Distances

Question 1

What is the actual distance photons transverse?

Answer 1

ct

Question 2

Why is it better to think of photon distances as comoving distances?

Answer 2

Their point of origin is further away than that now if Universe expands

Question 3

What is the Physical distance equation when z «1?

Answer 3

D_phys = cz / H_o

Question 4

What is the Physical distance equation when z tends to infinity?

Answer 4

D_phys = 2c / H_o Ω_o

Question 5

What is the photon horizon?

Answer 5

The furthest point of origin of a photon in comoving coordinates (to the limit of infinity of z by looking backwards)

Question 6

What is the radius of the visible horizon in comoving units?

Answer 6

6000/(Ω_o h) Mpc

Question 7

What is the real (not comoving) size of an object?

Answer 7

dl = ardθ

(dr = 0 as at a fixed distance r)

Question 8

What is the comoving size of the object?

Answer 8

R = rdθ

Question 9

What is the angular diameter distance?

Answer 9

d_A = ar = r / (1 + z)

Question 10

When is a scale factor added?

Answer 10

When something needs to become comoving

Question 11

What is the observed flux?

Answer 11

Received energy per unit time

Question 12

What is the luminosity distance ?

Answer 12

(1 + z)r

Question 13

What can test the relationship between d_A and redshift?

Answer 13

Radio jets from the black holes at the centre of galaxies which are thought to be similar sizes

Question 14

Which standard candle makes use of the luminosity and flux relationship?

Answer 14

Supernovae Ia which uses Ω _m = 0.1 , 0.3 and 1

(but observed data doesn’t appear to match the model)