Redshift

Quasars are extremely far away from us, and unlike nearby galaxies redshift becomes important. Redshift is the shift in frequency/wavelength of an object- obviously towards wavelengths that are more red in this particular case. If an object is moving away from us, the wavelength of light is redshifted. If it is moving towards us, the object is blueshifted. A parallel on earth is the doppler effect of a siren. As a siren approaches us (say, on a ambulance), the noise we hear is blueshifted towards higher frequencies. If the siren is speeding away, the noise will be redshifted towards lower frequencies. The base frequency the siren emits is not changing, but the shifts in frequency due to its movement cause us to here a different tone.

The same thing happens with the light from galaxies, but we also have to deal with the expansion of the universe when dealing with galaxies. Objects far away from us are receding due to the expansion of the universe, so we are primarily concerned with redshift when dealing with quasars. One of the reasons why redshift is so important is that we can use cosmological redhisft formulas to calculate the distance to an object, giving us a window back in time billions of years in the case of quasars. When we look at an object at high redshift, we are seeing light that was emitted billions of years ago. There are many equations involving redshift and the doppler effect, but the two we are concerned with are those involving the time and wavelength of an observation as shown below, with Z being the redshift of the object:

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