Please explain each part. 10. a) (10 points) Assume we live in a Universe that i
ID: 1770009 • Letter: P
Question
Please explain each part. 10. a) (10 points) Assume we live in a Universe that is NOT expanding, but is static (staying the same size). Three galaxies (Galaxy Hubble, Galaxy Rubin, and Galaxy Einstein) are llustrated below. Galaxy Hubble and Galaxy Rubin are separated by 10 billion lightyears. Galaxy Einstein is located between Galaxy Hubble and Galaxy Einstein, at a distance of 4 billion lightyears from Galaxy Hubble. When the Universe was 2 billion years old, could Galaxy Hubble see Galaxy Einstein? Could Galaxy Rubin see Galaxy Einstein? Explain b) (10 points) When the Universe was 9 billion years old, the supermassive black hole at the center of Galaxy Einstein began accreting gas and shining as a quasar. The quasar has continued shining to present av. At present day (when the age of the Universe is 14 billion years), does Galaxy Hubble see Galaxy Einstein? If so, does Galaxy Hubble see Galaxy Einstein as a galaxy with a quasar or a galaxy without a quasar? Does Galaxy Rubin see Galaxy Einstein? If so, does Galaxy Rubin see Galaxy Einstein as a galaxy with a quasar or a galaxy without a quasar? Galaxy Hubble Galaxy Rubin
Explanation / Answer
a) The distance between Galaxy Hubble and Galaxy Rubin is 10 billion lightyears. This means light from either of these galaxies will take 10 billion years to reach the other.
Similarly, light from either of Galaxy Hubble and Galaxy Einstein will take 4 billion years to reach the other.
And, light from either of Galaxy Rubin and Galaxy Einstein will take 6 billion years to reach the other.
Assuming, the universe is static, the distances between these galaxies have been the same all the while.
Now, the first light must have been emitted by these galaxies when the universe was just born.
So, Galaxy Hubble needs the universe to be at least 4 billion years old to see Galaxy Einstein and vice versa.
Also, Galaxy Rubin needs the universe to be at least 6 billion years old to see Galaxy Einstein and vice versa.
So, when the universe was 2 billion years old, neither of Galaxy Hubble or Galaxy Rubin could see Galaxy Einstein.
(Ans)
b) The present age of the universe is 14 billion years (> 4 light years). So, clearly, at the present day, Galaxy Hubble can see Galaxy Einstein.
Again, It's been (14-9) = 5 billion years (>4 billion years) since the quasar has been born within Galaxy Einstein or the first light has been emitted by the quasar. So, clearly, at the present day, Galaxy Hubble sees Galaxy Einstein with the quasar.
Again, the time needed by light to travel between galaxy Einstein and Galaxy Rubin (6 billion years) is less than the age of the universe (14 billion years) but more than the age of the quasar (5 billion years). Hence, Galaxy Rubin will see Galaxy Einstein at the present day, but without the quasar. (Ans)