What minimum distance is red shift detectable?

[Vern]

Based upon the currently accepted value for the Hubble constant, what would be a minimum distance to detect a red shift in starlight? Specifically, I am wondering if the 100 K light year size of the Milky Way galaxy would be enough distance to detect a red shift.

Edit: I know we could measure a Doppler shift at those distances; but I'm wondering if calculations based upon the Hubble constant alone would produce a detectable red shift.
Wiki article on Hubble's law

The most recent calculation of the proportionality constant used 2003 data from the satellite WMAP combined with other astronomical data, and yielded a value of H0 = 70.1 ± 1.3 (km/s)/Mpc. This value agrees well with that of H0 = 72 ± 8 km/s/Mpc obtained in 2001 by using NASA's Hubble Space Telescope[4]. In August, 2006, a less precise figure was obtained independently using data from NASA's orbital Chandra X-ray Observatory: H0 = 77 (km/s)/Mpc or about 2.5×10−18 s−1 with an uncertainty of ± 15%.[5] NASA summarizes existing data to indicate a constant of 70.8 ± 1.6 (km/s)/Mpc if space is assumed to be flat, or 70.8 ± 4.0 (km/s)/Mpc otherwise.[6] (2.3×10−18 s−1 = 1/(13.8 billion years))

 

[draqon]

try this calculator and see: http://www.star.uclan.ac.uk/~cph/redshift.html

 

[Vern]

Thanks; neat calculator !

 

[James R]

There's not much point in looking for cosmological red shift within our galaxy, since all stars in the galaxy are gravitationally bound to each other.

 

[Vern]

I was looking for evidence that the expansion of space is happening within our own galaxy. if my crude calculations are correct, It turns out that any red shift due to that expansion would be too small to detect.

We say that the universe is expanding, yet galaxies are not expanding because gravity overcomes the expansion. A question came up in another forum about what portion of the cosmological red shift could be attributed to space expanding, and what portion would be due to galaxies receding.

Hi jamesR; all galaxies should be gravitationally attracted to each other, wouldn't they. Then we have to say that at some unknown distance that attraction is too little to matter. We do not have a value for the cosmological expansion energy, or force, do we?

 

[Bishadi]

What minimum distance is red shift detectable?

Are you asking from a star?

Heck, a simply sunset is an example.

Now please, if Einstein's idea of light traveling at a constant speed, no matter the observance frame, then using hubble's frame is moot.

The change is not based on speed, but the interaction with what ever is in between the line of sight.

kind of like a sunset/sunrise.................

or even remember the Eddington experiment; well if the light was bending around the sun, anyone can duplicate a mirage to do the same thing.

Use your head folks!

 

[Vern]

I think what Eddington found was that light passing a star bends twice as much as the gravity of the star would bend it. This is a little different than a mirage.

 

[James R]

Vern:

We say that the universe is expanding, yet galaxies are not expanding because gravity overcomes the expansion. A question came up in another forum about what portion of the cosmological red shift could be attributed to space expanding, and what portion would be due to galaxies receding.

If galaxies are receding for some reason other than cosmological expansion, then why is it that all galaxies, in every direction we look, are receding from us? What is special about our galaxy?

Hi jamesR; all galaxies should be gravitationally attracted to each other, wouldn't they. Then we have to say that at some unknown distance that attraction is too little to matter.

Well, gravity drops off as the inverse square of the distance, and is directly proportional to mass. Take the average mass of a galaxy and you can decide at what distance the gravitational attraction of other galaxies becomes insignificant.

We do not have a value for the cosmological expansion energy, or force, do we?

We think we know the Hubble constant to reasonable accuracy.

I think what Eddington found was that light passing a star bends twice as much as the gravity of the star would bend it. This is a little different than a mirage.

Eddington confirmed Einstein's prediction of the amount that light is bent by a star, which is twice the amount that Newtonian physics predicts. This is additional evidence that Einstein's general theory of relativity is correct.


Bishadi:

What minimum distance is red shift detectable?

Are you asking from a star?

Heck, a simply sunset is an example.

Sunsets have nothing to do with red shift.

Now please, if Einstein's idea of light traveling at a constant speed, no matter the observance frame, then using hubble's frame is moot.

This appears to be meaningless nonsense.

or even remember the Eddington experiment; well if the light was bending around the sun, anyone can duplicate a mirage to do the same thing.

Use your head folks!

You are trolling. Please stop it.

 

[Bruinthor]

The Schwarzschild metric modified to account for a constant cosmological expansion has a dilation term of

1-2GM/c^2/r-H0^2/c^2r^2

This has a minimum at

GM/H0^2=r^3

ie beyond this r the pull of the mascon is less than the push of cosmological expansion

The coordinate r is not a true measure of distance but the area of the constant r and time surface is given by the classic formula

4Pi r^2

Using a solar gravitational parameter of 1.33*10^(20) m^3/s^2 and Hubble parameter of 1/(4.3*10^17 s) the assignment

r->(GM/H0^2)^(1/3)

places the minimum at M^(1/3)*2.9*10^18 m or M(1/3)*307 ly

(m->meter, s->second, ly->light-year,M->solar-mass)

This is meaningless for the sun

Using a Milky Way M of 10^12 the minimum occurs at roughly 3000000 light-years. The local group of galaxies makes this value almost useless but IMHO it is qualitatively instructive.

 

[krash661]

" What minimum distance is red shift detectable? "
at the point red shift is detected. shugs.
sorry im incapacitated due to vodka. , but still. increase your intelligence. shrugs.