The Hubble Constant
- By rearranging the equation for Hubble’s law, we can determine that the Hubble constant, H0, is:
H0 ≈ 
- Where:
- v = recessional velocity of an object (km s–1)
- d = distance between the object and the Earth (Mpc)
- Ho = Hubble constant (km s–1 Mpc–1)
- The value for the Hubble constant has been estimated using data for thousands of galaxies
- The latest estimate of the Hubble constant based on CMB observations by the Planck satellite is:
H0 = 67.4 ± 0.5 km s−1 Mpc−1 (Planck Collaboration VI 2020)
- It is difficult to be certain about just how accurate the values for the Hubble constant are
- This is due to the random and systematic errors involved when calculating the distance to a galaxy or star
Dark Matter
- We would expect the velocity of an object within a galaxy to decrease as it moves away from the galaxy's centre because of weakening gravitational field strength
- This is observed in smaller mass systems, like the solar system where planets orbiting furthest from the Sun have the slowest orbital velocity
- This is not the case in bigger mass systems like entire galaxies
- Mass is not actually concentrated in the centre of galaxies; it is spread out
- All the observable mass of a galaxy is, however, concentrated in its centre, so there must be another type of matter we can't see, called Dark Matter
- Dark matter is defined as:
Matter which cannot be seen and that does not emit or absorb electromagnetic radiation
- Dark matter cannot be detected directly through telescopes
- It should make up 27% of the mass in the universe
- It is detected based on its gravitational effects relating to either the rotation of galaxies or by the gravitational lensing of starlight
