6 ms·

For any, like me, wondering "how close"? 5 minutes of naive googling and extrapolation about the "z = 0.151" (which I learned refers to redshift) leads me to th

For any, like me, wondering "how close"? 5 minutes of naive googling and extrapolation about the "z = 0.151" (which I learned refers to redshift) leads me to the answer ~2B light years. For reference, the diameter of the Milky Way is a bit over 100k light years. Would love to be informed how wrong I got it!

defrost 4y ago
Large scale universe surveys rarely bother with light years as such, they're interested in the structure of the universe itself since birth and are looking to the properties of entire galaxies so far distant individual stars defy resolution (for the most part, stars with energetic burst behaviour stand out). EG: Planned Optical Srveys The Hector Galaxy Survey The Hector Galaxy Survey will use the Hector-I instrument on the AAT to collect resolved spectroscopy for 15,000 galaxies out to z < 0.1. The Taipan galaxy survey It will use the TAIPAN facility on the UK Schmidt Telescope at Siding Spring Observatory to collect spectroscopy for more than a million galaxies in the local Universe (z<0.3) that are brighter than i<17. https://astronomyaustralia.org.au/participating-australian-astronomical-surveys.html https://astronomyaustralia.org.au/participating-australian-a... So, uhh, yeah, to a certain class of astro-geek z<0.3 is absolutely "local universe"
AlbertCory 4y ago
thanks, I wondered that, too. I actually did a find on "year" and of course came up empty.
jen729w 4y ago
My exceptionally naive understanding is that "how long ago" and "how far away" aren't really things that cosmologists talk about. Because of relativity and the expansion of space neither really has a meaning. It's like the idea of common time: when did something happen on Alpha Centauri? You can't really answer that. Light cones and spheres of influence and so on. "When" is only a valid question when the thing you're asking about isn't relatavistically far away. Deep stuff. Source: I just read Carlo Rovelli's The order of time. Would be overjoyed if someone who knows what they're talking about would correct/enhance this response. :-)
- pdonis 4y ago
  > "how long ago" and "how far away" aren't really things that cosmologists talk about. In actual textbooks and technical papers, like this one, yes, that's correct; they don't talk about them because they're not directly observable, and deriving them from the things that are directly observable is model dependent. So they prefer to talk about the things that are directly observable, of which the key ones are redshift, apparent luminosity, and (for objects that are large enough, like galaxies) angular size. The relativistic effects you mention are additional reasons why those things are not usually talked about in technical writings. Pop science writings are another matter: many cosmologists do talk about how long ago and how far away in those contexts without taking the time to add the caveats about how we don't directly observe those things.
  - px43 4y ago
    I wonder how often things we observe are actually farther away than they seem, they're just rapidly moving towards us relative to their surroundings, or vice versa. Seems like it's totally possible that something could be 4 billion light years away, or 8 billion light years away, and have the same red shift. Maybe sometimes gravitational lensing would help determine if a star is behind or in front of another? I wonder if it's pretty common to observe objects that are more blue shifted than something that is clearly closer to us.
    - antihipocrat 4y ago
      I'd love to get an informed answer to this too. Is it more accurate to interpret inferred distances derived from redshift as an average estimate? If so it would be great if popular science reporting also included the confidence intervals
      - pdonis 4y ago
        > Is it more accurate to interpret inferred distances derived from redshift as an average estimate? We don't estimate distances just based on redshift. We use all three of the observables I mentioned--redshift, apparent luminosity, and angular size--as inputs to estimate parameters in our cosmological models, and one of those parameters is the scale factor--the "size" of the universe, which is what is used in the model to ultimately get actual distances--as a function of time. The models of course have error bars--no model is ever exact--and some of those error bars are due to variations of the motions of individual galaxies from the idealized cosmological expansion in the models. So in that sense distances given by the models are a sort of estimate, yes, based on the average motion.
habibur 4y ago
Better comparison, the furthest visible galaxy is at 13B LY. Therefore it's like at 20% distance towards the visible universe's edge.
- panzi 4y ago
  The observable universe is roughly 90B LY in diameter, which factors in how much the universe expanded the source of the light (or rather radio signal) away since its emission.
- mr_mitm 4y ago
  At this scale, you have to specify which distance measure you are using. I can infer that you mean light-travel distance, but a cosmologist would use the comoving distance unless specified otherwise. That's what the other comment was getting at with the 90B light-years and the observable universe. Reference: https://en.wikipedia.org/wiki/Distance_measure https://en.wikipedia.org/wiki/Distance_measure
  - habibur 4y ago
    Distances of stars and galaxies are mostly always put in light travel distance -- that was my assumption. Unless that's changed in recent times.
    - mr_mitm 4y ago
      In articles for the general population, you're right in both cases. For stars, you are also right in general. Well, we'd both be right, because for pretty much all observable stars light travel distance and comoving distance are equal (doesn't really make sense to talk about comoving distances or cosmological redshifts within a galaxy). In scientific populations, distances to galaxies are always given either directly in redshifts or perhaps in special cases in comoving distances.
photochemsyn 4y ago
From the paper: > "Spectroscopic observations of the afterglow and the host galaxy provided a redshift of z = 0.151 (de Ugarte Postigo et al. 2022; Castro-Tirado et al. 2022; Izzo et al. 2022), corresponding to a distance of 749.3 Mpc." 3.262e6 (so about 3.3 million light years per megaparsec) * 749.3 gives 2.445 billion light years, so in the ballpark. The key thing in getting these estimates is the complex business of standard candles, which are astronomical objects believed to have a well-defined luminosity so one can calibrate distances by looking at how faint they are. There are two major kinds used, Cepheid variables (which only cover the range from 1 kpc to 50 Mpc, so not useful here) and type 1a supernovae (which are rare): > "Beyond the question of whether conditions in the early universe affect the luminosity of a type 1a supernova, a second problem besets this standard candle that severely limits its use: supernovae are rare in any given galaxy. They therefore cannot be used to determine the distance to any galaxy that we may be interested in. Their only uses are in calibrating other distance measures, such as the cosmological redshift, in testing cosmological theories, and in studying the surrounding of these supernovae. Current studies with these standard candles are examining the expansion of the universe at redshifts between z = 0.01 and 1." https://astrophysicsspectator.org/topics/overview/DistanceExtragalactic.html https://astrophysicsspectator.org/topics/overview/DistanceEx... It might be interesting to look at those cited papers to see exactly how they got that measurement but it's bedtime. P.S. Crash Course Astronomy #25 (Distances) is a fun 10 min watch, great series overall: https://youtu.be/CWMh61yutjU https://youtu.be/CWMh61yutjU

defrost 4y ago

Large scale universe surveys rarely bother with light years as such, they're interested in the structure of the universe itself since birth and are looking to the properties of entire galaxies so far distant individual stars defy resolution (for the most part, stars with energetic burst behaviour stand out). EG: Planned Optical Srveys The Hector Galaxy Survey The Hector Galaxy Survey will use the Hector-I instrument on the AAT to collect resolved spectroscopy for 15,000 galaxies out to z < 0.1. The Taipan galaxy survey It will use the TAIPAN facility on the UK Schmidt Telescope at Siding Spring Observatory to collect spectroscopy for more than a million galaxies in the local Universe (z<0.3) that are brighter than i<17. https://astronomyaustralia.org.au/participating-australian-astronomical-surveys.html https://astronomyaustralia.org.au/participating-australian-a... So, uhh, yeah, to a certain class of astro-geek z<0.3 is absolutely "local universe"

AlbertCory 4y ago

thanks, I wondered that, too. I actually did a find on "year" and of course came up empty.

jen729w 4y ago

My exceptionally naive understanding is that "how long ago" and "how far away" aren't really things that cosmologists talk about. Because of relativity and the expansion of space neither really has a meaning. It's like the idea of common time: when did something happen on Alpha Centauri? You can't really answer that. Light cones and spheres of influence and so on. "When" is only a valid question when the thing you're asking about isn't relatavistically far away. Deep stuff. Source: I just read Carlo Rovelli's The order of time. Would be overjoyed if someone who knows what they're talking about would correct/enhance this response. :-)

pdonis 4y ago
> "how long ago" and "how far away" aren't really things that cosmologists talk about. In actual textbooks and technical papers, like this one, yes, that's correct; they don't talk about them because they're not directly observable, and deriving them from the things that are directly observable is model dependent. So they prefer to talk about the things that are directly observable, of which the key ones are redshift, apparent luminosity, and (for objects that are large enough, like galaxies) angular size. The relativistic effects you mention are additional reasons why those things are not usually talked about in technical writings. Pop science writings are another matter: many cosmologists do talk about how long ago and how far away in those contexts without taking the time to add the caveats about how we don't directly observe those things.
- px43 4y ago
  I wonder how often things we observe are actually farther away than they seem, they're just rapidly moving towards us relative to their surroundings, or vice versa. Seems like it's totally possible that something could be 4 billion light years away, or 8 billion light years away, and have the same red shift. Maybe sometimes gravitational lensing would help determine if a star is behind or in front of another? I wonder if it's pretty common to observe objects that are more blue shifted than something that is clearly closer to us.
  - antihipocrat 4y ago
    I'd love to get an informed answer to this too. Is it more accurate to interpret inferred distances derived from redshift as an average estimate? If so it would be great if popular science reporting also included the confidence intervals
    - pdonis 4y ago
      > Is it more accurate to interpret inferred distances derived from redshift as an average estimate? We don't estimate distances just based on redshift. We use all three of the observables I mentioned--redshift, apparent luminosity, and angular size--as inputs to estimate parameters in our cosmological models, and one of those parameters is the scale factor--the "size" of the universe, which is what is used in the model to ultimately get actual distances--as a function of time. The models of course have error bars--no model is ever exact--and some of those error bars are due to variations of the motions of individual galaxies from the idealized cosmological expansion in the models. So in that sense distances given by the models are a sort of estimate, yes, based on the average motion.

habibur 4y ago

Better comparison, the furthest visible galaxy is at 13B LY. Therefore it's like at 20% distance towards the visible universe's edge.

panzi 4y ago
The observable universe is roughly 90B LY in diameter, which factors in how much the universe expanded the source of the light (or rather radio signal) away since its emission.
mr_mitm 4y ago
At this scale, you have to specify which distance measure you are using. I can infer that you mean light-travel distance, but a cosmologist would use the comoving distance unless specified otherwise. That's what the other comment was getting at with the 90B light-years and the observable universe. Reference: https://en.wikipedia.org/wiki/Distance_measure https://en.wikipedia.org/wiki/Distance_measure
- habibur 4y ago
  Distances of stars and galaxies are mostly always put in light travel distance -- that was my assumption. Unless that's changed in recent times.
  - mr_mitm 4y ago
    In articles for the general population, you're right in both cases. For stars, you are also right in general. Well, we'd both be right, because for pretty much all observable stars light travel distance and comoving distance are equal (doesn't really make sense to talk about comoving distances or cosmological redshifts within a galaxy). In scientific populations, distances to galaxies are always given either directly in redshifts or perhaps in special cases in comoving distances.

photochemsyn 4y ago

From the paper: > "Spectroscopic observations of the afterglow and the host galaxy provided a redshift of z = 0.151 (de Ugarte Postigo et al. 2022; Castro-Tirado et al. 2022; Izzo et al. 2022), corresponding to a distance of 749.3 Mpc." 3.262e6 (so about 3.3 million light years per megaparsec) * 749.3 gives 2.445 billion light years, so in the ballpark. The key thing in getting these estimates is the complex business of standard candles, which are astronomical objects believed to have a well-defined luminosity so one can calibrate distances by looking at how faint they are. There are two major kinds used, Cepheid variables (which only cover the range from 1 kpc to 50 Mpc, so not useful here) and type 1a supernovae (which are rare): > "Beyond the question of whether conditions in the early universe affect the luminosity of a type 1a supernova, a second problem besets this standard candle that severely limits its use: supernovae are rare in any given galaxy. They therefore cannot be used to determine the distance to any galaxy that we may be interested in. Their only uses are in calibrating other distance measures, such as the cosmological redshift, in testing cosmological theories, and in studying the surrounding of these supernovae. Current studies with these standard candles are examining the expansion of the universe at redshifts between z = 0.01 and 1." https://astrophysicsspectator.org/topics/overview/DistanceExtragalactic.html https://astrophysicsspectator.org/topics/overview/DistanceEx... It might be interesting to look at those cited papers to see exactly how they got that measurement but it's bedtime. P.S. Crash Course Astronomy #25 (Distances) is a fun 10 min watch, great series overall: https://youtu.be/CWMh61yutjU https://youtu.be/CWMh61yutjU