Expanding Universe (L5)

Question 1

What needs to be true for the universe to be at critical density?

Answer 1

k =0 meaning universe is expanding at a decreasing rate forever. Also cosmological constant Λ needs to 0

Question 2

Which two parameters from the Friedmann equations determine the geometry and fate of the universe?

Answer 2

k (curvature parameter) and Ω (density parameter)

Question 3

What is the cosmological constant?

Answer 3

Homogeneous energy density that causes the expansion of the universe to accelerate

Question 4

what does k>0 and Ω>1 mean for the universe and its geometry?

Answer 4

The universe is a closed universe which will eventually collapse under its own gravity. It has spherical geometry

Question 5

what does k=0 and Ω = 1 mean for the universe and its geometry

Answer 5

The universe is at its critical point and will expand at a decreasing rate forever. It has flat geometry

Question 6

what does k<0 and Ω<1 mean for the universe and its geometry

Answer 6

The universe is an open universe and so will expand forever. It has is hyperbolic geometry

Question 7

What is Cosmic Background Radiation (CBR)?

Answer 7

Leftover radiation following the big bang

Question 8

Why did the universe have to cool before neutral matter could be condensed out of plasma?

Answer 8

Because each electron captured would emit a photon of high enough energy to ionise a newly formed atom. This meant that the rate of electron capture was only slightly larger than the rate of photodissociation

Question 9

What is the decoupling temperature?

Answer 9

The temperature at which the average photon energy was well below 13.6eV. This is roughly at 3500K