In fact though proper distance , it IS moving away from Earth with recession velocity V, caused by the expansion of the universe. So it should have a Doppler effect based on this recession velocity AND a cosmological expansion redshift should also take effect because the light gets stretched with the expansion of the universe.
Even though the recession velocity is not due to a peculiar movement, it means that the source of light is moving away from the observer and hence the light should be redshifted and on top the light gets redshifted on its way through expanding space.
Please correct me or tell me if I am right or wrong, I have spent a lot of time reading but still don't fully understand this. After considering benrg's comments, I realize that my first answer contained too strong statements about the relation between the two redshifts. I try here to moderate my answer, but you might want to accept their answer instead. It is common to think of the two redshifts as having nothing to do with each other. Because the cosmological redshift doesn't involve movement through space, it is often considered completely different from the Doppler.
However, it is also possible to derive the cosmological redshift by considering it as infinitely many infinitesimally small Doppler shift e. Lewis I admit that I'm not well enough versed in general relativity to be certain about my statements, but just because an infinitesimally small patch of spacetime is flat doesn't necessarily mean that infinitely many such patches add up to be flat.
However, as benrg says, in GR there is only one redshift. The reason I think it makes sense to view the Doppler shift and the cosmological redshift as two separate mechanisms is the following:. In principle you could have a universe non-capitalized, since it's not our universe, the Universe that were static when a distant galaxy emitted a photon, then at some point expanded quickly by a factor of 2, and then again is static.
In this hypothetical case, the observer would still measure the photon to have been redshifted by a factor of 2 i. That this is true can bee seen from considering the mathematical derivation of the cosmological redshift see e. In the real Universe, galaxies move through space i. Each of these terms give rise to a redshift, but through two very different mechanisms. Only the latter term is called a Doppler shift.
There's only one kind of redshift in general relativity. The cosmological redshift, gravitational redshift, and special-relativistic redshift formulas are special cases of it, which apply to spacetimes with certain symmetries. If you put approximate Minkowski coordinates on a patch of spacetime that's small enough to be approximately flat, you'll find that objects in that patch that are moving with the Hubble flow are moving away from each other in the special-relativistic sense with respect to to those coordinates.
If you use the special-relativistic redshift formula to calculate the redshift between objects A and B on that patch, then do the same in an approximately flat patch containing B and C, and keep doing that until you get to a very distant object Z, and multiply all those redshift factors together, you'll get the correct cosmological redshift between A and Z, up to an error arising from the deviation of each patch from flatness.
Redshift, on the other hand, is a property determined by the shift in wavelength of spectral lines emitted by the gas in an object. Now, a redshifted object will also have a redshifted peak to its continuum emission, so it also will appear redder.
Edwin Hubble used the Doppler effect to determine that the universe is expanding. Hubble found that the light from distant galaxies was shifted toward lower frequencies, to the red end of the spectrum.
If the galaxies were moving toward Hubble, the light would have been blue-shifted. Definition: Doppler Effect refers to the change in wave frequency during the relative motion between a wave source and its observer.
For instance, when a sound object moves towards you, the frequency of the sound waves increases, leading to a higher pitch. Measuring the Doppler Shift is a satellite tracking technique for determining the distance between the satellite and the receiver at the time of closest approach as well as the time itself.
As a satellite approaches, the frequency appears raised relative to the actual transmission frequency. Doppler shift is an apparent change in frequency and, correspondingly, wavelength due to the relative motion of two objects. For a Doppler radar system to measure speed, an accurate measurement of the original transmitted frequency and the reflected return frequency is required. The Doppler effect occurs for light as well as sound. The Doppler effect is important in astronomy because it enables the velocity of light-emitting objects in space, such as stars or galaxies, to be worked out.
When wave energy like sound or radio waves travels from two objects, the wavelength can seem to be changed if one or both of them are moving. For a moving object to create an appreciable redshift or blueshift requires some pretty serious speeds. Studying light from galaxies throughout our universe, astronomers have noticed something surprising: almost all of it is redshifted.
In fact, not only is it redshifted, galaxies that are farther away are more redshifted than closer ones. So it seems that not only are all the galaxies in the universe moving away from us, the farther ones are moving away from us the fastest. On first glance, this seems to put us at ground zero of a major cosmological exodus. In fact, we aren't really at the center of the expansion. In an expanding universe, anyone standing anywhere in the universe would see everything as moving away, or redshifted.
What puzzles astronomers most now is not that the universe is expanding, but that the rate of this expansion seems to be increasing. Using data from the Hubble Space Telescope, astronomers hope to be able to figure out the likely fate of our universe: Will it expand forever, or will the expansion reverse and cause the universe to collapse back into another Big Bang?
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