SME
Grade 12 Science · Problem type
Space stretching: every galaxy moves apart
Use this for “Space doubles in size. How far does a galaxy 100 million light-years away move?”
Diagram
Every distance grows 2 times: a galaxy twice as far moves twice as far in the same time, so it recedes twice as fast.
No galaxy is the center.
This is an example. Type your own values to start a new problem.
D (million light-years)
=
a
×
d (million light-years)
Δ
How far it moved
worked out (not typed) · million light-years
Formulas
D
=
a
×
d
200 = 2 × 100
Δ
=
D
−
d
100 = 200 − 100
Assumptions
•
Space itself stretches, carrying the galaxies apart.
•
Every distance grows by the same factor, so from any galaxy the others all seem to move away.
•
Twice as far moves twice as far: that is Hubble’s law.
Step-by-step
WE KNOW
a = 2, d = 100 million light-years
FIND
distance after (D), how far it moved (Δ)
Step 1 · Find distance after (D)
Use D = a × d
Every distance grows by the stretch factor.
D
=
2
×
100
D
=
200
million
light-years
Step 2 · Find how far it moved (Δ)
Use Δ = D − d
How far it moved is the growth in its distance.
Δ
=
200
−
100
Δ
=
100
million
light-years
Check
Put the numbers back in. Both sides should match.
200 = 2 × 100 ✓
100 = 200 − 100 ✓
Topic check
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