Hubble's law and the Big Bang theory — common mistakes

To calculate the age of the universe in years, you must convert Hubble's constant from $\text{km s}^{-1}\text{Mpc}^{-1}$ to $\text{s}^{-1}$. Remember that $1 \text{ Mpc} \approx 3.09 \times 10^{19} \text{ km}$. After finding the age in seconds, convert it to years ($1 \text{ year} \approx 3.15 \times 10^7 \text{ s}$).

When explaining redshift in an exam, remember to explicitly link it to the expansion of space (cosmological redshift) and state that it indicates recessional velocity. For Hubble's Law, ensure you use the correct units for $H_0$ and maintain consistency in your calculations, especially with distance (e.g., Mpc).

The primary evidence supporting the Big Bang Theory includes the observed galactic redshift (indicating universal expansion), the cosmic microwave background radiation (a remnant heat from the early universe), and the observed abundance of light elements like hydrogen and helium in the universe.

While called 'Hubble's constant,' its value has been refined over time as measurement techniques improve. More significantly, it represents the expansion rate at our current moment in cosmic time. Evidence suggests the expansion rate has changed over the universe's history. However, for A-Level Physics calculations, it's treated as a fixed constant.

Astronomers identify specific absorption or emission lines in galactic spectra that correspond to known elements (e.g., hydrogen, helium). They can measure the exact wavelengths of these lines in laboratories on Earth (the 'original' or 'rest' wavelength) and then compare them to the shifted wavelengths observed from distant galaxies to determine the redshift.