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Layperson's question: The article talks about "a possible galaxy that exists some 13.5 billion light-years from Earth". But isn't that just the light traveling

Layperson's question: The article talks about "a possible galaxy that exists some 13.5 billion light-years from Earth". But isn't that just the light traveling distance, and the object should actually be much further away due to the expansion of the universe?

simonh 5y ago
Well spotted yes, by now that object will be about 45 bn ly away. The light reaching us from it has only traversed 13.5 bn ly though.
- sdeframond 5y ago
  Define "now" https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
  - simonh 5y ago
    While that’s a problem in theory, to moderate accuracy it’s not a practical issue. Most objects in the universe are roughly at rest relative to the cosmic background radiation (in other words they don’t see it as Doppler shifted in any particular direction), so we can use that as a common frame of reference. When we’re talking about ballpark integer billions of years, or even to a few decimal places, that’s easily good enough.
    - uoaei 5y ago
      Accuracy? "Now" is an inaccurate (nonsensical, actually) concept except in extremely local conditions.
      - simonh 5y ago
        It's perfectly sensible given a common frame of reference, which we have thanks to the CMB. Frames of reference aren't local. We have clocks and can measure time on them just fine, and 'now' is just a time on a clock.
        deleted 5y ago
        [deleted]
- cygx 5y ago
  What are you basing the 45 bn ly on? A redshift of z~13 should correspond to a comoving distance of about 33 bn ly, whereas 45 bn ly would correspond to z~600.
  - simonh 5y ago
    You may be right, I've just seen ~45 bn ly given as the radius of the observable universe accounting for expansion. The discrepancy could be due to changes in the rate of expansion over time, as it hasn't been constant.
    - cygx 5y ago
      45.5 bn ly is the approximate comoving distance of the surface of last scattering (the source of the microwave background and the point where the universe becomes opaque). This is the boundary of the universe observable via electromagnetic radiation.
layer8 5y ago
“Light-years” literally is light-traveling distance, though. :)
spindle 5y ago
Speaking as someone who used to teach introductory cosmology at a good university ... your question is not a layperson's question! (That's a compliment, BTW.)
- quadcore 5y ago
  What is the expansion of the universe? Or should I first ask, is there a center?
  - hotpotamus 5y ago
    The best analogy I've ever gotten is to think of an expanding balloon. Where on the surface of the balloon would you call the center? And if you imagine yourself standing on the surface while it expands, you will be still while all other points of the surface move away from you. It's far from a perfect analogy, but probably the best way to visualize in human terms.
  - cygx 5y ago
    What is the expansion of the universe? The distance between galaxies (that aren't gravitationally bound to each other) increases over time. Or should I first ask, is there a center? Probably not. If you inflate a balloon, any point on its surface will move away from any other point on the surface. And yet, no surface point can claim to be central. Note that in the balloon example, there is of course a central point within the volume of the balloon. This is where the analogy breaks down: In case of the universe, we do not assume the existence of an ambient higher-dimensional space.
    - deltaonefour 5y ago
      A perfect geometric surface of a sphere that is expanding has no center, but a balloon has a center. That's obviously the nozzle. The analogy breaks down in both places. But who knows? Maybe the analogy is correct. I don't think we can say. In fact I would even wager that a "center" is more likely... because can you even imagine a real world analogy where something expanding has no center? If you can't then there probably is some sort of center.
sph 5y ago
To add my dumb question: since the universe expands faster than light, how is this considered a very far object and not just that we're looking at something in the past? Is this confusion of mine between distance and time what physicists mean by space-time? If that's the case, why isn't the article titled "astronomers just discovered the oldest object in the known universe"?
- spindle 5y ago
  Distances of this sort of thing are not measured directly - what's measured is its red shift, which tells us both how distant it is (very) and how old it is (very). That inference is indirect and depends on the model of cosmology, but the relevant bit of cosmology is relatively uncontentious. (But if it really was only 13.5 billion light-years from Earth then you'd be right. In fact it's much further away than that.)
karldcampbell 5y ago
It sound odd, but that depends on what you mean by "now"[1]. If you mean, where would the galaxy be if I traveled there at the speed of light, you would actually travel less distance due to Length Contraction [2]. [1] https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity [2] https://en.wikipedia.org/wiki/Length_contraction https://en.wikipedia.org/wiki/Length_contraction
- throw82473751 5y ago
  It sounds odd to interpret it like that and not as usually a layperson ignoring relativity&physical limitations would mean it? "Now" as in if I could just teleport there!

simonh 5y ago

Well spotted yes, by now that object will be about 45 bn ly away. The light reaching us from it has only traversed 13.5 bn ly though.

sdeframond 5y ago
Define "now" https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
- simonh 5y ago
  While that’s a problem in theory, to moderate accuracy it’s not a practical issue. Most objects in the universe are roughly at rest relative to the cosmic background radiation (in other words they don’t see it as Doppler shifted in any particular direction), so we can use that as a common frame of reference. When we’re talking about ballpark integer billions of years, or even to a few decimal places, that’s easily good enough.
  - uoaei 5y ago
    Accuracy? "Now" is an inaccurate (nonsensical, actually) concept except in extremely local conditions.
    - simonh 5y ago
      It's perfectly sensible given a common frame of reference, which we have thanks to the CMB. Frames of reference aren't local. We have clocks and can measure time on them just fine, and 'now' is just a time on a clock.
      - deleted 5y ago
        [deleted]
cygx 5y ago
What are you basing the 45 bn ly on? A redshift of z~13 should correspond to a comoving distance of about 33 bn ly, whereas 45 bn ly would correspond to z~600.
- simonh 5y ago
  You may be right, I've just seen ~45 bn ly given as the radius of the observable universe accounting for expansion. The discrepancy could be due to changes in the rate of expansion over time, as it hasn't been constant.
  - cygx 5y ago
    45.5 bn ly is the approximate comoving distance of the surface of last scattering (the source of the microwave background and the point where the universe becomes opaque). This is the boundary of the universe observable via electromagnetic radiation.

layer8 5y ago

“Light-years” literally is light-traveling distance, though. :)

spindle 5y ago

Speaking as someone who used to teach introductory cosmology at a good university ... your question is not a layperson's question! (That's a compliment, BTW.)

quadcore 5y ago
What is the expansion of the universe? Or should I first ask, is there a center?
- hotpotamus 5y ago
  The best analogy I've ever gotten is to think of an expanding balloon. Where on the surface of the balloon would you call the center? And if you imagine yourself standing on the surface while it expands, you will be still while all other points of the surface move away from you. It's far from a perfect analogy, but probably the best way to visualize in human terms.
- cygx 5y ago
  What is the expansion of the universe? The distance between galaxies (that aren't gravitationally bound to each other) increases over time. Or should I first ask, is there a center? Probably not. If you inflate a balloon, any point on its surface will move away from any other point on the surface. And yet, no surface point can claim to be central. Note that in the balloon example, there is of course a central point within the volume of the balloon. This is where the analogy breaks down: In case of the universe, we do not assume the existence of an ambient higher-dimensional space.
  - deltaonefour 5y ago
    A perfect geometric surface of a sphere that is expanding has no center, but a balloon has a center. That's obviously the nozzle. The analogy breaks down in both places. But who knows? Maybe the analogy is correct. I don't think we can say. In fact I would even wager that a "center" is more likely... because can you even imagine a real world analogy where something expanding has no center? If you can't then there probably is some sort of center.

sph 5y ago

To add my dumb question: since the universe expands faster than light, how is this considered a very far object and not just that we're looking at something in the past? Is this confusion of mine between distance and time what physicists mean by space-time? If that's the case, why isn't the article titled "astronomers just discovered the oldest object in the known universe"?

spindle 5y ago
Distances of this sort of thing are not measured directly - what's measured is its red shift, which tells us both how distant it is (very) and how old it is (very). That inference is indirect and depends on the model of cosmology, but the relevant bit of cosmology is relatively uncontentious. (But if it really was only 13.5 billion light-years from Earth then you'd be right. In fact it's much further away than that.)

karldcampbell 5y ago

It sound odd, but that depends on what you mean by "now"[1]. If you mean, where would the galaxy be if I traveled there at the speed of light, you would actually travel less distance due to Length Contraction [2]. [1] https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity [2] https://en.wikipedia.org/wiki/Length_contraction https://en.wikipedia.org/wiki/Length_contraction

throw82473751 5y ago
It sounds odd to interpret it like that and not as usually a layperson ignoring relativity&physical limitations would mean it? "Now" as in if I could just teleport there!