4 ms·

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 ve

by 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.
- mr_mitm 4y ago
  The effect you are talking about is real, but it's generally small. The cause is called peculiar velocity, and it could introduce uncertainties of a few percent, which is relatively small for cosmological standards. We know how velocities are distributed inside galaxy clusters. It leads to the "finger of god" effect, which makes it appear as if objects would be distributed such that they tend to align along our line of sight. See: https://en.wikipedia.org/wiki/Peculiar_velocity https://en.wikipedia.org/wiki/Peculiar_velocity
pdonis 4y ago
> 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. In theory, of course this is possible. But in our actual universe, that's not what happens. In our actual universe, there is a close relationship between how far away something is and its redshift. Or, to phrase it in terms of direct observables, in theory we could have an object that was 4 billion light years away and an object that was 8 billion light years away, both with the same redshift--but they would be very different in terms of other observables like apparent luminosity and angular size. So in the general case, in theory, we would not expect any particular relationship between those variables. But in our actual universe, there is a close relationship between all three of those variables: redshift, apparent luminosity, and angular size all vary in concert and are closely correlated. That kind of observation is what leads cosmologists to conclude that our universe is expanding. > I wonder if it's pretty common to observe objects that are more blue shifted than something that is clearly closer to us. It's not only not "pretty common", it never happens except for objects very close to us, cosmologically speaking--for example, the Andromeda galaxy is somewhat blueshifted, indicating that it is moving towards us, while the Magellanic Clouds, which are closer (they are satellite galaxies of our own galaxy), are not, AFAIK, blueshifted. But all of these objects are in the same local, gravitationally bound group as our galaxy, so their relative velocities don't tell us anything useful about the large scale behavior of the universe.

by px43 4y ago

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.
mr_mitm 4y ago
The effect you are talking about is real, but it's generally small. The cause is called peculiar velocity, and it could introduce uncertainties of a few percent, which is relatively small for cosmological standards. We know how velocities are distributed inside galaxy clusters. It leads to the "finger of god" effect, which makes it appear as if objects would be distributed such that they tend to align along our line of sight. See: https://en.wikipedia.org/wiki/Peculiar_velocity https://en.wikipedia.org/wiki/Peculiar_velocity

pdonis 4y ago

> 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. In theory, of course this is possible. But in our actual universe, that's not what happens. In our actual universe, there is a close relationship between how far away something is and its redshift. Or, to phrase it in terms of direct observables, in theory we could have an object that was 4 billion light years away and an object that was 8 billion light years away, both with the same redshift--but they would be very different in terms of other observables like apparent luminosity and angular size. So in the general case, in theory, we would not expect any particular relationship between those variables. But in our actual universe, there is a close relationship between all three of those variables: redshift, apparent luminosity, and angular size all vary in concert and are closely correlated. That kind of observation is what leads cosmologists to conclude that our universe is expanding. > I wonder if it's pretty common to observe objects that are more blue shifted than something that is clearly closer to us. It's not only not "pretty common", it never happens except for objects very close to us, cosmologically speaking--for example, the Andromeda galaxy is somewhat blueshifted, indicating that it is moving towards us, while the Magellanic Clouds, which are closer (they are satellite galaxies of our own galaxy), are not, AFAIK, blueshifted. But all of these objects are in the same local, gravitationally bound group as our galaxy, so their relative velocities don't tell us anything useful about the large scale behavior of the universe.