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    1.

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

    [2]
    Verified
    Solution

    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»

    2.

    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]
    Verified
    Solution

    ALTERNATIVE 1 z=392−122122=2.21z=\frac{392-122}{122}=2.21z=122392−122​=2.21 ✔ RR0=«2.21+1=»3.21\frac{R}{R_0}=« 2.21+1=» 3.21R0​R​=«2.21+1=»3.21 ✔ ALTERNATIVE 2 RR0=392122\frac{R}{R_0}=\frac{392}{122}R0​R​=122392​ =3.21=3.21=3.21 ✔

    3.

    Estimate the age of the universe in seconds using the Hubble constant H_0 = 70 km s^(-1) Mpc^(-1)

    [2]
    Verified
    Solution

    H=«70×103(106×3.26×9.46×1015)=»2.27×10−18H=« \frac{70 \times 10^{3}}{(10^{6} \times 3.26 \times 9.46 \times 10^{15})}=» 2.27 \times 10^{-18}H=«(106×3.26×9.46×1015)70×103​=»2.27×10−18 « s−1^{-1}−1 » ✔ T=«12.27×10−18=»4.4×1017T=« \frac{1}{2.27 \times 10^{-18}}=» 4.4 \times 10^{17}T=«2.27×10−181​=»4.4×1017 s ✔

    4.

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

    [1]
    Verified
    Solution

    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 H0H_0H0​ ✔

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