Redshift

Aim: To investigate whether the shift in wavelength depends on how far away the galaxy is.

Part A — Is the line shifted at all?

1. Select Galaxy A.
2. Drag the cursor onto the dark hydrogen line in the laboratory reference spectrum and read its wavelength. This is λ₀.
3. Now drag the cursor onto the dark line in the galaxy spectrum and read its wavelength. This is λ.
4. Is the galaxy's line at the same wavelength as the reference line? Towards which end of the scale has it moved — shorter or longer wavelengths?

Part B — Does the shift depend on distance?

1. Work through all four galaxies. For each one, record the galaxy name, its stated distance, λ₀ and λ in your table.
2. Look down your table: as the stated distance increases, what happens to the galaxy wavelength λ?
3. Predict what λ would read for a galaxy at 1500 Mly, then explain your reasoning.

Part C — How big is the shift?

1. For each galaxy, calculate the change in wavelength and the fractional change, and type them into your table.
2. Calculate the speed at which each galaxy is moving away (you will need the speed of light, 3 × 10⁸ m/s) and add it to your table.
3. On paper, plot speed against distance for your four galaxies. What shape is your graph? Does doubling the distance double the speed?

Part D — What does this tell us about the universe?

1. Astronomers see this same pattern for distant galaxies in every direction they look. What does your table say about the direction all four galaxies are moving — towards us or away from us?
2. If more distant galaxies are receding faster, what does that suggest is happening to the space between galaxies?
3. Imagine running your graph backwards in time. Where would all the galaxies have been long ago, and which theory of the universe's origin does your data support?