5 ms·

Replying to the other replies here - this regards the observable universe. Speed of light limits and all that. Of course we have no reason to believe the univer

by causal 2y ago

Replying to the other replies here - this regards the observable universe. Speed of light limits and all that. Of course we have no reason to believe the universe just stops at the point where we happen to lack the ability to observe.

sandworm101 2y ago
Well, no. The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe. The size of that universe is extrapolated from the rate of expansion and the time since the big bang. The size and shape of the observable universe also changes. A moving observer, say someone doing 30% of lightspeed, will see further in one direction than another. Accelerate quickly enough and the "dark" side of your custom observable universe might catch up with you, causing all sorts of havoc. https://en.wikipedia.org/wiki/Unruh_effect https://en.wikipedia.org/wiki/Unruh_effect
- layer8 2y ago
  As far as we know, the total universe may have infinite size, and thus contain infinitely many galaxies.
  - an-honest-moose 2y ago
    That doesn't necessarily follow - the universe can be infinite in size, but contain a finite amount of matter.
    - layer8 2y ago
      True, I was keeping the reasoning about the average density. A homogeneous universe is still the null hypothesis.
    - pdonis 2y ago
      Not the universe we observe, no. There is no valid model in GR that has this property and matches our observations of the universe as a whole. Models with a finite amount of matter surrounded by an infinite region of vacuum exist in GR, but they are not homogeneous and isotropic on large scales, while our observations indicate that our universe is.
- nilkn 2y ago
  You’re assuming that space was compressed into a single point at the Big Bang. However, this is not implied by the Big Bang or cosmology. All we can truly infer is that the universe was very hot and dense and that spacetime experienced rapid expansion. We do not know the size, extent, or shape of space at that time, and we don’t even know how much matter and energy were present. We only have a notion of the density.
  - causal 2y ago
    Yeah this is a difficult concept, and I think the way the big bang is commonly portrayed in media often leads to this misconception of the big bang as starting at a point in space rather than a density. I uncovered this for myself when asking, "where is that point now?" and discovering it was never a point at all, space is expanding from all points simultaneously.
    - sandworm101 2y ago
      The easy answer to the hard concept is that the big bang is not the increase in size of a thing. It is an increase in dimensions, including time. Our notions of size, of dimension, might not exist outside the bubble. We would therefore never perceive an edge, but that doesn't mean that one does not exist nor that there may be a finite size.
      - Baeocystin 2y ago
        I explain it to folks as if one was trying to go further south than the south pole. There's nothing physically impeding you; it's simply that once on the pole, all directions are north.
      - regularfry 2y ago
        Even that's not especially easy, because you then need to deal with "if the dimensions themselves are changing, why aren't protons the size of planets?"
  - sandworm101 2y ago
- pdonis 2y ago
  > The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe. As has already been pointed out, our best current model of our universe is that it is spatially infinite. That means an infinite number of galaxies. The finite galaxy numbers that astronomers give are for the observable universe. > The size and shape of the observable universe also changes. Not the way you are describing, no. The observable universe does increase in size as time goes on, because there is more time for light to travel so the light we see can come from objects further distant. Its shape, however, does not change. A good reference is Davis & Lineweaver's 2003 paper: https://arxiv.org/abs/astro-ph/0310808 https://arxiv.org/abs/astro-ph/0310808 > A moving observer, say someone doing 30% of lightspeed, will see further in one direction than another. I don't know where you're getting this from. What part of the universe you can observe from a given point does not depend on your state of motion. > Accelerate quickly enough and the "dark" side of your custom observable universe might catch up with you, causing all sorts of havoc. This is nonsense. The Unruh effect is (a) nothing like what you are describing, and (b) irrelevant to this discussion anyway, since the Unruh effect only applies to objects which have nonzero proper acceleration, which is not the case for any galaxies, stars, or planets in the universe.
- Nevermark 2y ago
  > The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe. The unobserved universe is likely to be many orders of magnitude larger than the observed universe. It is possible that it is unimaginably larger. Technically, it is possible that the unobserved universe is infinite, however whether that is a credible option depends on individual scientists informed intuitions. We simply have no experimental or theoretical evidence either way at this point. So there is no estimate of how many galaxies there are in the universe in toto.

by causal 2y ago

sandworm101 2y ago

Well, no. The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe. The size of that universe is extrapolated from the rate of expansion and the time since the big bang. The size and shape of the observable universe also changes. A moving observer, say someone doing 30% of lightspeed, will see further in one direction than another. Accelerate quickly enough and the "dark" side of your custom observable universe might catch up with you, causing all sorts of havoc. https://en.wikipedia.org/wiki/Unruh_effect https://en.wikipedia.org/wiki/Unruh_effect

layer8 2y ago
As far as we know, the total universe may have infinite size, and thus contain infinitely many galaxies.
- an-honest-moose 2y ago
  That doesn't necessarily follow - the universe can be infinite in size, but contain a finite amount of matter.
  - layer8 2y ago
    True, I was keeping the reasoning about the average density. A homogeneous universe is still the null hypothesis.
  - pdonis 2y ago
    Not the universe we observe, no. There is no valid model in GR that has this property and matches our observations of the universe as a whole. Models with a finite amount of matter surrounded by an infinite region of vacuum exist in GR, but they are not homogeneous and isotropic on large scales, while our observations indicate that our universe is.
nilkn 2y ago
You’re assuming that space was compressed into a single point at the Big Bang. However, this is not implied by the Big Bang or cosmology. All we can truly infer is that the universe was very hot and dense and that spacetime experienced rapid expansion. We do not know the size, extent, or shape of space at that time, and we don’t even know how much matter and energy were present. We only have a notion of the density.
- causal 2y ago
  Yeah this is a difficult concept, and I think the way the big bang is commonly portrayed in media often leads to this misconception of the big bang as starting at a point in space rather than a density. I uncovered this for myself when asking, "where is that point now?" and discovering it was never a point at all, space is expanding from all points simultaneously.
  - sandworm101 2y ago
    The easy answer to the hard concept is that the big bang is not the increase in size of a thing. It is an increase in dimensions, including time. Our notions of size, of dimension, might not exist outside the bubble. We would therefore never perceive an edge, but that doesn't mean that one does not exist nor that there may be a finite size.
    - Baeocystin 2y ago
      I explain it to folks as if one was trying to go further south than the south pole. There's nothing physically impeding you; it's simply that once on the pole, all directions are north.
    - regularfry 2y ago
      Even that's not especially easy, because you then need to deal with "if the dimensions themselves are changing, why aren't protons the size of planets?"
- sandworm101 2y ago
pdonis 2y ago
> The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe. As has already been pointed out, our best current model of our universe is that it is spatially infinite. That means an infinite number of galaxies. The finite galaxy numbers that astronomers give are for the observable universe. > The size and shape of the observable universe also changes. Not the way you are describing, no. The observable universe does increase in size as time goes on, because there is more time for light to travel so the light we see can come from objects further distant. Its shape, however, does not change. A good reference is Davis & Lineweaver's 2003 paper: https://arxiv.org/abs/astro-ph/0310808 https://arxiv.org/abs/astro-ph/0310808 > A moving observer, say someone doing 30% of lightspeed, will see further in one direction than another. I don't know where you're getting this from. What part of the universe you can observe from a given point does not depend on your state of motion. > Accelerate quickly enough and the "dark" side of your custom observable universe might catch up with you, causing all sorts of havoc. This is nonsense. The Unruh effect is (a) nothing like what you are describing, and (b) irrelevant to this discussion anyway, since the Unruh effect only applies to objects which have nonzero proper acceleration, which is not the case for any galaxies, stars, or planets in the universe.
Nevermark 2y ago
> The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe. The unobserved universe is likely to be many orders of magnitude larger than the observed universe. It is possible that it is unimaginably larger. Technically, it is possible that the unobserved universe is infinite, however whether that is a credible option depends on individual scientists informed intuitions. We simply have no experimental or theoretical evidence either way at this point. So there is no estimate of how many galaxies there are in the universe in toto.