17 ms·

Number of stars in the Milky Way: 100 billion Number of galaxies in the universe: 200 billion to 2,000 billion

by afterburner 2y ago

markus_zhang 2y ago
Is 2,000 billion some theoretical limit or something else? Thanks.
- sandworm101 2y ago
  Yes. Take the age of the universe, multiple by the rate of expansion to get the total size of the universe, then multiple by the average density of galaxies in the observable universe. There are some further complications, but at the root it is basic algebra.
  - 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.
  - worldsayshi 2y ago
    Is that really the way to see it? As I understand it, the Big Bang didn't happen in "one place". The Universe is expanding from an compressed state - the Big Bang state. But there is no center point. We can only see that there's expansion but it's not from a single point. The only known "center point" is us. And the only reason it's a center point is because we can only see as far away as light has traveled since the Big Bang.
    - reactordev 2y ago
      This theory of multiple points supports the big ring and other structures outside the “this shouldn’t exist” bubble. The bubble is the Big Bang + rate of expansion. It was thought that there was nothing outside of the farthest point… but there is!
    - pdonis 2y ago
      > As I understand it, the Big Bang didn't happen in "one place"...there is no center point. That is correct. The only tenable answer to "where did the Big Bang take place" is "everywhere".
  - injidup 2y ago
    So if the universe has a size then what do you see if you are on the edge of it? Do you see stars to the left and nothing to the right? I mean given the speed of light and the age of the universe and the rate of expansion there are regions inaccessible to us but that doesn't quite mean the universe has a finite size.
    - dragonwriter 2y ago
      Finite size doesn’t require an edge. Consider the surface of a balloon for a 2-D case (or the perimeter of a sphere, for a 1-D case): it has finite extent, but no edge.
      - lupire 2y ago
        It has a surface, though, which is what PP was asking about.An answer to the question is, yes, nesr the edge/face, one side is dark. But relativity and expansion makes the situation a bit more complicated.
    - dotnet00 2y ago
      The observable universe has a size, the cosmic microwave background is what we 'see' at the 'edge' in terms of photons (~400k years after the big bang). We could see further if we could map out the gravitational wave or neutrino backgrounds (1 sec after the big bang). But for now we can't really say if the universe in its entirety has a finite size.
      - mvkel 2y ago
        Fascinating. Do you think it's possible that we can map these out in the next 50 years?
        dotnet00 2y ago
        For the gravitational wave background, maybe with LISA we might be able to get a glimpse, but the neutrino background seems like it'd take some truly unprecedented breakthroughs in our ability to detect neutrinos to have any chance of mapping it out.
  - reactordev 2y ago
    What about the big ring [0]? Or other mega structures of galaxies outside that “bubble”? [0] https://en.m.wikipedia.org/wiki/Big_Ring https://en.m.wikipedia.org/wiki/Big_Ring
  - mvkel 2y ago
    Isn't the rate of the expansion of the universe increasing? And that assumes the observable universe is homogeneous, which it isn't
    - pdonis 2y ago
      > Isn't the rate of the expansion of the universe increasing? It is now, but up until a few billion years ago, it wasn't, it was decreasing. Many of the objects we currently see are far enough away that the light we are now seeing from them was emitted while the universe's expansion was still decelerating. > that assumes the observable universe is homogeneous, which it isn't No, the models cosmologists use do not assume the universe is homogeneous period. They only assume it is homogeneous on average, on large distance scales (roughly scales larger than the size of the largest galaxy clusters).
  - glenstein 2y ago
    >Take the age of the universe, multiple by the rate of expansion to get the total size of the universe, then multiple by the average density of galaxies in the observable universe My understanding is that, at the largest scales, clusters of galaxies are organized along a series of gravitationally bound filaments, sometimes called the cosmic web. So they aren't distributed like random noise, but more like a web. I have no reason to think this changes anything about calculating average densities, but it is notable that there's the general density but probably a significantly different density within that structure.
hyperliner 2y ago
[dead]
layer8 2y ago
*observable universe
- afterburner 2y ago
  Yes. I find it amusing I sparked a debate on what might be beyond the observable universe, when my point was entirely about what you could theoretically observe in the night sky.
bufferoverflow 2y ago
100 billion is the low end estimate for our galaxy. 400 billion is the high end.

Number of stars in the Milky Way: 100 billion Number of galaxies in the universe: 200 billion to 2,000 billion

markus_zhang 2y ago

Is 2,000 billion some theoretical limit or something else? Thanks.

sandworm101 2y ago
Yes. Take the age of the universe, multiple by the rate of expansion to get the total size of the universe, then multiple by the average density of galaxies in the observable universe. There are some further complications, but at the root it is basic algebra.
- 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.
- worldsayshi 2y ago
  Is that really the way to see it? As I understand it, the Big Bang didn't happen in "one place". The Universe is expanding from an compressed state - the Big Bang state. But there is no center point. We can only see that there's expansion but it's not from a single point. The only known "center point" is us. And the only reason it's a center point is because we can only see as far away as light has traveled since the Big Bang.
  - reactordev 2y ago
    This theory of multiple points supports the big ring and other structures outside the “this shouldn’t exist” bubble. The bubble is the Big Bang + rate of expansion. It was thought that there was nothing outside of the farthest point… but there is!
  - pdonis 2y ago
    > As I understand it, the Big Bang didn't happen in "one place"...there is no center point. That is correct. The only tenable answer to "where did the Big Bang take place" is "everywhere".
- injidup 2y ago
  So if the universe has a size then what do you see if you are on the edge of it? Do you see stars to the left and nothing to the right? I mean given the speed of light and the age of the universe and the rate of expansion there are regions inaccessible to us but that doesn't quite mean the universe has a finite size.
  - dragonwriter 2y ago
    Finite size doesn’t require an edge. Consider the surface of a balloon for a 2-D case (or the perimeter of a sphere, for a 1-D case): it has finite extent, but no edge.
    - lupire 2y ago
      It has a surface, though, which is what PP was asking about.An answer to the question is, yes, nesr the edge/face, one side is dark. But relativity and expansion makes the situation a bit more complicated.
  - dotnet00 2y ago
    The observable universe has a size, the cosmic microwave background is what we 'see' at the 'edge' in terms of photons (~400k years after the big bang). We could see further if we could map out the gravitational wave or neutrino backgrounds (1 sec after the big bang). But for now we can't really say if the universe in its entirety has a finite size.
    - mvkel 2y ago
      Fascinating. Do you think it's possible that we can map these out in the next 50 years?
      - dotnet00 2y ago
        For the gravitational wave background, maybe with LISA we might be able to get a glimpse, but the neutrino background seems like it'd take some truly unprecedented breakthroughs in our ability to detect neutrinos to have any chance of mapping it out.
- reactordev 2y ago
  What about the big ring [0]? Or other mega structures of galaxies outside that “bubble”? [0] https://en.m.wikipedia.org/wiki/Big_Ring https://en.m.wikipedia.org/wiki/Big_Ring
- mvkel 2y ago
  Isn't the rate of the expansion of the universe increasing? And that assumes the observable universe is homogeneous, which it isn't
  - pdonis 2y ago
    > Isn't the rate of the expansion of the universe increasing? It is now, but up until a few billion years ago, it wasn't, it was decreasing. Many of the objects we currently see are far enough away that the light we are now seeing from them was emitted while the universe's expansion was still decelerating. > that assumes the observable universe is homogeneous, which it isn't No, the models cosmologists use do not assume the universe is homogeneous period. They only assume it is homogeneous on average, on large distance scales (roughly scales larger than the size of the largest galaxy clusters).
- glenstein 2y ago
  >Take the age of the universe, multiple by the rate of expansion to get the total size of the universe, then multiple by the average density of galaxies in the observable universe My understanding is that, at the largest scales, clusters of galaxies are organized along a series of gravitationally bound filaments, sometimes called the cosmic web. So they aren't distributed like random noise, but more like a web. I have no reason to think this changes anything about calculating average densities, but it is notable that there's the general density but probably a significantly different density within that structure.

hyperliner 2y ago

[dead]

layer8 2y ago

*observable universe

afterburner 2y ago
Yes. I find it amusing I sparked a debate on what might be beyond the observable universe, when my point was entirely about what you could theoretically observe in the night sky.

bufferoverflow 2y ago

100 billion is the low end estimate for our galaxy. 400 billion is the high end.