Calculating Redshift to Using Hubble Constant and Distance

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Homework Help Overview

The discussion revolves around calculating the redshift of a galaxy cluster based on its distance and the Hubble constant. The original poster presents a scenario involving a distance of 520 Mpc and questions the need for a relativistic speed to determine the redshift.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to connect the distance to the redshift calculation and raises a question about the necessity of knowing the speed of the galaxy. Some participants confirm the approach and discuss the conditions under which classical redshift can be applied.

Discussion Status

The discussion is active, with participants confirming the original poster's edit regarding the calculation of speed from distance using the Hubble constant. There is an exploration of when to apply classical redshift versus considering relativistic effects.

Contextual Notes

Participants note the importance of the speed being much less than the speed of light for classical redshift calculations, as well as the implications of redshifts approaching or exceeding 1.

elliottmarter
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A redshift survey has identified a galaxy cluster for which it infers a di stance of 520 Mpc. At what redshift is this cluster observed to be? You may use H0 = 72 km s−1 Mpc−1

Now call me old fashioned but don't i need some kind of relativistic speed to calculate the redshift, i mean i have a distance of 520Mpc (=16.12x10^21m) and the Hubble constant...but I am sure i need a value for how fast its traveling away from me...

Thanks

EDIT i think i just realized that i times its distance by Ho to get its speed as its Kms-1 Mpc-1
 
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Your edit is correct.
 
Then as long as this speed is much less than 'c' you can just use classical redshift.
When you get redshifts approaching (or larger than) 1 you need to take into account the expansion of the universe.
 
yeah the Lorentz factor was 1.00... so i just used z=v/c
 

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