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Hubble’s Law
Hubble's Law is a description of an observation made by Edwin Hubble in 1929.
Hubble noted that objects in the universe:
1) appeared to be moving apart from each other
2) and that the further a galaxy was from an observer, the faster it would move.
This observation is used as the basis for Hubble's constant, H0, an estimate of the galaxy's rate of expansion
which is an important concept in cosmology, used to support a number of discussions about the nature of the
universe. Hubble's constant is actually better termed “Hubble's Variable,” because it is time dependent.
V=H0D
where: V is the velocity of the galaxy, determined by the red shift/Doppler effect (in km/s)
H0 is the Hubble Constant (≈75 km/s/Mpc), determined by the Hubble Telescope in 2009
D is the distance from Earth in Mpc (megaparsecs)
The Hubble constant represents the expansion rate of the Universe.
NOTE: Both velocity and distance cannot be measured directly!!! This means the equation could have drastic
errors (with big implications for the age of our universe and the Hubble constant).
Exercise 1: Determining the Age of the Universe
The Hubble constant is expressed in units of km/(s × Mpc). Since km and Mpc are both units of distance, we
can cancel them out and express H in terms of 1/sec. Simply convert the Mpc into km, and cancel the units of
distance. The conversion factor is 1 Mpc = 3.086×1019 km.
a) The Hubble constant is theorized to be approximately 75 km/s/Mpc. Convert your value of H into units of
1/s by converting Mpc into km.
H=
b) Now convert this into seconds by inverting it (1/H from part a):
Age of the Universe =
s
c) How many years is this? (convert from seconds to years by knowing there are 60 seconds in a minute, 60
minutes in an hour, etc.)
Age of the Universe=
yrs
Exercise 2:
Use the above table to determine the speed of each galaxy using the Hubble
constant.
Exercise 3:
Use the table above to determine the distance to each galaxy in mpc.
Exercise 4:
Use your answers in question 2 to determine the distance of each galaxy in km. The conversion factor is 1 Mpc =
3.086×1019 km.
Exercise 5:
Use your answers in question 3 to determine the distance of each galaxy in light years. The conversion factor
is 1 light years = 9.4605284×1012 km.