How many galaxies do we have red shift data for?

[windy miller]

Im wondering does anyone know roughly how many galaxies we have red shift data for, just a ball park. Is it hundreds? thousands? millions?

 

[kimbyd]

Im wondering does anyone know roughly how many galaxies we have red shift data for, just a ball park. Is it hundreds? thousands? millions?

For spectroscopic redshifts, the answer is millions. For photometric redshifts (which are quicker but less accurate), hundreds of millions. The Sloan Digital Sky Survey is one of the largest surveys, and it alone measured numbers in this range, but there have been other surveys:
https://en.wikipedia.org/wiki/Sloan_Digital_Sky_Survey

 

[windy miller]

thanks for that , very helpful

 

[TEFLing]

https://www.universetoday.com/30305/how-many-galaxies-in-the-universe/

estimated 2,000 billion galaxies in the known Universe

200 billion would be observable

0.5 billion have been catalogued by SDSS ?
0.003 billion have scrutinized spectra ?

 

[kimbyd]

https://www.universetoday.com/30305/how-many-galaxies-in-the-universe/

estimated 2,000 billion galaxies in the known Universe

200 billion would be observable

0.5 billion have been catalogued by SDSS ?
0.003 billion have scrutinized spectra ?

Yup. It's a really, really big universe and it takes a lot of telescope time to image far-away galaxies in depth.

 

[TEFLing]

Yup. It's a really, really big universe and it takes a lot of telescope time to image far-away galaxies in depth.

I understand our current map of the universe is somewhat like a "Lindisfarne map" of the heavens

Lots could be "lurking out there between the lines" and we would as yet have very scant knowledge

 

[kimbyd]

I understand our current map of the universe is somewhat like a "Lindisfarne map" of the heavens

Lots could be "lurking out there between the lines" and we would as yet have very scant knowledge

It's possible, but unlikely. Enough observations have been made to demonstrate that the universe is highly homogeneous on large scales. The CMB observations alone are very strong evidence of that.

 

[PeterDonis]

@TEFLing you didn't violate the rules against speculation, but you did violate the rules against shouting, which is why your post got deleted. There's no need to shout.

Also, the question of what might be lurking in the data we do not have is not on topic here, since the OP's question was only about how many galaxies are included in the data we do have.

 

[TEFLing]

It's possible, but unlikely. Enough observations have been made to demonstrate that the universe is highly homogeneous on large scales. The CMB observations alone are very strong evidence of that.

Yes, homogeneous, and there could hypothetically be technosignatures homogeneously distributed across the sky and we would only have a 0.003/2000 chance of noticing

I need a reminder on how to format text so as not to have to use all caps for emphasis?

 

[berkeman]

I need a reminder on how to format text so as not to have to use all caps for emphasis?

In the Edit window (or Reply or whatever), at the left of the top toolbar there is a bold B. Highlight the text you want to Bold and click that B. You can do the same thing for Bold and Underline. It's best to only bold a word or two for emphasis, though. Lots of bolding or large fonts is usually distracting (at least in the technical forums).

 

[TEFLing]

@TEFLing you didn't violate the rules against speculation, but you did violate the rules against shouting, which is why your post got deleted. There's no need to shout.

Also, the question of what might be lurking in the data we do not have is not on topic here, since the OP's question was only about how many galaxies are included in the data we do have.

Found the Latex / Bbcode guides link, will use text formatting instead of all caps for emphasis, to be easier to read :)

 

[kimbyd]

Yes, homogeneous, and there could hypothetically be technosignatures homogeneously distributed across the sky and we would only have a 0.003/2000 chance of noticing

The problem with that analysis is that it's extraordinarily unlikely that we could ever detect an extra-galactic civilization. Our chances of detecting another civilization are pretty limited to a quite small fraction of our own galaxy close to us.

 

[TEFLing]

The problem with that analysis is that it's extraordinarily unlikely that we could ever detect an extra-galactic civilization. Our chances of detecting another civilization are pretty limited to a quite small fraction of our own galaxy close to us.

frighteningly true

 

[Lindsayforbes]

It's possible, but unlikely. Enough observations have been made to demonstrate that the universe is highly homogeneous on large scales. The CMB observations alone are very strong evidence of that.

I'm sure I listened to an interview with George Efstathiou where he admitted that the Planck data suggested significant differences in the CMB on each side of the map. It was 5 years ago though.

 

[mfb]

The chance to miss something that happens in 0.1% of all galaxies, if you observe 1 million randomly picked galaxies, is 3*10^-435.

We miss a lot due to the limits of our telescopes. But to miss something obvious just from our limited number of observations it has to be extremely rare.

 

[Bandersnatch]

I'm sure I listened to an interview with George Efstathiou where he admitted that the Planck data suggested significant differences in the CMB on each side of the map. It was 5 years ago though.

This is true insofar as these 'significant differences' are observable. They are not an indication that the universe is not highly homogeneous on large scales. Quite the opposite, in fact. The small magnitude of these anisotropies is what let's one say that the homogeneity is high.
Furthermore, this particular anisotropy, the dipole* anisotropy, is expected, and fully accounted for by the relative motion of our vantage point w/r to the rest frame of the gas that emitted the CMB. Meaning, this is not even an indication of physical inhomogeneities in the distribution of matter in the universe, but an artefact of our reference frame.
The smaller-scale anisotropies (BAOs) do indicate differences in distribution of matter - but we did know that already without having to look at CMB, since there are both galaxies and the roughly empty space in-between them.
There's more about those in here:
http://www.astro.ucla.edu/~wright/CMB-DT.html

*I don't know what lecture or talk you had watched, but it's a pretty safe bet that this is what was discussed