Explanation:
Doppler effect is observed in the difference between the frequencies emitted by an object and the frequencies received by an observer due to the relative speed between them. The first to describe such an effect mathematically was Johann Christian Doppler. Studies show us that the Big Bang or "Big Bang" theory explosion ", a term suggested by Fred Hoyle in 1950, means that the Universe started in a small space with all its mass and high concentrated density. After 1923, scientists found that the Universe was more than just the Milky Way, and today it is known that it is just one of over 400 billion galaxies observed.
Since light is also a wave, it behaves in the same way, which is why through this frequency difference one can determine the speed of stars and galaxies relative to the earth. In the case of light, however, because it is highly fast, the difference in frequency is noticeable only if the source is also fast. In this case, when the star/galaxy is moving away from Earth, the light we receive has an apparent frequency less than the actual frequency emitted, with a redshift deviation, which has the lowest visible frequency. When the star/galaxy is approaching Earth, the light we receive has an apparent frequency greater than the actual frequency emitted, with a deviation to blue (blueshift), which has the highest visible frequency.
With the expansion of the universe is the same thing. The universe expands without increasing the size of objects such as stars, planets, galaxies, etc, but increases the distance between them, and the wavelength of light also stretches and gets bigger, causing the redshift seen by an observer. This explanation is different from that of the Doppler Effect since here the objects are not moving relative to each other so the name is changed to cosmological redshift, but the physical treatment is the same as the Doppler effect, so this theory is still valid.
