10 ms·

Assuming the universe is mainly uniform, why would distant galaxies have different starts than nearby ones? Is it related to time (the light from those farther

Assuming the universe is mainly uniform, why would distant galaxies have different starts than nearby ones? Is it related to time (the light from those farther away that reaches us being from the distant past)?

disconcision 4y ago
distant means younger means different composition (less heavy elements), dunno if that's the deal here though
- mrits 4y ago
  Unless we are the center of the universe I don't see how distant could mean younger
  - Cthulhu_ 4y ago
    That's an interesting one to think about; every galaxy moves away from every one, so (if I got this right) from every other galaxy's point of view, all the other galaxies are younger... or older, either / or I don't know anymore.
    - ianai 4y ago
      Light and thus causality and information travel at a constant speed of "c." As far as we know, nothing can travel faster than "c" - other than the universe itself as in during the inflationary epoch. So the further out any observer would look the longer that light would have had to travel relative to that observer. Seeing something 3 billion light years away would mean the light took 3 billion years to arrive at the observer.
      - Maursault 4y ago
        > As far as we know, nothing can travel faster than "c" What Relativity forbids is anything traveling as fast as c, as the increase in mass as c is approached would require infinite energy to reach. Relativity says absolutely nothing about faster than c, i.e. it does not forbid things such as the theoretical tachyon.
    - cmehdy 4y ago
      It's more like every point of view looks at the past. Which is also true if you look at your screen right now, it's just that you're looking at your screen some nanoseconds ago.
  - ncallaway 4y ago
    The further away it is, the more time light has spent traveling to us. So, more distant means we are viewing events that happened further in the past. We are (almost certainly) not at the center of the universe, but we are, by definition, at the center of our observable universe.
  - Jenk 4y ago
    The light took far longer to reach us, thus what we are _observing_ is from a much younger (than it really, currently, is) star.
  - Archelaos 4y ago
    You are the oldest thing in the universe you can observe. Even the person next to you, as you observe her or him, is younger, because the speed of light is finite. In this respect, you are the centre of your universe. But objectively speaking, there is no centre.
    - p1esk 4y ago
      This doesn’t make any sense
      - deleted 4y ago
        [deleted]
      - deleted 4y ago
        [deleted]
      - tromp 4y ago
        When you see someone standing 3m away from you, the light took 10 nanoseconds to reach your eye, so you see them as they were a very short time ago. Of course, they're only younger when measuring their age not from their birth but from the big bang.
      - jpollock 4y ago
        I find it useful to think of photons as vector clocks: https://en.wikipedia.org/wiki/Vector_clock https://en.wikipedia.org/wiki/Vector_clock Take a photon starting from your chest and give it t1. * Assume that the speed of light is symmetrical. * When the photon hits the next person's eyes, it is t1 + distance/speed of light. Meaning, the photon is "from the past". Since all photons arriving are from the past, the photons travelling the shortest distance are the newest (representing the largest timestamp). So, the thing with the largest observable timestamp is yourself.
        p1esk 4y ago
        We are not talking about the age of photons. We are talking about the age of the thing that emits/reflects photons. The confusion arose because the OP said "you are the oldest thing in the universe you can observe". They should have said "you are the most recent thing in the universe you can observe".
  - BurningFrog 4y ago
    The universe has no center. All places are equally old. So a galaxy a billion light years away is as old as ours. But we see it as it was 1 billion years ago.
    - samstave 4y ago
      I’ve always had trouble understanding this; if everything is the same age, how did they get billions of light years away from the Big Bang? How do we know where the Big Bang occurred? Unles it means that our galaxy is on the spherical plane(?) directly projected out in all directions from the bang, and it’s the diameter of this sphere that is billions across?
      - x3n0ph3n3 4y ago
        The big bang occurred _everywhere_.
        JumpCrisscross 4y ago
        To add to this: we are used to seeing explosions propagating across space and time. The Big Bang was an explosion of spacetime.
        hdjjhhvvhga 4y ago
        Why did it happen?
        sdmike1 4y ago
        Good question, my understanding is that we frankly just don't know, it may be one of those things that we can't know, but that is getting awfully close to the realm of philosophy
        JumpCrisscross 4y ago
        > it may be one of those things that we can't know This is true for anything we don’t know. For the Big Bang, we know the intermediate problem: black holes. Gravity and quantum mechanics interacting at the same scale. The singularities there are accessible in a way the singularity of the Big Bang is not.
SemanticStrengh 4y ago
The universe is not uniform/isotropic at large scales
- gizmo686 4y ago
  The universe is not isotropic in the time dimension. However, across space the prevailing thinking is that the universe is isotropic on a large scale in the spatial dimensions.
  - spookthesunset 4y ago
    If it wasn’t isotopic across space, that would be a huge deal.
    - SemanticStrengh 4y ago
      As you can see, it clearly isn't homogenous https://en.wikipedia.org/wiki/List_of_largest_cosmic_structures#:~:text=Edit-,List%20of%20the%20largest%20cosmic%20structures,-Structure%20name%0A(year https://en.wikipedia.org/wiki/List_of_largest_cosmic_structu...
    - enriquepablo 4y ago
      As far as I understand (I'm not a physicist), to be able to say that the universe is isotropic across space, you need a point of view that just doesn't exist. You cannot speak about an absolute synchronicity among events happening in locations scattered among an arbitrary subset of the set of all visible galaxies, and without that, you cannot abstract time out of the picture, which you'd need to do to speak about isotropicity across space. I don't think that, in this regard, you can say much more than "the universe is not isotropic across space-time".
      - cygx 4y ago
        You cannot speak about an absolute synchronicity among events happening in locations scattered among an arbitrary subset of the set of all visible galaxies The cosmic microwave background gives you a physical realization of just that (but of course only approximately so), at least as far as cosmologists are concerned. The rules of relativity of course still apply, making this particular synchronicity convention just one of many others...
yk 4y ago
They are older, so they had less time to produce heavier elements. Additionally lighter stars live longer, so it may be that the heavy stars just burned out in nearby galaxies.
- jeremyjh 4y ago
  Why wouldn't that have already been the assumption? When we look at distant galaxies we are looking billions of years in the past; so it would seem natural to find more heavy stars there that haven't burned out like they have more locally.
  - superjan 4y ago
    I expect this effect is not ignored. But please note that heavy stars do not burn in billions of years but in millions. Not seeing them locally suggests that we have slower starformation, not that we had more of them (heavy stars) billions of years ago. https://en.wikipedia.org/wiki/Stellar_evolution https://en.wikipedia.org/wiki/Stellar_evolution
  - denton-scratch 4y ago
    > it would seem natural to find more heavy stars there that haven't burned out Is that right? I thought the really, really big stars in the early universe burned-out in as little as a few million years.
JumpCrisscross 4y ago
> why would distant galaxies have different starts than nearby ones? I know one reason: metallicity. The very early universe didn’t have clumps of heavy elements to create nucleation points of sort for fusion. So big clouds of hydrogen had to very slowly draw together via “molecular…cooling in the gas phase,” which meant more massive stars that burned furiously for a short while [1]. [1] https://iopscience.iop.org/article/10.1088/0004-637X/745/1/50 https://iopscience.iop.org/article/10.1088/0004-637X/745/1/5...

disconcision 4y ago

distant means younger means different composition (less heavy elements), dunno if that's the deal here though

mrits 4y ago
Unless we are the center of the universe I don't see how distant could mean younger
- Cthulhu_ 4y ago
  That's an interesting one to think about; every galaxy moves away from every one, so (if I got this right) from every other galaxy's point of view, all the other galaxies are younger... or older, either / or I don't know anymore.
  - ianai 4y ago
    Light and thus causality and information travel at a constant speed of "c." As far as we know, nothing can travel faster than "c" - other than the universe itself as in during the inflationary epoch. So the further out any observer would look the longer that light would have had to travel relative to that observer. Seeing something 3 billion light years away would mean the light took 3 billion years to arrive at the observer.
    - Maursault 4y ago
      > As far as we know, nothing can travel faster than "c" What Relativity forbids is anything traveling as fast as c, as the increase in mass as c is approached would require infinite energy to reach. Relativity says absolutely nothing about faster than c, i.e. it does not forbid things such as the theoretical tachyon.
  - cmehdy 4y ago
    It's more like every point of view looks at the past. Which is also true if you look at your screen right now, it's just that you're looking at your screen some nanoseconds ago.
- ncallaway 4y ago
  The further away it is, the more time light has spent traveling to us. So, more distant means we are viewing events that happened further in the past. We are (almost certainly) not at the center of the universe, but we are, by definition, at the center of our observable universe.
- Jenk 4y ago
  The light took far longer to reach us, thus what we are _observing_ is from a much younger (than it really, currently, is) star.
- Archelaos 4y ago
  You are the oldest thing in the universe you can observe. Even the person next to you, as you observe her or him, is younger, because the speed of light is finite. In this respect, you are the centre of your universe. But objectively speaking, there is no centre.
  - p1esk 4y ago
    This doesn’t make any sense
    - deleted 4y ago
      [deleted]
    - deleted 4y ago
      [deleted]
    - tromp 4y ago
      When you see someone standing 3m away from you, the light took 10 nanoseconds to reach your eye, so you see them as they were a very short time ago. Of course, they're only younger when measuring their age not from their birth but from the big bang.
    - jpollock 4y ago
      I find it useful to think of photons as vector clocks: https://en.wikipedia.org/wiki/Vector_clock https://en.wikipedia.org/wiki/Vector_clock Take a photon starting from your chest and give it t1. * Assume that the speed of light is symmetrical. * When the photon hits the next person's eyes, it is t1 + distance/speed of light. Meaning, the photon is "from the past". Since all photons arriving are from the past, the photons travelling the shortest distance are the newest (representing the largest timestamp). So, the thing with the largest observable timestamp is yourself.
      - p1esk 4y ago
        We are not talking about the age of photons. We are talking about the age of the thing that emits/reflects photons. The confusion arose because the OP said "you are the oldest thing in the universe you can observe". They should have said "you are the most recent thing in the universe you can observe".
- BurningFrog 4y ago
  The universe has no center. All places are equally old. So a galaxy a billion light years away is as old as ours. But we see it as it was 1 billion years ago.
  - samstave 4y ago
    I’ve always had trouble understanding this; if everything is the same age, how did they get billions of light years away from the Big Bang? How do we know where the Big Bang occurred? Unles it means that our galaxy is on the spherical plane(?) directly projected out in all directions from the bang, and it’s the diameter of this sphere that is billions across?
    - x3n0ph3n3 4y ago
      The big bang occurred _everywhere_.
      - JumpCrisscross 4y ago
        To add to this: we are used to seeing explosions propagating across space and time. The Big Bang was an explosion of spacetime.
        hdjjhhvvhga 4y ago
        Why did it happen?
        sdmike1 4y ago
        Good question, my understanding is that we frankly just don't know, it may be one of those things that we can't know, but that is getting awfully close to the realm of philosophy
        JumpCrisscross 4y ago
        > it may be one of those things that we can't know This is true for anything we don’t know. For the Big Bang, we know the intermediate problem: black holes. Gravity and quantum mechanics interacting at the same scale. The singularities there are accessible in a way the singularity of the Big Bang is not.

SemanticStrengh 4y ago

The universe is not uniform/isotropic at large scales

gizmo686 4y ago
The universe is not isotropic in the time dimension. However, across space the prevailing thinking is that the universe is isotropic on a large scale in the spatial dimensions.
- spookthesunset 4y ago
  If it wasn’t isotopic across space, that would be a huge deal.
  - SemanticStrengh 4y ago
    As you can see, it clearly isn't homogenous https://en.wikipedia.org/wiki/List_of_largest_cosmic_structures#:~:text=Edit-,List%20of%20the%20largest%20cosmic%20structures,-Structure%20name%0A(year https://en.wikipedia.org/wiki/List_of_largest_cosmic_structu...
  - enriquepablo 4y ago
    As far as I understand (I'm not a physicist), to be able to say that the universe is isotropic across space, you need a point of view that just doesn't exist. You cannot speak about an absolute synchronicity among events happening in locations scattered among an arbitrary subset of the set of all visible galaxies, and without that, you cannot abstract time out of the picture, which you'd need to do to speak about isotropicity across space. I don't think that, in this regard, you can say much more than "the universe is not isotropic across space-time".
    - cygx 4y ago
      You cannot speak about an absolute synchronicity among events happening in locations scattered among an arbitrary subset of the set of all visible galaxies The cosmic microwave background gives you a physical realization of just that (but of course only approximately so), at least as far as cosmologists are concerned. The rules of relativity of course still apply, making this particular synchronicity convention just one of many others...

yk 4y ago

They are older, so they had less time to produce heavier elements. Additionally lighter stars live longer, so it may be that the heavy stars just burned out in nearby galaxies.

jeremyjh 4y ago
Why wouldn't that have already been the assumption? When we look at distant galaxies we are looking billions of years in the past; so it would seem natural to find more heavy stars there that haven't burned out like they have more locally.
- superjan 4y ago
  I expect this effect is not ignored. But please note that heavy stars do not burn in billions of years but in millions. Not seeing them locally suggests that we have slower starformation, not that we had more of them (heavy stars) billions of years ago. https://en.wikipedia.org/wiki/Stellar_evolution https://en.wikipedia.org/wiki/Stellar_evolution
- denton-scratch 4y ago
  > it would seem natural to find more heavy stars there that haven't burned out Is that right? I thought the really, really big stars in the early universe burned-out in as little as a few million years.

JumpCrisscross 4y ago

> why would distant galaxies have different starts than nearby ones? I know one reason: metallicity. The very early universe didn’t have clumps of heavy elements to create nucleation points of sort for fusion. So big clouds of hydrogen had to very slowly draw together via “molecular…cooling in the gas phase,” which meant more massive stars that burned furiously for a short while [1]. [1] https://iopscience.iop.org/article/10.1088/0004-637X/745/1/50 https://iopscience.iop.org/article/10.1088/0004-637X/745/1/5...