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Question 20N.3.HL.TZ0.21

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Date November 2020 Marks available [Maximum mark: 5] Reference code 20N.3.HL.TZ0.21
Level HL Paper 3 Time zone TZ0
Command term Calculate, Explain, Outline Question number 21 Adapted from N/A
21.
[Maximum mark: 5]
20N.3.HL.TZ0.21
(a)

The light from a distant galaxy shows that z=0.11.

Calculate the ratio size of the universe when the light was emittedsize of the universe at present.

[1]

Markscheme

«R0R=»

11.11  OR  0.90  OR  90% ✓

 

Examiners report

Many candidates got the ratio upside down and ended up with R/R0 as 1.11.


This was not accepted as it would have required an identification of the variables. Perhaps candidates need to look more carefully at which R is which here. R is the current value of the scale factor in the data book, so R0/R = 0.9 was required.

(b)

Outline how Hubble’s law is related to z.

[1]

Markscheme

«Hubble’s » measure of v/recessional speed uses redshift which is z
OR
redshift (z) of galaxies is proportional to distance «from earth»
OR
combines v=Hd  AND  z=vc into one expression, e.g. z=Hdc ✓

OWTTE

Examiners report

To show the link between z and Hubble's law many rearranged formulae to obtain zc = Hd or similar. Others stated that Hubble used redshift z to determine that v was proportional to distance. Either approach was allowed.

(c)

Hubble originally linked galactic redshift to a Doppler effect arising from galactic recession. Hubble’s law is now regarded as being due to cosmological redshift, not the Doppler effect. Explain the observed galactic redshift in cosmological terms.

[3]

Markscheme

reference to «redshift due to» expansion of the universe, «not recessional speed» ✓

expansion of universe stretches spacetime / increases distance between objects ✓

«so» wavelength stretches / increases leading to observed redshift ✓

Examiners report

The galactic redshift was successfully explained by many in terms of the stretching of spacetime.

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