Astro 321 Set 1: FRW Cosmology - Wayne Hu's Tutorials

Hubble Parameter
• Useful to define the expansion rate or Hubble parameter
H(t) ≡ 1
a
da
dt
= d ln a
dt
fractional change in the scale factor per unit time - ln a = N is also
known as the e-folds of the expansion
• Cosmic time becomes
t =
dt =
• Conformal time becomes
dt
η =
a =
d ln a
H(a)
d ln a
aH(a)