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Here is an example of Doppler sound effect. You can see the lines of sound waves get stretch apart as the source of the sound moves away from the person hearing the sound.

Because the source of the sound is moving away, the pitch or frequency of the sound gets lower as the waves get further apart.

The red shift of light from distant galaxies works in a similar way. The universe expanding. Space itself is literally stretching apart from itself. Light or electromagnetic radiation comes in many forms as shown in the image below;

Notice red light has a longer wavelength (700 nm) in the visible spectrum than green or blue/purple (400 nm). As space gets stretched, the light from distant galaxies also gets stretched. We see this by looking at light spectra from these galaxies. The absorption and emission lines from this light is shifted toward the red end of the spectrum:

The CMB or Cosmic Microwave Background radiation is the first light that was emitted by the earliest elements when the early universe cooled enough to allow for light to travel freely. We see it in all directions of the night sky. You can think of it as a glowing electromagnetic fossil left over from the beginnings of the universe. Those early photons were emitted about 300,000 years after creation. It is this light that we ‘see’ with radio telescopes. In fact, about 1% of static on your radio is this background radiation from the early universe. Scientists have even mapped it. See the image below

The green, yellow, and red spots are areas of greater concentration. We think these are due to random quantum fluctuations from particles in the early universe. They are also, likely, regions where the earliest stars and galaxies formed.

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