When the Universe doubles its size, the distance between two galaxies is doubled. Galaxies are scattered in the Universe, and are actually rather stationary, but because space itself expands, all galaxies recede from each other. The best spectrographs can measure the speed of stars many light-years away with an accuracy of 1 m/s. If light if shifted from, say, ultraviolet to blue, or from red to infrared, we still say that it has been redshifted, even though the "final" wavelength is not red.Īn analogous effect is met when the sirene from an ambulance approaching us sounds more high-pitched, while it's more low-pitched when it recedes again.įor light, however, the cause is fundamentally different, and is described by the theory of relativity.įrom lab experiments we know at which wavelengths the different elements emit light.īy measuring how much the light from a distant object has been shifted, we can (very accurately) calculate how fast it moves. The terms describe the "direction" of the shift In both cases, the reason is the Doppler effect. In the first case the light is said the be blueshifted, in the second it's redshifted. a star or a flashlight - is moving toward us, the wavelength of the light gets shorter and will hence be shifted toward the blue end of the spectrum.Ĭonversely, if the source is moving away from us, the wavelength gets longer, and hence redder.
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