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Calculating distance using redshift 
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#1
Feb2612, 10:54 AM

P: 2

Hi. I have pretty rudimentary math skills, and am hoping someone can explain this formula for me, found online a number of places including:
http://www.astrophysicsspectator.com...agalactic.html for calculating distance (d) in megaparsecs using redshift (z), the speed of light (c), and Hubble's constant (H). d = c z/H It's the (units in) Hubble's constant that have me confused. In the example from the link, "we see that objects with a redshift of 0.1 are about 4.6 gigaparsecs [a]way", assuming an H of 65 km s1 Mpc1. So I assumed c here would be in km/s, but: 300000 * (0.1/65) = 461 Which is 0.46 gigaparsecs, not 4.6. What is it about the units/Hubble's constant here that I don't get? Here's what I guessing: units ^ 1 mean 65 should be multiplied by 0.1, which is different than 65 ^ 1. This would make H 6.5, and the result would be correct. Is that right? If so, why is the notation km/s1/Mpc1? How would that be different from km/s/Mpc1? 


#2
Feb2612, 11:57 AM

P: 1,261

I think your calculation (as it was) is correct. Your guess about the units is definitely not rightthe units are separate from the number attached to them.
Maybe their result is just a typo... 


#3
Feb2612, 02:42 PM

P: 2

Yeah, you're right. The next thing I tried this on was the Large Magellanic Cloud, which produced even more bizarre results, but I found a calculator for this online:
http://hyperphysics.phyastr.gsu.edu...hubble.html#c3 And eventually got some data to match up (it seems the LMC's redshift is an anomaly, so my first test case was a typo and my second anomalous _). 


#4
Feb2612, 05:54 PM

P: 1,261

Calculating distance using redshift
Haha, just some bad luck I guess!
Way to be thoroughgood job 


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