On a cosmic scale the expansion of the universe means that almost everything outside our own galactic supercluster is moving away from us at high speed - and the further away the faster it's received. Whether in our own galaxy or elsewhere, most objects in space are moving towards or away from us. In this case, the red/blueshift is ultimately due to special relativity (the movement of objects relative to an observer in spacetime). In astronomical terms, the most common cause by far for red/blueshifting is an object's relative velocity towards or away from Earth. (The other possible cause is gravitational redshift, see next section) Causes So a cosmologist can tell from the spectra they detect, what original frequencies were emitted, and they can also be absolutely sure whether the light or radio or other waves they detect always were that frequency, or were originally emitted at a different frequency but has been red or blue shifted by some amount (=received at a lower or higher frequency), and that this is due to their high relative velocities. The difference between that and the frequency we actually detected, is the red or blue shift that the radiation has experienced. Knowing this, we can be sure what the original frequencies for that interaction or element would "really" have been. It is so specific, that we can often even identify the exact interaction that gave rise to the radiation (specific interactions usually have well known energies for the photons they produce). In simple terms, it's possible to look at a pattern of frequencies (often drawn as " spectral lines" in graphical form), and be sure which lines represent what element. These patterns of frequencies are different for each element and act like a "fingerprint". The frequencies related to each source commonly show up as a pattern of very specific frequencies, or a distribution of frequencies, not just one frequency. We also know exactly what frequencies hydrogen gas clouds can absorb as light travels through them (so that specific frequency is "missing" when we see it). For example, we know exactly what frequencies of light, excited hydrogen can emit when it loses energy. Many kinds of light we see in the universe are very well defined. The same kind of statements, but inverted, are true for blue shifted light (orange light seen as yellow/white, xrays received as gamma rays, etc) Importance So long as the explanation is based on the relative velocity of the emitter and observer, the effect of gravity, or the expansion of space (which actually "stretch" or "compress" the wavelength of each photon), and not due to factors such as filtering of light, which just bias the photons received. ultraviolet light perceived as blue visible light.yellowish/white visible light from a star like our sun that an extragalactic observer detects as visible but more orange tinted due to their velocity,.gamma rays emitted from a distant galaxy that we detect as x rays,.In simple terms, light, radio, gamma waves - any electromagnetic radiation at all - from a source that is "red shifted" is a way to say that it is received (by us) at a lower energy than it "really" was emitted (assuming a suitable reference frame).īut it could be any radiation at all. "Red" and "blue" in this context are shorthand ways to say "towards longer wavelengths/lower energies" (red) and "towards shorter wavelengths/higher energies" (blue), because in the visible light spectrum, red is at the low energy end of what we can see, and blue at the high energy end. Redshifts doesn't actually mean the light is red, or was ever red. It is the fixed locations of the absorption lines in the spectrum that allow identifying the element absorbing those lines. A lower frequency shift is called a "red shift". In contrast, if a star is moving away from the earth, its light is shifted to lower frequencies on the color spectrum (towards the orange/red/infrared/microwave/radio end of the spectrum). The faster a star moves towards the earth, the more its light is shifted to higher frequencies. A higher frequency shift is called a "blue shift". If a star is moving towards the earth, its light is shifted to higher frequencies on the color spectrum (towards the green/blue/violet/ultraviolet/x-ray/gamma-ray end of the spectrum). When a star emits light, the color of its light as observed on earth depends on its motion relative to earth. Note that this is a schematic diagram and not actual data. It just means that the entire spectrum is shifted up in frequency. A complementary answer to Chris's, the middle row is the spectrum at rest.Ī blue shift does not mean that the object ends up blue.
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