An astrophysicist compiles observational data of Type Ia supernovae in eight distant galaxies to estimate the Hubble constant (H0H_0H0). The table below shows the distances of these galaxies from Earth, ddd, in Megaparsecs (Mpc\text{Mpc}Mpc), and their recessional velocities, vvv, in kilometres per second (km s−1\text{km s}^{-1}km s−1):
| Distance, ddd (Mpc\text{Mpc}Mpc) | Recessional velocity, vvv (km s−1\text{km s}^{-1}km s−1) |
|---|---|
| 80 | 5800 |
| 160 | 11500 |
| 240 | 16100 |
| 320 | 23000 |
| 400 | 27500 |
| 480 | 34200 |
| 560 | 38500 |
| 640 | 45100 |
The astrophysicist plots these data points on a scatter graph and draws a line of best fit through the origin, as shown below:
The gradient of this line of best fit is used to calculate the Hubble constant, which can then be used to estimate the age of the Universe.
Explain why using the Hubble constant calculated from this gradient can only provide an estimate of the age of the Universe.