If the distance of a galaxy at a redshift z = 0.5 is 1,800 Mpc,how many years back into the past are we looking when we observe this galaxy?
A) 500 million years
B) 2 billion years
C) 6 billion years
D) 9 billion years

Question

Quizplus · Accepted Answer

To calculate the age of the galaxy, we need to use the formula:

Age of the universe at redshift z = (1/H0) x (1/sqrt(Omega_m)) x ln[(1+z)/sqrt(1+z^2 x (1-Omega_m))]

Where:
H0 = Hubble constant (in km/s/Mpc)
Omega_m = mass density parameter

Assuming H0 = 70 km/s/Mpc and Omega_m = 0.3 (values commonly used in cosmology), we get:

Age of the universe at z = 0.5 = (1/70) x (1/sqrt(0.3)) x ln[(1+0.5)/sqrt(1+0.5^2 x (1-0.3))] 
= 6.02 billion years

Therefore, we are observing this galaxy as it was 6 billion years ago, when the universe was 6.02 billion years old.

If the Distance of a Galaxy at a Redshift Z