4 ms·

This appears to be the paper actually stating the headline [0]. Interestingly the statement is a bit different: "The star formation activity over the last ∼11

This appears to be the paper actually stating the headline [0]. Interestingly the statement is a bit different: "The star formation activity over the last ∼11 Gyr is responsible for producing ∼95 per cent of the total stellar mass density observed locally, with half of that being assembled in 2 Gyr between z = 1.2 and 2.2, and the other half in 8 Gyr (since z < 1.2). If the star formation rate density continues to decline with time in the same way as seen in the past ∼11 Gyr, then the stellar mass density of the Universe will reach a maximum which is only 5 per cent higher than the present-day value." To me, this says a lot more about the expansion of the universe, in that we have an ever smaller window into the observable universe, than anything about black holes. Is that a reasonable interpretation or am I missing something? [0] https://academic.oup.com/mnras/article/428/2/1128/1000290 https://academic.oup.com/mnras/article/428/2/1128/1000290

pdonis 6y ago
> To me, this says a lot more about the expansion of the universe, in that we have an ever smaller window into the observable universe I don't think that's what the actual paper is saying. The "stellar mass density" it talks about is by definition homogeneous across the entire universe; it's not something that is affected by how much of the entire universe we can actually see.
- alex_young 6y ago
  “Observed locally” seems to be doing a lot of heavy lifting if I read things right. If the observable universe is say 1/300th of the total, wouldn’t this actually represent an increase?
  - pdonis 6y ago
    We can only make direct observations of stellar mass density locally. We have no direct way of knowing what the stellar mass density is "now" a billion light years away; all we can see is what it was a billion years ago. We have to reconstruct what things are like "now" elsewhere based on past data, which is all we can see.
    - alex_young 6y ago
      That makes sense, but isn’t it at least more likely than not that there is relatively normal distribution?
      - pdonis 6y ago
        > isn’t it at least more likely than not that there is relatively normal distribution? No, because that would make our particular spatial location in the universe special, and that is an additional hypothesis that would require some kind of evidence in its favor, and we have none. The simplest hypothesis is that the universe is pretty much the same everywhere, and that means the average stellar density "now" should be pretty much the same everywhere.
        alex_young 6y ago
        That’s what I meant by normal distribution :)
        pdonis 6y ago
        Ah, ok. I was interpreting "normal distribution" in the sense that term is used in statistics, as "Gaussian centered on our spatial location".
        Akababa 6y ago
        That's a uniform distribution
        falseprofit 6y ago
        This type of casualty is to be expected when a word like 'normal' takes on a technical definition :)
  - pdonis 6y ago
    > If the observable universe is say 1/300th of the total, wouldn’t this actually represent an increase? I'm not sure what you mean.
cheez 6y ago
Nope, I thought the same thing and said it in my comment. But you are smarter than me, clearly :-)

pdonis 6y ago

> To me, this says a lot more about the expansion of the universe, in that we have an ever smaller window into the observable universe I don't think that's what the actual paper is saying. The "stellar mass density" it talks about is by definition homogeneous across the entire universe; it's not something that is affected by how much of the entire universe we can actually see.

alex_young 6y ago
“Observed locally” seems to be doing a lot of heavy lifting if I read things right. If the observable universe is say 1/300th of the total, wouldn’t this actually represent an increase?
- pdonis 6y ago
  We can only make direct observations of stellar mass density locally. We have no direct way of knowing what the stellar mass density is "now" a billion light years away; all we can see is what it was a billion years ago. We have to reconstruct what things are like "now" elsewhere based on past data, which is all we can see.
  - alex_young 6y ago
    That makes sense, but isn’t it at least more likely than not that there is relatively normal distribution?
    - pdonis 6y ago
      > isn’t it at least more likely than not that there is relatively normal distribution? No, because that would make our particular spatial location in the universe special, and that is an additional hypothesis that would require some kind of evidence in its favor, and we have none. The simplest hypothesis is that the universe is pretty much the same everywhere, and that means the average stellar density "now" should be pretty much the same everywhere.
      - alex_young 6y ago
        That’s what I meant by normal distribution :)
        pdonis 6y ago
        Ah, ok. I was interpreting "normal distribution" in the sense that term is used in statistics, as "Gaussian centered on our spatial location".
        Akababa 6y ago
        That's a uniform distribution
        falseprofit 6y ago
        This type of casualty is to be expected when a word like 'normal' takes on a technical definition :)
- pdonis 6y ago
  > If the observable universe is say 1/300th of the total, wouldn’t this actually represent an increase? I'm not sure what you mean.

cheez 6y ago

Nope, I thought the same thing and said it in my comment. But you are smarter than me, clearly :-)