3 ms·

> "we can be absolutely confident of their fantastic distances" My current layperson's understanding is: 1) Redshift-to-distance conversion is a calibration b

> "we can be absolutely confident of their fantastic distances" My current layperson's understanding is: 1) Redshift-to-distance conversion is a calibration based on the distance ladder. 2) And the distance ladder itself seems to have come under some doubt recently due to the "cosmological crisis" [1]. Can someone here ELI5 how they can be absolutely confident of these distances? [1] "JWST just made the Crisis in Cosmology WORSE" https://www.youtube.com/watch?v=hps-HfpL1vc https://www.youtube.com/watch?v=hps-HfpL1vc

CrimsonRain 4y ago
I love astronomy since I was a kid. It is amazing how much we've figured out and still doing so everyday. However, I realized this long ago (in my teens) that much of the stats, eg numbers of stars/planets in the universe, their distances/compositions/ages etc, while calculated, are built on top of multiple instances of approximation/assumption/insufficient data. We know very well how a simple early rounding can lead to widely different end result in a chain of equations. Imagine that in a gigantic situation. In my teen years, I've always wondered why they are not launching Hubble 2.0 3.0 etc every 5 years. Because that's what we need. More state of the art observatories out there in space to gather more data. Without that, we're just increasing the chain of assumptions and approximations.
- sandworm101 4y ago
  We have. In fact since the 1960s new space telescopes have been launched far more often than every five years, some bigger than hubble. https://en.wikipedia.org/wiki/List_of_space_telescopes https://en.wikipedia.org/wiki/List_of_space_telescopes
magicalhippo 4y ago
> Can someone here ELI5 how they can be absolutely confident of these distances? The way I read it is that they're absolutely confident of their fantastic young age, that is, their redshift. As you point out, the age-to-distance conversion isn't quite settled, but that doesn't really matter in this context I think, as younger will still mean further away. So these galaxies will still be the furthest away (for now) even if the distance ladder gets adjusted "inward", ie closer to us.
- gradschool 4y ago
  Why couldn't a relatively nearby galaxy recede from us faster than a more distant one due to causes beyond the limits of our observation? I'm thinking primarily of gravitational attraction to a massive object past the boundary of our observable universe, but wouldn't dark matter also imply in any case that all bets are off? What objection could anyone raise to the idea of a big load of dark matter right next to some galaxy making it recede faster?
  - magicalhippo 4y ago
    The recession speed is a statistical average given the distance. Individual galaxies might vary some, but the trend is clear. At these extreme distances, I'm pretty sure any motion due to the local (to the distant galaxy) gravitational field is negligible to the perceived recession due to the expansion of the universe[1]. [1]: https://en.wikipedia.org/wiki/Hubble%27s_law https://en.wikipedia.org/wiki/Hubble%27s_law
graycat 4y ago
> Can someone here ELI5 how they can be absolutely confident of these distances? In physics, it is common for the writing to say that some fact was "shown" or "proven". Maybe look at the "absolutely confident" as similar! I'm a skeptical and often dissatisfied reader of the fundamentals in physics: E.g., as a ugrad, I tried to be a physics major but too frequently got totally torqued off at the sloppy math so majored only in math. To try to make a long story and explanation short, it appears that a common approach is just to go ahead and say "shown" and synonyms and keep saying that until there is some serious contradiction, conflict, new news, etc. Still, "shown", ..., "absolutely confident" are not easy to achieve: It appears that we have a right to assume that such work did build on the accepted physics, got the arithmetic right, got the math right or close enough, etc. And, there are some assumptions rarely explained: E.g., for these observations claimed to be from 13.5 billion years ago, we have to assume, maybe, things like the spectral lines of hydrogen, deuterium, etc. have not changed, the clock ticks and meter sticks mean the same as on Earth now, etc. Heck, maybe 13.5 billion years ago there was no hydrogen yet and dark matter radiated what we now believe came from hydrogen??? Here I'm making just wild stabs: A real conflict in the accepted results of physics would be big news and is not easy to find -- I can't give real examples here or at all. So, for now, and until there is some big reason otherwise, we just let physics say "shown", ..., "absolutely confident". These are not nearly the worst ways the English language is sometimes used!

CrimsonRain 4y ago

I love astronomy since I was a kid. It is amazing how much we've figured out and still doing so everyday. However, I realized this long ago (in my teens) that much of the stats, eg numbers of stars/planets in the universe, their distances/compositions/ages etc, while calculated, are built on top of multiple instances of approximation/assumption/insufficient data. We know very well how a simple early rounding can lead to widely different end result in a chain of equations. Imagine that in a gigantic situation. In my teen years, I've always wondered why they are not launching Hubble 2.0 3.0 etc every 5 years. Because that's what we need. More state of the art observatories out there in space to gather more data. Without that, we're just increasing the chain of assumptions and approximations.

sandworm101 4y ago
We have. In fact since the 1960s new space telescopes have been launched far more often than every five years, some bigger than hubble. https://en.wikipedia.org/wiki/List_of_space_telescopes https://en.wikipedia.org/wiki/List_of_space_telescopes

magicalhippo 4y ago

> Can someone here ELI5 how they can be absolutely confident of these distances? The way I read it is that they're absolutely confident of their fantastic young age, that is, their redshift. As you point out, the age-to-distance conversion isn't quite settled, but that doesn't really matter in this context I think, as younger will still mean further away. So these galaxies will still be the furthest away (for now) even if the distance ladder gets adjusted "inward", ie closer to us.

gradschool 4y ago
Why couldn't a relatively nearby galaxy recede from us faster than a more distant one due to causes beyond the limits of our observation? I'm thinking primarily of gravitational attraction to a massive object past the boundary of our observable universe, but wouldn't dark matter also imply in any case that all bets are off? What objection could anyone raise to the idea of a big load of dark matter right next to some galaxy making it recede faster?
- magicalhippo 4y ago
  The recession speed is a statistical average given the distance. Individual galaxies might vary some, but the trend is clear. At these extreme distances, I'm pretty sure any motion due to the local (to the distant galaxy) gravitational field is negligible to the perceived recession due to the expansion of the universe[1]. [1]: https://en.wikipedia.org/wiki/Hubble%27s_law https://en.wikipedia.org/wiki/Hubble%27s_law

graycat 4y ago

> Can someone here ELI5 how they can be absolutely confident of these distances? In physics, it is common for the writing to say that some fact was "shown" or "proven". Maybe look at the "absolutely confident" as similar! I'm a skeptical and often dissatisfied reader of the fundamentals in physics: E.g., as a ugrad, I tried to be a physics major but too frequently got totally torqued off at the sloppy math so majored only in math. To try to make a long story and explanation short, it appears that a common approach is just to go ahead and say "shown" and synonyms and keep saying that until there is some serious contradiction, conflict, new news, etc. Still, "shown", ..., "absolutely confident" are not easy to achieve: It appears that we have a right to assume that such work did build on the accepted physics, got the arithmetic right, got the math right or close enough, etc. And, there are some assumptions rarely explained: E.g., for these observations claimed to be from 13.5 billion years ago, we have to assume, maybe, things like the spectral lines of hydrogen, deuterium, etc. have not changed, the clock ticks and meter sticks mean the same as on Earth now, etc. Heck, maybe 13.5 billion years ago there was no hydrogen yet and dark matter radiated what we now believe came from hydrogen??? Here I'm making just wild stabs: A real conflict in the accepted results of physics would be big news and is not easy to find -- I can't give real examples here or at all. So, for now, and until there is some big reason otherwise, we just let physics say "shown", ..., "absolutely confident". These are not nearly the worst ways the English language is sometimes used!