SME
Grade 12 Science · Earth & Space Science
The Milky Way, galaxies, the Big Bang and the expanding universe
Use this for “A galaxy is 200 Mpc away, and H₀ = 70 km/s per Mpc. How fast is it moving away?” and the age 1/H₀ gives.
Diagram
The farther a galaxy, the faster it moves away: 14,000 km/s at 200 Mpc.
This is an example. Type your own values to start a new problem.
v (km/s)
=
H₀ (km/s per Mpc)
×
d (Mpc)
t
Age from 1/H₀
worked out (not typed) · billion years
Formulas
v
=
H₀
×
d
14,000 = 70 × 200
t
=
977.8
H₀
14
=
977.8
70
Assumptions
•
Farther galaxies move away faster because space itself stretches.
•
1/H₀ is the age if expansion had never changed speed.
•
Nearby galaxies such as Andromeda can approach us.
Step-by-step
WE KNOW
H₀ = 70 km/s per Mpc, d = 200 Mpc
FIND
speed away (v), age from 1/H₀ (t)
Step 1 · Find speed away (v)
Use v = H₀ × d
Each
megaparsec
of
distance
adds
H₀
km/s
of
speed.
v
=
70
×
200
v
=
14,000
km/s
Step 2 · Find age from 1/H₀ (t)
Use t = 977.8 ÷ H₀
1
H₀
is
a
time;
977.8
turns
km/s
per
Mpc
into
billions
of
years.
t
=
977.8
70
t
=
14
billion
years
Check
Put the numbers back in. Both sides should match.
14,000 = 70 × 200 ✓
14
≈
977.8
70
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Refresh
More problem types
A galaxy’s redshift and distance
Use this for “A galaxy’s light is shifted to longer wavelengths, z = 0.03. How fast is it moving away, and how far is it?”
Space stretching: every galaxy moves apart
Use this for “Space doubles in size. How far does a galaxy 100 million light-years away move?”
Kinds of galaxies
Use this for “What do all galaxies have in common?” and for sorting galaxies by shape.
The history of the universe
Use this for “Put the events since the Big Bang in order. What is the microwave background?”