What is redshift?

Redshift refers to the phenomenon where light is shifted to longer wavelengths, which typically occurs when an object emitting light is moving away from the observer. This shift can be observed in the light from distant galaxies and is a key piece of evidence for the expanding universe model. The term "redshift" comes from the fact that as light moves away, its wavelengths stretch, making the light appear more red on the spectrum, which is at the longer wavelength end.

This effect is primarily due to the Doppler effect, where the frequency of waves changes based on the motion of the source relative to an observer. In astronomy, redshift is often used to measure how fast galaxies are receding from us and helps cosmologists understand the rate of expansion of the universe.

In contrast, the other options either incorrectly describe redshift or relate to different concepts in astrophysics. The shifting of light to shorter wavelengths pertains to "blueshift," which occurs when objects are moving closer to the observer. The measure of a star's temperature is typically determined using its color and spectral lines, not redshift. Lastly, the event horizon of a black hole is a boundary beyond which nothing can escape, which is unrelated to the concept of redshift at all.