1. The Doppler Effect:
* Imagine a speeding ambulance. As it approaches, the sound waves it emits get compressed, making the siren sound higher pitched. When it moves away, the sound waves get stretched, making the siren sound lower pitched. This is called the Doppler effect.
* Light waves behave similarly. If a light source is moving towards us, its light waves get compressed, causing a shift towards the blue end of the spectrum (blueshift). If it's moving away, the light waves get stretched, causing a shift towards the red end of the spectrum (redshift).
2. Redshift and the Expanding Universe:
* The universe is expanding, meaning galaxies are moving further apart.
* As galaxies move away from us, their light gets stretched, causing a redshift.
* The amount of redshift (how much the light is shifted towards red) is directly proportional to the speed at which the galaxy is moving away from us.
3. Measuring Redshift:
* Astronomers use spectrometers to analyze the light from distant galaxies.
* Spectrometers split light into its different wavelengths, allowing scientists to identify specific spectral lines (unique patterns of light emitted by specific elements).
* By comparing the observed wavelengths of these lines to their known laboratory values, astronomers can determine how much the light has been shifted and calculate the galaxy's recessional velocity (how fast it's moving away).
4. Hubble's Law:
* Edwin Hubble discovered a relationship between a galaxy's distance and its recessional velocity: the further away a galaxy is, the faster it's moving away from us.
* This relationship is known as Hubble's Law, and it's a cornerstone of modern cosmology.
In summary, redshift provides a direct measure of how fast a galaxy is moving away from us due to the expansion of the universe. The greater the redshift, the faster the galaxy is receding.
Important Note: While redshift tells us how fast galaxies are moving *away* from us, it doesn't tell us about their movement *within* their local groups.
