Observations of distant galaxies provide evidence that the universe is expanding, supporting the Big Bang theory.
Redshift
When a source of light moves away from an observer, the observed wavelength of the light is longer (shifted toward the red end of the spectrum). This is called redshift. It is the same principle as the Doppler effect for sound, applied to light.
Evidence for the expanding universe
- Light from nearly all distant galaxies is redshifted.
- The further away a galaxy is, the greater its redshift.
- This means more distant galaxies are moving away faster.
These observations show that the universe is expanding: space itself is stretching, carrying the galaxies apart.
The Big Bang theory
The Big Bang theory states that the universe began from a single point of extremely high temperature and density approximately 13.8 billion years ago. It has been expanding and cooling ever since.
Key evidence:
- Redshift of galaxies: shows the universe is expanding, consistent with everything starting from one point.
- Cosmic microwave background (CMB) radiation: a faint microwave radiation detected from all directions in space. It is the cooled remnant of the intense radiation from the early universe, now stretched to microwave wavelengths by the expansion of space.
What the Big Bang theory does not say
- It does not describe an explosion into existing space. Space itself was created in the Big Bang.
- It does not explain what happened before the Big Bang or what caused it.
Common errors and how to correct them
Saying redshift means galaxies are moving through space. The galaxies are being carried apart by the expansion of space itself, not moving through a fixed space.
Describing the Big Bang as an explosion. The Big Bang was the rapid expansion of space from a very hot, dense state. It is not an explosion in the conventional sense.
Confusing blueshift and redshift. Redshift = moving away (most galaxies). Blueshift = moving toward (some nearby galaxies, like Andromeda).
How to apply this in an exam
State two pieces of evidence for the Big Bang: redshift of galaxies and cosmic microwave background radiation. Explain what each one tells us. When describing redshift, link it to the expansion of space and the increasing distance between galaxies.
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