Cosmology
5 marks · NormalA student is studying two galaxies, Galaxy X and Galaxy Y. She notices that the absorption lines in the spectrum of Galaxy Y are shifted more towards the red end of the spectrum than those of Galaxy X.
Describe what is meant by the term 'red-shift' in the context of light spectra from galaxies.
Explain what the observation about the absorption lines tells us about the relative velocities and distances of Galaxy X and Galaxy Y from Earth.
State how this evidence is related to the theory that the universe began with a Big Bang.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Red-shift refers to the increase in wavelength (shift towards the red end of the spectrum) of light observed from distant galaxies, caused by these galaxies moving away from the observer. · must show definition of red-shift with wavelength and movement
(b) A greater red-shift in Galaxy Y means it is moving away from us faster than Galaxy X. Since greater red-shift also occurs in more distant galaxies, Galaxy Y is further from Earth than Galaxy X and receding at a higher speed. · must show comparison of velocities, comparison of distances
(c) It supports the idea that the universe is expanding from a single origin, as predicted by the Big Bang theory. · must show link to Big Bang and expansion
Common mistakes: Confusing red-shift with blue-shift; Not linking red-shift to universe expansion
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Defines red-shift as shift toward longer wavelength/red end, or galaxies moving away.
B2 (a) · 1 mark · Describes this as being due to galaxies moving away from the observer.
M1 (b) · 1 mark · Uses observation of greater red-shift to compare velocities correctly.
B3 (b) · 1 mark · Correctly links greater red-shift to greater distance from Earth.
B4 (c) · 1 mark · Links red-shift evidence to universe expansion/the Big Bang theory.
Worked solution
a) 'Red-shift' means observed light from a galaxy has increased wavelength, shifted towards the red end of the spectrum, because the galaxy is moving away from us.
b) Galaxy Y showing a greater red-shift than Galaxy X indicates Galaxy Y is moving away from Earth faster than Galaxy X. Because greater red-shift is seen in more distant galaxies, Galaxy Y is also further away.
c) This supports the theory that the universe is expanding (as predicted by the Big Bang) because most galaxies show red-shift, meaning they are moving away from a common point of origin.