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Date	November 2019	Marks available	2	Reference code	19N.3.SL.TZ0.11
Level	Standard level	Paper	Paper 3	Time zone	0 - no time zone
Command term	Outline	Question number	11	Adapted from	N/A

Question

Outline how the light spectra of distant galaxies are used to confirm hypotheses about the expansion of the universe.

[2]

a(i).

Light from a hydrogen source in a laboratory on Earth contains a spectral line of wavelength 122 nm. Light from the same spectral line reaching Earth from a distant galaxy has a wavelength of 392 nm. Determine the ratio of the present size of the universe to the size of the universe when the light was emitted by the galaxy.

[2]

a(ii).

Estimate the age of the universe in seconds using the Hubble constant H₀ = 70 km s^–1Mpc^–1.

[2]

b(i).

Outline why the estimate made in (b)(i) is unlikely to be the actual age of the universe.

[1]

b(ii).

Markscheme

spectra of galaxies are redshifted «compared to spectra on Earth» ✔

redshift/longer wavelength implies galaxies recede/ move away from us
OR
redshift is interpreted as cosmological expansion of space ✔

«hence universe expands»

NOTE: Universe expansion is given, so no mark for repeating this.
Do not accept answers based on CMB radiation.

a(i).

ALTERNATIVE 1

$z = \frac{392 - 122}{122} = 2.21$ ✔

$\frac{R}{R_{0}} = « 2.21 + 1 = » 3.21$ ✔

ALTERNATIVE 2

$\frac{R}{R_{0}} = \frac{392}{122}$ ✔

=3.21 ✔

a(ii).

$H = « \frac{70 \times 10^{3}}{(10^{6} \times 3.26 \times 9.46 \times 10^{15})} = » 2.27 \times 10^{- 18} « s^{- 1} »$ ✔

$T = « \frac{1}{2.27 \times 10^{- 18}} = » 4.4 \times 10^{17} s$ ✔

b(i).

because estimate assumes the «present» constant rate of expansion ✔
it is unlikely that the expansion rate of the universe was ever constant ✔
there is uncertainty in the value of H₀ ✔

NOTE: OWTTE

b(ii).

Examiners report

[N/A]

a(i).

[N/A]

a(ii).

[N/A]

b(i).

[N/A]

b(ii).

Syllabus sections

Option D: Astrophysics » Option D: Astrophysics (Core topics) » D.3 – Cosmology

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