gravity pulls them back together), but everything seems to be moving away from everything else.
- JackFr 7y ago
  What's still not clear is how I know the other point on the surface of the ballon is moving away from me? I measure it as 9 tick marks away. After inflation it's still nine tick marks. That is to say, what is my ruler that is untethered to the surface of the balloon?
  - Tenemo 7y ago
    Your ruler is absolute distances, like the speed of light. You can inflate or deflate the balloon and the number of marks on its surface won't change. But the time required for the light to travel between them will.
    - zaroth 7y ago
      Stupid question - How do you know for certain it isn’t just that light is slowing down? Relativity probably doesn’t work this way at all, but if light did actually slow down, would it be perceived by us as light moving at the same speed across a greater distance?
roywiggins 7y ago
So, imagine the box is filled with hydrogen gas. The space inside the box gets bigger. But protons don't, they're little packets of quarks that hold each other very powerfully. You'd have to expand space very, very, very fast to rip those quarks apart from each other, and once you did, you would have a bunch of bare quarks, not hydrogen. The quarks aren't getting bigger, since they appear to be indivisible and essentially pointlike, and even if they were made of something smaller, those things will hold tight to each other, too. A nice gentle expansion will just expand the space in between the protons. This scales up to forces in between atoms, molecules, and bits of matter. It's only at the very large scales (larger than galaxies) that all the forces that are holding the stuff together starts getting overwhelmed by the expansion of space and it starts to pull things apart. The expansion is so gentle that even relatively weak gravity can hold galaxy clusters together even as space grows. In other words: space gets bigger, but matter doesn't. It more or less stays the same size.
- galaxyLogic 7y ago
  Well so in other words "space" is NOT expanding. It's particles of matter that are moving away from each other. Right? So fine, but then why would we say that space is expanding, that would seem to imply we know there is some edge of space which is moving away from us, but what evidence do we have that space has such an edge? Saying that "space is expanding" seems to hold the assumption that space has a (limited) width and height.
  - cygx 7y ago
    Specific volume is increasing. If space was finite (but still without boundary - think eg surface of a sphere), so would total volume.
  - Certhas 7y ago
    No space really is expanding. Think of it this way: You have a rubber band and you pull it apart, if you place two beads on the rubber band the distance between the beads will increase (even though they don't move relative to the rubber). But if you connect the two beads by a strong spring, then their distance will stay the same, despite space expanding. The force of the spring makes them move (relative to the rubber), counteracting the expansion of space between them.
mcv 7y ago
This makes me wonder whether we would be able to tell the difference between the universe getting larger and matter in the universe getting smaller.
- gus_massa 7y ago
  Someone has published that idea a few year ago https://news.ycombinator.com/item?id=19588996 https://news.ycombinator.com/item?id=19588996 I think it's more a mathematical trick than something deep about the universe. If you tweak the change of the universal "constants" correctly in a universe that doesn't expand, it is equivalent to a universe that expands where the "constants" don't change. Note that there is some research about the change of the universal constant, but in most models they don't have just the rights values to make the universe look like expanding. https://en.wikipedia.org/wiki/Time-variation_of_fundamental_constants https://en.wikipedia.org/wiki/Time-variation_of_fundamental_...
wolfram74 7y ago
Think about drawing two circles on a balloon, one with radius 1cm, and the other with radius 2cm. the distance between the two perimeters is 1cm, but if we inflate the balloon so that the surfaces double, now the inner circle has a radius of 2, and the outer circle a radius of 4, the distance between the perimeters has grown along side the rest of all the lengths. That's my best understanding of the situation, I mostly do non-linear classical mechanics.
- shpeak 7y ago
  Who pumping the balloon? Where is source of this energy? Can we use this source of energy for our own needs?
  - orestarod 7y ago
    We do not know why the balloon is inflating. I think "dark energy" is a form of energy assigned for being responsible for it, but other than the name I don't think we have reached conclusions as to what it actually is.
317070 7y ago
It boils down to this: You take a bowling ball, and put it 1 meter stick away from you at zero velocity compared to you. Assume that at this distance, no forces are exchanged between you and the ball. You wait a billion billion gazillion years, take your meter stick again and measure the distance between you and the ball, and notice it is now 2 meters away. It is the distance between you and the ball that has grown. Note that the ball has not grown twice its size. The sizes of atoms and molecules have not changed (as they have mechanisms to keep them exactly the size they are, despite space growing). It is only when there are no mechanisms or forces to keep distances between objects fixed, they seem to drift apart very slowly. The question the article addresses, is whether that growth is accelerating or not.
blechinger 7y ago
I am not a physicist. My understanding is that the fundamental forces are strong enough to resist the local expansion of space. So the box gets bigger but particles do not get ripped apart or themselves expand. Space is expanding. Not matter. The particles occupy the same space and the fundamental forces keep them together. Space just keeps moving and expanding around them. This (coupled with the fact that, at least currently, the expansion is happening very slowly) is why the expansion isn't observable unless you're looking at a very large scale.
yllorepap 7y ago
Imagine space is the surface of a balloon and you are blowing it up. Everything is in the middle of the surface, or at the centre. Everything is expanding. However we can’t appreciate this as at human scale electromagnetic and nuclear forces are dominant.

by dieppe 7y ago

cjonas 7y ago

My (limited) understanding is that only space is expanding. Iow there is more space but the space that matter takes up stays constant. Maybe I'm also confused though

simcop2387 7y ago
That's basically my lay-person's understanding also. It's because the expansion isn't significant at small scales, so gravity and electromagnetism overpower it. This causes existing objects and clouds to effectively remain the same size [I have to assume that there's an effect, stealing potential energy at very small scales].
- shpeak 7y ago
  Gravity and EM are holding our galaxy, our Laniakea, our Universe. See https://www.youtube.com/watch?v=rENyyRwxpHo https://www.youtube.com/watch?v=rENyyRwxpHo .
adrianN 7y ago
As I understand it, the forces that keep matter together are strong enough to counteract any expansion.
- Koshkin 7y ago
  Unlikely, because expansion of space is equivalent to the slowing of light, as other commenters have mentioned.
  - cygx 7y ago
    Such heuristic explanations are distractions: Locally, gravitational effects (including spatial expansion) manifest as pseudo-forces and hence can be counteracted.

Hendrikto 7y ago

Your interpretation of the expansion is incorrect. Basically, it looks like all points are moving away from each other over large distances. It is easier to understand in 2D: Imagine a deflated balloon with two dots drawn on it. When you inflate the balloon, its 2D surface increases, which results in the two points "moving away" from each other, without either of them actually moving. It‘s just that the surface on which they lie grows. This is also the reason why distant galaxies are getting more distant faster than the speed of light: They are not actually moving, but the space in between us and them grows. Notice that the farther two points on the surface of the balloon are apart, the faster they will appear to be moving away from each other, which is why the effect is unnoticeable over small (in astronomical terms) distances. Gravity pulls things together much faster than space expansion pulls them apart. Bringing this back to your example: Imagine "blowing up" the box. Things inside it are growing a little bit (but gravity pulls them back together), but everything seems to be moving away from everything else.

JackFr 7y ago
What's still not clear is how I know the other point on the surface of the ballon is moving away from me? I measure it as 9 tick marks away. After inflation it's still nine tick marks. That is to say, what is my ruler that is untethered to the surface of the balloon?
- Tenemo 7y ago
  Your ruler is absolute distances, like the speed of light. You can inflate or deflate the balloon and the number of marks on its surface won't change. But the time required for the light to travel between them will.
  - zaroth 7y ago
    Stupid question - How do you know for certain it isn’t just that light is slowing down? Relativity probably doesn’t work this way at all, but if light did actually slow down, would it be perceived by us as light moving at the same speed across a greater distance?

roywiggins 7y ago

So, imagine the box is filled with hydrogen gas. The space inside the box gets bigger. But protons don't, they're little packets of quarks that hold each other very powerfully. You'd have to expand space very, very, very fast to rip those quarks apart from each other, and once you did, you would have a bunch of bare quarks, not hydrogen. The quarks aren't getting bigger, since they appear to be indivisible and essentially pointlike, and even if they were made of something smaller, those things will hold tight to each other, too. A nice gentle expansion will just expand the space in between the protons. This scales up to forces in between atoms, molecules, and bits of matter. It's only at the very large scales (larger than galaxies) that all the forces that are holding the stuff together starts getting overwhelmed by the expansion of space and it starts to pull things apart. The expansion is so gentle that even relatively weak gravity can hold galaxy clusters together even as space grows. In other words: space gets bigger, but matter doesn't. It more or less stays the same size.

galaxyLogic 7y ago
Well so in other words "space" is NOT expanding. It's particles of matter that are moving away from each other. Right? So fine, but then why would we say that space is expanding, that would seem to imply we know there is some edge of space which is moving away from us, but what evidence do we have that space has such an edge? Saying that "space is expanding" seems to hold the assumption that space has a (limited) width and height.
- cygx 7y ago
  Specific volume is increasing. If space was finite (but still without boundary - think eg surface of a sphere), so would total volume.
- Certhas 7y ago
  No space really is expanding. Think of it this way: You have a rubber band and you pull it apart, if you place two beads on the rubber band the distance between the beads will increase (even though they don't move relative to the rubber). But if you connect the two beads by a strong spring, then their distance will stay the same, despite space expanding. The force of the spring makes them move (relative to the rubber), counteracting the expansion of space between them.

mcv 7y ago

This makes me wonder whether we would be able to tell the difference between the universe getting larger and matter in the universe getting smaller.

gus_massa 7y ago
Someone has published that idea a few year ago https://news.ycombinator.com/item?id=19588996 https://news.ycombinator.com/item?id=19588996 I think it's more a mathematical trick than something deep about the universe. If you tweak the change of the universal "constants" correctly in a universe that doesn't expand, it is equivalent to a universe that expands where the "constants" don't change. Note that there is some research about the change of the universal constant, but in most models they don't have just the rights values to make the universe look like expanding. https://en.wikipedia.org/wiki/Time-variation_of_fundamental_constants https://en.wikipedia.org/wiki/Time-variation_of_fundamental_...

wolfram74 7y ago

Think about drawing two circles on a balloon, one with radius 1cm, and the other with radius 2cm. the distance between the two perimeters is 1cm, but if we inflate the balloon so that the surfaces double, now the inner circle has a radius of 2, and the outer circle a radius of 4, the distance between the perimeters has grown along side the rest of all the lengths. That's my best understanding of the situation, I mostly do non-linear classical mechanics.

shpeak 7y ago
Who pumping the balloon? Where is source of this energy? Can we use this source of energy for our own needs?
- orestarod 7y ago
  We do not know why the balloon is inflating. I think "dark energy" is a form of energy assigned for being responsible for it, but other than the name I don't think we have reached conclusions as to what it actually is.

317070 7y ago

It boils down to this: You take a bowling ball, and put it 1 meter stick away from you at zero velocity compared to you. Assume that at this distance, no forces are exchanged between you and the ball. You wait a billion billion gazillion years, take your meter stick again and measure the distance between you and the ball, and notice it is now 2 meters away. It is the distance between you and the ball that has grown. Note that the ball has not grown twice its size. The sizes of atoms and molecules have not changed (as they have mechanisms to keep them exactly the size they are, despite space growing). It is only when there are no mechanisms or forces to keep distances between objects fixed, they seem to drift apart very slowly. The question the article addresses, is whether that growth is accelerating or not.

blechinger 7y ago

I am not a physicist. My understanding is that the fundamental forces are strong enough to resist the local expansion of space. So the box gets bigger but particles do not get ripped apart or themselves expand. Space is expanding. Not matter. The particles occupy the same space and the fundamental forces keep them together. Space just keeps moving and expanding around them. This (coupled with the fact that, at least currently, the expansion is happening very slowly) is why the expansion isn't observable unless you're looking at a very large scale.

yllorepap 7y ago

Imagine space is the surface of a balloon and you are blowing it up. Everything is in the middle of the surface, or at the centre. Everything is expanding. However we can’t appreciate this as at human scale electromagnetic and nuclear forces are dominant.