- #1
Thomas Smith
- 12
- 0
- Homework Statement
- The current average density of the universe is roughly 3 x 10^-27 km m^-3. What was the average density of the universe at the time the light was emitted by a galaxy with the red shift of z=11.1? Express your answer in terms of a number of H atoms per cubic meter.
- Relevant Equations
- R(t)/R(t0) = 1/1+z
Na x p/mH Where Na is Avogardo's number in terms of atoms, mH is the mass of the hydrogen atom in kg and p is the average density.
Firstly i worked out the scale factor of the universe
R(t)/R(t0) = 1/1+z = 1/1+11.1 = 1/12.1 = 12.1^3 = 1/1772
The distance between the galaxies were 12.1 times less than today and the volume was 1772 times smaller than today.
Then I think the average density in the universe at that time is (3×10^-27 )×1772 = 5.32 × 10^-24kg m^-3
then the average density in terms of the hydrogen atom
= 6.023×10^23 × (5.32×10^-24/1.67×10^-24) = 1.92×10^24 hydrogen atoms per cubic meter.
This does not seem right to me at all!
R(t)/R(t0) = 1/1+z = 1/1+11.1 = 1/12.1 = 12.1^3 = 1/1772
The distance between the galaxies were 12.1 times less than today and the volume was 1772 times smaller than today.
Then I think the average density in the universe at that time is (3×10^-27 )×1772 = 5.32 × 10^-24kg m^-3
then the average density in terms of the hydrogen atom
= 6.023×10^23 × (5.32×10^-24/1.67×10^-24) = 1.92×10^24 hydrogen atoms per cubic meter.
This does not seem right to me at all!
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