The Hubble constant

This lesson covers:

  1. The relationship between distance and velocity of galaxies
  2. Hubble's law
  3. Hubble's constant
  4. How to use Hubble's constant to estimate the age of the universe

Distant galaxies are receding at a faster rate


Earth is located in the Milky Way galaxy. 

We have already seen that galaxies are moving away from one another, as shown by red shift. 

The graph below shows the relationship between the recessional velocity of these galaxies vs the distance from Earth.

Graph showing the relationship between the velocity of distant galaxies and their distance from Earth, demonstrating Hubble's law.

The graph shows a directly proportional relationship between the velocity of distant galaxies and their distance from Earth.

The greater the distance from Earth, the greater the velocity.

Hubble's law


Hubbles law states that the recessional velocity of distant galaxies is proportional to their distance from Earth.

v = Ho x d


Where:

v = recessional velocity of galaxy (km/s)

Ho = Hubble's constant (s-1) = 2.3 x 10-18 s-1

d = distance from Earth (km)

Worked example: Hubble's law


A galaxy is receding with a velocity of 7,500 km/s.

Calculate the distance of the distant galaxy from Earth.

Estimating the age of the universe


Hubble's law can be used to estimate the age of the universe. 

Hubble's constant has the units s-1. Calculating the reciprocal of Hubble's constant gives an estimate for the age of the universe in s.

According to Hubble's law, what is the relationship between the recessional velocity of distant galaxies and their distance from Earth?

They are directly proportional

They are inversely proportional

They are unrelated

There is no relationship

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How can Hubble's constant be used to estimate the age of the universe?

By measuring the velocity of galaxies

By comparing the recessional velocity of galaxies

By measuring the distance of galaxies

By calculating 1 divided by Hubble's constant

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A galaxy has a recessional velocity of 3,200 km/s. Ho = 2.3 x 10-18 s-1

How far away from Earth is the galaxy?

7.36 x 10-15 km

1.39 x 1021 km

7.19 x 10-22 km

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Galaxy A has a recessional velocity of 6,500 km/s. Galaxy B has a recessional velocity of 4,000 km/s.

Which of the following is true?

Galaxy A is closer to Earth than galaxy B

Galaxy A is further away from Earth than Galaxy B

Galaxy B is travelling faster than galaxy A

Galaxy A is travelling slower than galaxy B

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