The galaxies in the Universe may be assumed to be distributed uniformly through space. In a simplified model, the average separation between two neighbouring galaxies of equal mass is 1.6×1023 m1.6 \times 10^{23}\text{ m}1.6×1023 m. Each galaxy is situated at the center of a cubical region of space of volume 4.1×1069 m34.1 \times 10^{69}\text{ m}^34.1×1069 m3.
There are, on average, 1.5×10111.5 \times 10^{11}1.5×1011 stars in each galaxy, and the mass of an average star is estimated to be 1.8×1030 kg1.8 \times 10^{30}\text{ kg}1.8×1030 kg.
Calculate the mass of an average galaxy.
Estimate the gravitational force between two neighbouring galaxies.
Calculate the mean density of the Universe in this model.
Suggest why the actual mean density of the Universe might differ from the value calculated in (iii).
66 exam-style questions on OCR A A Level Physics Astrophysics and cosmology, covering Stars, Electromagnetic radiation from stars, and Cosmology. Each one has a worked solution and a mark scheme showing where the marks go.