5 ms·

Interesting, thanks! Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same di

Interesting, thanks! Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?

dmd 3y ago
You would have to come up with an explanation for such drifting to be occurring across the entire visible universe in precisely the same way, across billions and billions of light years.
- deleted 3y ago
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- xeonmc 3y ago
  Just a fringe speculation without any physical basis: maybe the Big Bang emits a 4-dimensional spherical shell of matter at c? With the direction in which we are traveling at c called “time”, and the other three which we're not moving through called “space”; diverging rays having spatial velocity relative to each other equal to tan(alpha) i.e. flat projection; and at any given point on the 4-sphere's 3D surface everything appear to be diverging in 3D because the emission is spherically symmetric; and trajectories sufficiently paraxial to us fits the small angle approximation but those diverging widely has much more apparent perspective distortion, with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) = 1/Lorentz_Factor?
  - dmd 3y ago
    [flagged]
    - deleted 3y ago
      [deleted]
    - dvsfish 3y ago
      While its probably totally wrong, I think your tone and general attitude to people throwing around imaginative and creative ideas is extremely unproductive and frankly rude. If you want to stifle creative discussions, this is exactly how you'd do it. I found the post to be very stimulating regardless of its validity. And it's not like they're trying to spread it as gospel. There was a lot of disclaimer.
      - mrguyorama 3y ago
        If you want creativity, take an art class. This is cosmology, ostensibly a science. Pulling wild ideas out of thin air and basically ad-libbing a "theory" is not.
        dvsfish 3y ago
        So we have to put everyone who dares to imagine down about it? I see value in sharing a wild idea purely for the inspiration it may give others to think about things from a unique perspective. A lot of great science started as a strange idea.
        mrguyorama 3y ago
        What's the value in """daring to imagine""" about cosmology? This is nothing more than reality based fan fiction. >A lot of great science started as a strange idea. Disagree. A lot of great science started with "This weird math I'm doing is the only way to fit this data", which is an extremely different thing. Do you not think physicists and cosmologists and their nerdy friends have thought of wild ideas? Do you not think undergrad students spend drunken friday nights theorycrafting universes? It's pretty disrespectful to "why don't you just" a profession without showing that you at least did your homework first, or that you have looked at whoever definitely asked that question before you. There are people in this post reinventing Tired light and Aether based theories for crying out loud, theories with fairly conclusive reasons we abandoned them. When you just ass-pull theorycrafting without any of the fundamentals, all you do is talk in circles. It's no better than all the dumb "the brain is like a computer" arguments by analogy in neuroscience posts. It's not even wrong. We live in a world where the people who had to cheat off me in high school biology angrily insist that the vaccine doesn't work. Do we really benefit from playing such anti-scientific games?
    - root_axis 3y ago
      I think if you're going to dismiss the comment in such a flippant and condescending manner you should at least explain what's wrong with it.
      - dmd 3y ago
        How does what they said differ from simply saying the universe is expanding - but in a word-salady way that obfuscates it?
  - jabits 3y ago
    This is a really interesting thought. I will be noodling on this tonight. Thanks
    - deleted 3y ago
      [deleted]
arandomusername 3y ago
I think we observed some of the galaxies moving away from us at rates faster than speed of light, multiple times faster, which wouldn't work with objects drifting away if we hold by that speed of light is max speed matter can move at.
- brabel 3y ago
  That's physically impossible. You can't observe something that's beyond your "light cone", as any galaxy "moving" (it's not really moving, it's the space that's getting expanding) faster than light would be. What you're referring to is the fact that we can confidently predict that galaxies at the edge of the observable universe, which we currently see moving away from us really fast, but as they were in the distant past due to their light taking billions of years to arrive at us, are currently, if we could actually see them where they are right now (again: we can't), "moving" faster than light away from us. Once a galaxy has moved beyond or "light cone", it's lost forever: you won't see it again even if you try moving towards it at light speed for all eternity.
  - cygx 3y ago
    The Hubble sphere (the place where recession velocities hit the speed of light) is not the same as the particle horizon (our past lightcone at current cosmological time, the boundary of the observable universe) or the cosmic event horizon (our past lightcone at infinite cosmological time, the boundary of the asymptotically observable universe). Cf the last paragraph of https://en.wikipedia.org/wiki/Hubble_volume https://en.wikipedia.org/wiki/Hubble_volume Observations indicate that the expansion of the universe is accelerating, and the Hubble constant is thought to be decreasing. Thus, sources of light outside the Hubble horizon but inside the cosmological event horizon can eventually reach us. A fairly counter-intuitive result is that photons we observe from the first ~5 billion years of the universe come from regions that are, and always have been, receding from us at superluminal speeds.
  - tzs 3y ago
    It's a bit more complicated than that. The PBS Space Time episode "How Much Of The Universe Can Humanity Ever See?" [1] might be interesting to you. The next episode, "How Far Beyond Earth Could Humanity Spread?" [2] might also be interesting. [1] https://www.youtube.com/watch?v=eVoh27gJgME https://www.youtube.com/watch?v=eVoh27gJgME [2] https://www.youtube.com/watch?v=R-eIUxwcQo4 https://www.youtube.com/watch?v=R-eIUxwcQo4
mr_mitm 3y ago
Yes, the "drifting" component you speak of is called the peculiar velocity. It's responsible for the "finger of god" effect if we don't account for it (galaxies appear to be aligned along the line of sight of the observer) and becomes negligible at large enough distances. I guess technically it's hard to differentiate because it's usually random, but it enters the error analysis and is accounted for.

Interesting, thanks! Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?

dmd 3y ago

You would have to come up with an explanation for such drifting to be occurring across the entire visible universe in precisely the same way, across billions and billions of light years.

deleted 3y ago
[deleted]
xeonmc 3y ago
Just a fringe speculation without any physical basis: maybe the Big Bang emits a 4-dimensional spherical shell of matter at c? With the direction in which we are traveling at c called “time”, and the other three which we're not moving through called “space”; diverging rays having spatial velocity relative to each other equal to tan(alpha) i.e. flat projection; and at any given point on the 4-sphere's 3D surface everything appear to be diverging in 3D because the emission is spherically symmetric; and trajectories sufficiently paraxial to us fits the small angle approximation but those diverging widely has much more apparent perspective distortion, with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) = 1/Lorentz_Factor?
- dmd 3y ago
  [flagged]
  - deleted 3y ago
    [deleted]
  - dvsfish 3y ago
    While its probably totally wrong, I think your tone and general attitude to people throwing around imaginative and creative ideas is extremely unproductive and frankly rude. If you want to stifle creative discussions, this is exactly how you'd do it. I found the post to be very stimulating regardless of its validity. And it's not like they're trying to spread it as gospel. There was a lot of disclaimer.
    - mrguyorama 3y ago
      If you want creativity, take an art class. This is cosmology, ostensibly a science. Pulling wild ideas out of thin air and basically ad-libbing a "theory" is not.
      - dvsfish 3y ago
        So we have to put everyone who dares to imagine down about it? I see value in sharing a wild idea purely for the inspiration it may give others to think about things from a unique perspective. A lot of great science started as a strange idea.
        mrguyorama 3y ago
        What's the value in """daring to imagine""" about cosmology? This is nothing more than reality based fan fiction. >A lot of great science started as a strange idea. Disagree. A lot of great science started with "This weird math I'm doing is the only way to fit this data", which is an extremely different thing. Do you not think physicists and cosmologists and their nerdy friends have thought of wild ideas? Do you not think undergrad students spend drunken friday nights theorycrafting universes? It's pretty disrespectful to "why don't you just" a profession without showing that you at least did your homework first, or that you have looked at whoever definitely asked that question before you. There are people in this post reinventing Tired light and Aether based theories for crying out loud, theories with fairly conclusive reasons we abandoned them. When you just ass-pull theorycrafting without any of the fundamentals, all you do is talk in circles. It's no better than all the dumb "the brain is like a computer" arguments by analogy in neuroscience posts. It's not even wrong. We live in a world where the people who had to cheat off me in high school biology angrily insist that the vaccine doesn't work. Do we really benefit from playing such anti-scientific games?
  - root_axis 3y ago
    I think if you're going to dismiss the comment in such a flippant and condescending manner you should at least explain what's wrong with it.
    - dmd 3y ago
      How does what they said differ from simply saying the universe is expanding - but in a word-salady way that obfuscates it?
- jabits 3y ago
  This is a really interesting thought. I will be noodling on this tonight. Thanks
  - deleted 3y ago
    [deleted]

arandomusername 3y ago

I think we observed some of the galaxies moving away from us at rates faster than speed of light, multiple times faster, which wouldn't work with objects drifting away if we hold by that speed of light is max speed matter can move at.

brabel 3y ago
That's physically impossible. You can't observe something that's beyond your "light cone", as any galaxy "moving" (it's not really moving, it's the space that's getting expanding) faster than light would be. What you're referring to is the fact that we can confidently predict that galaxies at the edge of the observable universe, which we currently see moving away from us really fast, but as they were in the distant past due to their light taking billions of years to arrive at us, are currently, if we could actually see them where they are right now (again: we can't), "moving" faster than light away from us. Once a galaxy has moved beyond or "light cone", it's lost forever: you won't see it again even if you try moving towards it at light speed for all eternity.
- cygx 3y ago
  The Hubble sphere (the place where recession velocities hit the speed of light) is not the same as the particle horizon (our past lightcone at current cosmological time, the boundary of the observable universe) or the cosmic event horizon (our past lightcone at infinite cosmological time, the boundary of the asymptotically observable universe). Cf the last paragraph of https://en.wikipedia.org/wiki/Hubble_volume https://en.wikipedia.org/wiki/Hubble_volume Observations indicate that the expansion of the universe is accelerating, and the Hubble constant is thought to be decreasing. Thus, sources of light outside the Hubble horizon but inside the cosmological event horizon can eventually reach us. A fairly counter-intuitive result is that photons we observe from the first ~5 billion years of the universe come from regions that are, and always have been, receding from us at superluminal speeds.
- tzs 3y ago
  It's a bit more complicated than that. The PBS Space Time episode "How Much Of The Universe Can Humanity Ever See?" [1] might be interesting to you. The next episode, "How Far Beyond Earth Could Humanity Spread?" [2] might also be interesting. [1] https://www.youtube.com/watch?v=eVoh27gJgME https://www.youtube.com/watch?v=eVoh27gJgME [2] https://www.youtube.com/watch?v=R-eIUxwcQo4 https://www.youtube.com/watch?v=R-eIUxwcQo4

mr_mitm 3y ago

Yes, the "drifting" component you speak of is called the peculiar velocity. It's responsible for the "finger of god" effect if we don't account for it (galaxies appear to be aligned along the line of sight of the observer) and becomes negligible at large enough distances. I guess technically it's hard to differentiate because it's usually random, but it enters the error analysis and is accounted for.