This deep dive covers Evidence for an Expanding Universe within Red Shift & Big Bang for GCSE Physics. Revise Red Shift & Big Bang in Space Physics for GCSE Physics with 13 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 3 of 14 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 3 of 14
Practice
13 questions
Recall
12 flashcards
🚀 Evidence for an Expanding Universe
Figure 2: Hubble's observations — recession velocity is proportional to distance, meaning more distant galaxies move away faster
Hubble's Key Observations
By measuring the red shift of hundreds of galaxies, Hubble established three key facts:
- The light from almost all distant galaxies is red-shifted
- This means almost all galaxies are moving away from us
- The greater the distance to a galaxy, the greater its red shift — meaning more distant galaxies are moving away faster
This relationship is described by Hubble's Law: recession velocity is proportional to distance. The constant of proportionality is the Hubble constant, which can be used to estimate the age of the universe.
What the Expansion Really Means
It is important to understand that galaxies are not moving through space away from us. Space itself is expanding, carrying the galaxies with it. This is why every galaxy (beyond our local group) is receding, and why more distant ones appear to recede faster — there is simply more space between them and us that is expanding.
Quick Check: Astronomers measure the spectrum of a distant galaxy and find that all absorption lines are shifted towards longer wavelengths compared to laboratory measurements. What does this tell them about the galaxy?
The galaxy is red-shifted, meaning it is moving away from us. The light waves have been stretched to longer (redder) wavelengths as the galaxy recedes. This is evidence that the universe is expanding.