The Doppler Effect explains how the relative motion between a source and an observer changes the observed frequency of waves. When a light source moves away from an observer, the wavefronts are 'stretched' out, increasing the wavelength.
In cosmology, red-shift is primarily caused by the expansion of space itself. As the space between galaxies expands, the light traveling through that space is stretched along with it, a phenomenon known as cosmological red-shift.
The relationship between the change in wavelength () and the velocity of the source () is given by the ratio: where is the speed of light. This formula applies when the recession speed is much lower than the speed of light.
Observations show that light from more distant galaxies exhibits a greater red-shift than light from closer galaxies. This implies that more distant galaxies are receding from us at higher velocities.
This proportional relationship between distance and speed suggests that the universe is expanding uniformly in all directions. If every point is moving away from every other point, the entire 'fabric' of the universe must be growing.
By 'rewinding' this expansion, scientists conclude that the universe must have originated from a single, extremely hot and dense point. This concept forms the basis of the Big Bang Theory, with red-shift serving as one of its strongest pieces of evidence.
| Feature | Red-shift | Blue-shift |
|---|---|---|
| Relative Motion | Source moving away from observer | Source moving toward observer |
| Wavelength Change | Increases (Stretches) | Decreases (Compresses) |
| Frequency Change | Decreases | Increases |
| Spectral Position | Moves toward the red end | Moves toward the blue end |
It is vital to distinguish between Doppler red-shift (caused by local motion through space) and Cosmological red-shift (caused by the expansion of space itself). While they look similar in a spectrum, the latter affects all distant objects regardless of their individual 'peculiar' velocities.
Red-shift is an observation of light properties, whereas the Big Bang is the theoretical model that explains why that red-shift occurs on a universal scale.
Identify the Shift: In exam questions, look for the pattern of black absorption lines. If the pattern is identical but shifted to the right (longer wavelengths), it is a red-shift.
Recall the Relationship: Always remember the 'Distance-Speed' rule: The further away a galaxy is, the faster it moves, and the larger its red-shift will be. This is often tested using graphs of Red-shift vs. Distance.
Check the Units: When using the Doppler formula, ensure that the change in wavelength () and the original wavelength () are in the same units (e.g., nanometers) so they cancel out.
Common Mistake: Do not assume a red-shifted galaxy actually 'looks red' to the naked eye. Red-shift refers to the displacement of spectral lines, not necessarily the overall perceived color of the object.
