Galaxy Database with distances from Earth

[Buzz Bloom]

TL;DR

I am assuming that there is a database somewhere listing all of the galaxies which astronomers have found. If that is correct, my question is:
What fraction of these galaxies have a distance from Earth value associated with it?

Summary: I am assuming that there is a database somewhere listing all of the galaxies which astronomers have found. If that is correct, my question is:
What fraction of these galaxies in the database have a distance from Earth value associated with it, other than a distance based only on the value of z using a standard value for H₀?

My guess is that it is a small fraction since for most of them a special kind of supernova must be detected whose brightness curve enables a calculation of distance D independent of z. My curiosity about this is from an idea I have that it might be possible to use all (or most of) the galaxies in the database knowing only the z values for the galaxies to calculate the radius of curvature of the universe.

I am also guessing that only galaxies with a value for z, and a value D which is not based only on z (using H₀), are used to calculate the values for the five universe model parameters: h₀ and four Ωs.

 

[jim mcnamara]

Yes there are galactic DB's: https://pages.astronomy.ua.edu/keel/galaxies/catalogs.html

Short answer - I believe most of the currently catalogued galaxies have a z value - red shift. This z value approximates the distance to the object at a point in the past. When light left the object. Not now, millions or billions of years after the light left. Long ago.

Note that 'nearby' galaxies have their distances based on Cepheid variables as a standard candle.
As well as a z value. Some more remote object have Type II supernovae data or quasars as a standard candle. And a z value.

And since there are vast numbers of galaxies compared to a few observatories and astronomers I'm fairly sure that there are guesses/estimates on the fraction you want. Simply because we cannot reasonably gotten them all. Plus, intervening objects can obscure or refract the light from objects "behind" them.

Cannot quickly find a reference. Someone, whose field this is, can give you a better answer. Try a google scholar search. I tried 'percent of galaxies not catalogued' and got some not so great hits.

I am hoping for the James Webb telescope... probably in vain. May not live that long.
https://www.skyandtelescope.com/astronomy-news/james-webb-space-telescope-march-2021-launch/

 

[Buzz Bloom]

And since there are vast numbers of galaxies compared to a few observatories and astronomers I'm fairly sure that there are guesses/estimates on the fraction you want. Simply because we cannot reasonably gotten them all. Plus, intervening objects can obscure or refract the light from objects "behind" them.

Hi Jim:

Thank you for y our post.

I think I did not phrase my question clearly. The fraction I am seeking is that fraction of galaxies in the database that have an associated value for distance D (from Earth) other than a distance calculated just from z (presumably based on H₀).

I will edit my post #1 to make this clearer.

Regards,
Buzz

 

[jim mcnamara]

Try this - about determining the flatness of the universe in another way:

 

[Buzz Bloom]

Try this

Hi Jim:

My idea is to calculate the universe radius of curvature independently of model building for the five parameters (including Ω_k). If the fraction of database galaxies that have a D value (independent of z) is a small fraction, the my idea allows for using (almost) all of the database, which might well give a more accurate value for Ω_k with a smaller error range.

Regards,
Buzz

 

[jim mcnamara]

Listen to the claims in the video: H₀ is flat to .04% - This is based on data from the Planck satellite CMB data, and starts post-inflation, i.e., much further back than type II supernovae. YMMV.
Ignore - completely wrong.

 

[Bandersnatch]

@Buzz Bloom check out this page on Ned Wright's site:
http://www.astro.ucla.edu/~wright/sne_cosmology.htmlI think it describes exactly the kind of (ongoing) studies that you had in mind, or at least related. You may want to follow the references there for more details on the data and methods used - if you can handle them (they're over my head).

The parameters for the LCDM model that you mention in post #1 are, I believe, based mostly on CMB data from COBE, WMAP, and Planck - not on galactic (SNe) data.

 

[Buzz Bloom]

H0 is flat to .04%

Hi Jim:

I do not understand what the ".04%" means. In another thread I discuss the value from

http://planck.caltech.edu/pub/2015results/Planck_2015_Results_XIII_Cosmological_Parameters.pdf .​

In particular, from pg 39 eq 50.

The constraint can be sharpened further by adding external data that break the main geometric degeneracy. Combining the Planck data with BAO, we find​

Ω_k=0.000±0.005 (95%,PlanckTT+lowP+lensing+BAO). (50)​

In the thread

https://www.physicsforums.com/threads/what-is-the-mean-expected-volume-of-a-finite-universe.972608/​

I assume that the standard deviation of a Gaussian distribution (with mean=0) is σ=0.0025, and then I calculate M_x from the left half of this distribution. M_x is the mean value of Ω_k (assuming that Ω_k<0). I got

M_x = -0.00199471.​

From this I calculate the corresponding value of r = radius of curvature:

r = 3.23384*10¹¹ ly .​

Note how close this value is to r_ou, the radius of the observable universe:

r_ou = 9.3x10¹⁰ ly.​

-0.00199471 may look like a value very close to zero, but the corresponding radius is only a little larger (factor of about 3) than the radius of the observable universe. This is my motivation for wanting to have a different way to calculate r based on a lot more data.

Regards,
Buzz

 

[Buzz Bloom]

I think it describes exactly the kind of (ongoing) studies that you had in mind, or at least related.

Hi Bandersnatch:

That graph is based on data points that have both a value for z and a value for D based on luminosity.

I want to calculate the radius of curvature from data only needing z, hopefully a lot more data.

My idea is related to the number of galaxies in portions of a cone. The summary below is somewhat over-brief. I plan to start a new thread to elaborate after I find out about the fraction of galaxies in the data base that do not have a D value independent of z, if it turns out that this fraction is very small.

The portions are arranged along the axis of the cone. All portions of the cone share the same specified thickness adjusted for the scale factor, a, calculated from z. Each portion is at a variable distance r from the origin (Earth). I assume that the number of galaxies in each portion will be (approximately) proportional to the volume of each section. If (and only if) the universe is flat, the volume of each section will be (approximately) proportional to r².

Regards,
Buzz

 

[phyzguy]

Summary: I am assuming that there is a database somewhere listing all of the galaxies which astronomers have found. If that is correct, my question is:
What fraction of these galaxies have a distance from Earth value associated with it?

Summary: I am assuming that there is a database somewhere listing all of the galaxies which astronomers have found. If that is correct, my question is:
What fraction of these galaxies in the database have a distance from Earth value associated with it, other than a distance based only on the value of z using a standard value for H₀?

My guess is that it is a small fraction since for most of them a special kind of supernova must be detected whose brightness curve enables a calculation of distance D independent of z. My curiosity about this is from an idea I have that it might be possible to use all (or most of) the galaxies in the database knowing only the z values for the galaxies to calculate the radius of curvature of the universe.

I am also guessing that only galaxies with a value for z, and a value D which is not based only on z (using H₀), are used to calculate the values for the five universe model parameters: h₀ and four Ωs.

You need to understand that, except for the nearest galaxies, there is no way to know the distance except by calculating it given the observables and having a model of the cosmology of the universe. There are many different cosmological distances, co-moving distance, luminosity distance, angular size distance, ... Calculating any of these distances, and the relations between them, requires that you have a model of the universe, which gives the expansion history as a function of time. This is one of the main reasons why astronomers focus on redshift. Redshift is a direct observable, and is not subject to interpretation or dependent on which cosmological model you use. So your goal of having distances which are independent of the cosmological model cannot be satisfied.

 

[Buzz Bloom]

So your goal of having distances which are independent of the cosmological model cannot be satisfied.

Hi phyzguy:

I think you have misunderstood my intention. An approximation of distance D can be made based on z using the best current estimated value for h₀. However, if that is the only estimate of D available, that galaxy cannot be used (with others) to calculate best fit values for the five model parameters, one of which being h₀. As I understand it (possibly wrongly) this is because it is not reasonable to assume a value for h₀ and then use the result in calculating a best value for h₀.

My idea is to use the z value for galaxies (and a corresponding D calculated only from z and H₀) to count the number of galaxies, N(D), in a series of equivalent distance ranges (ΔD adjusted for a) which vary with D. Assuming the number density is (approximately) uniform, if the universe is flat, then

N(D) = BD².​

If Ω_k<0, a radius of curvature |R| will affect this result,

N(D) = B(D sin(D/|R|))².​

If Ω_k>0, a radius of curvature R will affect this result,

N(D) = B(D sinh(D/|R|))².​

Thus an estimate for R can be calculated as the value of R which produces a best value of fit to these functions on the right with the appropriate proportionality constant B. The value of B will depend on (a) the particular size of the solid angle of the sky in which the counted galaxies are included for the calculation, and (b) the value used for ΔD.

I would much appreciate your comments on this explanation.

Regards,
Buzz

 

[kimbyd]

Hi Jim:

My idea is to calculate the universe radius of curvature independently of model building for the five parameters (including Ω_k). If the fraction of database galaxies that have a D value (independent of z) is a small fraction, the my idea allows for using (almost) all of the database, which might well give a more accurate value for Ω_k with a smaller error range.

Regards,
Buzz

The most precise way to measure curvature is to use Baryon Acoustic Oscillations, which correlate length scales measured on the Cosmic Microwave Background to average distances between visible galaxies. While model building is used for this result, it is largely independent of most of the model parameters.

The main reason why you still need model building is that the length scales on the CMB aren't precisely the same thing as with the average distances between nearby galaxies. The connection between the two involves a model. The nice thing is that because the measurement is based on comparing two length scales, the outcome of the curvature measurement is highly independent of the other model parameters.

 

[Buzz Bloom]

The most precise way to measure curvature is to use Baryon Acoustic Oscillations

Hi kimbyd:

Thank you much for your post. I found the Wikipedia article on BOA, and from that the 2004 article

DETECTION OF THE BARYON ACOUSTIC PEAK IN THE LARGE-SCALE CORRELATION FUNCTION OF SDSS LUMINOUS RED GALAXIES​

https://arxiv.org/pdf/astro-ph/0501171.pdf .​

It looks like I will doing a lot of reading for a while. I am hoping I will be able to understand what I read.

Regards,
Buzz