Redshift, Hubble's Law and the Expanding Universe

Light stretched by distance

When we split the light from a distant galaxy, its features are shifted toward the red end of the spectrum. This redshift is the key clue that the Universe is not standing still. This module measures redshift, uses Hubble's law to link a galaxy's speed to its distance, and shows why the whole Universe must be expanding.

The language of cosmology

Four ideas run through this module:

Why is it redshifted?

Why is the light from distant galaxies redshifted?

• Because the galaxies are moving away from us
• Because the galaxies are moving toward us
• Because the galaxies are extremely hot
• Because the galaxies are made only of red stars

Work out the redshift

A galaxy emits light at a wavelength of 500 nm, but we observe it at 510 nm. Using redshift z = (lambda - lambda0) / lambda0, where lambda0 is 500 and lambda is 510, what is the redshift z?

Two ways to explain expansion

An expanding Universe once fitted two rival theories. Later observations came to favour the Big Bang.

Hubble's law

Hubble's law says a galaxy's recession speed is proportional to its distance: v = H0 x d, where H0 is the Hubble constant. So the further away a galaxy is, the faster it is moving away from us. Dividing 1 by the Hubble constant gives a rough estimate of the age of the Universe.

Work out the recession speed

A galaxy is 100 Mpc away. Using Hubble's law v = H0 x d, with a Hubble constant H0 of 70 km/s per Mpc, what is its recession speed, in km/s?

Match each symbol to its meaning

Cosmology in words

Light from a distant galaxy is shifted to longer, _____ wavelengths because the galaxy is _____ from us. The greater a galaxy's distance, the greater its _____. Hubble's law links recession speed to distance through the Hubble _____. That the galaxies are all receding is evidence that the Universe is _____.

Which two are true?

Select the TWO statements that are true about redshift and Hubble's law.

• A larger redshift means a galaxy is moving away faster
• The further away a galaxy, the greater its recession speed
• Redshift shows that galaxies are moving toward us
• Hubble's law says all galaxies are the same distance away

From redshift to the Big Bang

Put the steps of the reasoning into a logical order.

• Light from distant galaxies is redshifted
• This means those galaxies are receding from us
• The further a galaxy, the greater its redshift and speed
• So the whole Universe is expanding
• An expanding Universe supports the Big Bang theory

Redshift and recession

Tap the TWO statements that are TRUE.

• Redshift is caused by galaxies moving away from us.
• Hubble's law states that recession speed is proportional to distance.
• A blueshift would mean a galaxy is moving away from us.
• The Hubble constant tells us the temperature of a galaxy.
• Nearby galaxies always show the largest redshift.

Measure the Universe

Work through three cosmology decisions.

• A galaxy recedes at 14000 km/s. With a Hubble constant of 70 km/s per Mpc, how far away is it? Use d = v divided by H0.
• Galaxy A has a larger redshift than Galaxy B. What can you conclude?
• Galaxies in every direction are rushing apart. What does this suggest?

Evidence for an expanding Universe

Explain how the redshift of distant galaxies provides evidence that the Universe is expanding. Refer to Hubble's law in your answer.

• Explain what redshift is and why it happens
• Explain that more distant galaxies show greater redshifts
• State Hubble's law, linking recession speed to distance
• Explain how this shows the Universe is expanding and supports the Big Bang