• By rearranging the equation for Hubble’s law, we can determine that the Hubble constant,?H0, is:

• 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 dif?cult 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

Step 1: From the data booklet:

Hubble’s Law:?v?≈ H0d

Step 2: Determine the Hubble constant,?H0, from the graph:

• • y–axis =?v?= 20, 000
  • x–axis =?d?= 305
  • gradient =?H0

Step 3: Calculate the gradient of the graph:

• • H0 = ?= 66 km s–1?Mpc–1

Step 4: Confirm your answer:

• • The Hubble Constant = 66 km s?1?Mpc?1

• 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