Expanding Universe ::: Doppler Shift

The sudden change in pitch of a car horn as a car passes by an observer, was first explained in 1842 by Christian Doppler. This effect came to be known as Doppler Effect is the shift in frequency and wavelength of waves which results from a source’s or target’s motion.Although first discovered for sound waves, the Doppler effect holds true for all types of waves including light (and other electromagnetic waves). The Doppler effect for light waves is usually described in terms of colors rather than frequency. A red shift occurs when the source and observer are moving away from each other, and a blue shift occurs when the source and observer are moving towards each other. The red shift of light from remote galaxies is proof that the universe is expanding.Astronomers had previously noticed that nebulae emitted spectral lines of unidentified wavelength. It turned out that these actually were common emission wavelengths, severely Doppler-shifted because their sources were moving away from us, at speeds which could be an appreciable fraction of c. In fact, this "red shift" seemed a universal feature, present in all directions. It also suggested that all distant galaxies are moving away from us - the more distant they were, the faster their motion. Light is a wave that can be described in terms of its frequency, the number of wave peaks that pass by each second. A star zooming toward earth has its light waves squeezed together. We see these light waves as having a higher frequency than normal. Since blue is at the high-frequency end of the visible spectrum, we say the light from an approaching star is shifted toward blue, or blueshifted. Likewise, if a star is zooming away from us, any light it emits gets stretched. You see these stretched-out light waves as having a lower frequency. Since red is at the low-frequency end of the visible spectrum, we say that light from a receding star is shifted toward red, or redshifted.