2. Key Observations Flashcards
What is the equation for the flux from a star?
f = L_* / 4 pi r^2
- Luminosity of the star
- distance r away
What are the units of the flux from a star?
W/m^2
What is the “horizon distance”?
That we can’t see stars/light hasn’t reached us yet beyond the distance c*t_0 where t_0 is the age of the universe
What is the consequence of the Universe being finite in age in relation to the night sky?
Light hasn’t reached us from all stars so it is not infinitely bright
State the isotropic assumption
There is no preferred direction in the Universe
state the homogenenous assumption
There is no preferred location in the Universe
What is the equation for redshift?
z = Δλ / λ
State Doppler’s law
z = v/c
What is the equation for the Hubble constant?
v = H_0 r z = H_0 r/c
What is Hubbles constant?
H_0 = 68 ± 2 km/s Mpc
In which regime is Hubble’s constant, H_0, valid?
for 0 < z < 0.2
What is “Hubble flow”
Galaxies moving away from each other
Define the scale factor
A scalar quantity that is a function of time
It relates the distance between two galaxies which increases over time due to expansion.
r(t) = a(t) r(t_0)
State the equation for Hubble’s law
H = a dot (t) / a(t)
H - Hubble parameter
How is the Hubble constant obtained from Hubble’s law?
Evaluate H = a dot (t) / a(t) at t = t_0
How can the age of the Universe be estimated from Hubble’s constant, and what is it?
t_0 = r / v = r / H_0 r = (H_0) ^ -1
(14.4 ± 0.4) Gyr
What is the Hubble distance?
c t_0 = c / H_0 = (4380 ± 130) Mpc
State the relationship between redshift, wavelength and the scalefactor
1 + z = λ_obs / λ_em = a_obs / a_em
Describe how CMBR has evolved over time
Used to be much hotter (2970 K), now 2.73 K
T(t) is proportional to 1/a(t)
