What is the difference between cosmological and doppler red shift

Although cosmological redshift at first appears to be a similar effect to the more familiar Doppler shift, there is a distinction. In Doppler Shift, the wavelength of the emitted radiation depends on the motion of the object at the instant the photons are emitted. If the object is travelling towards us, the wavelength is shifted towards the blue end of the spectrum, if the object is travelling away from us, the wavelength is shifted towards the red end.

In cosmological redshift, the wavelength at which the radiation is originally emitted is lengthened as it travels through expanding space. Cosmological redshift results from the expansion of space itself and not from the motion of an individual body. Please see my comments on pela's answer, my answer, and for more details the other answers linked from it.

Add a comment. Active Oldest Votes. Different or the same? 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.

Improve this answer. For many cosmological red shift is a doppler shift. I think focusing on comoving and proper coordinates remove the confusion or whatever the need to call things a way or another.

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We will now follow steps 2 and 3 in our procedure, as we did with Milne, to derive the cosmological redshift in de Sitter based on the Doppler and gravitational effects. The cosmological redshift has the same form in terms of the expansion factor regardless of whether the space—time curvature is constant or not.

When calculated using an alternative set of coordinates, the redshift has precisely the same contributions — Doppler and gravitational shifts — that one would expect from the calculation of the lapse function in other applications of general relativity. This association may break down for the non-static FRW metrics, but it would be difficult to see why, given that the formulation of z in terms of the expansion factor a t is identical in all cases.

Still, the proof we have presented here is only partial. It remains to be seen whether the cosmological redshift is a lapse function even when the space—time curvature changes with time.

There are many reasons why the distinction between an expanding space and a fixed space through which particles move is dynamically important. For example, one sometimes hears statements to the effect that light in cosmology can be transported over vast distances faster than one would infer on the basis of c alone.

The justification for this is that the speed of light is limited to c only in an inertial frame, but if space is expanding, then light can be carried along with the expansion at even higher speeds. However, it is not difficult to understand why such notions arise from the improper use of the coordinates. In general relativity, the velocity measured by an observer is the proper velocity e. For light, d s satisfies the null condition i.

Though we have only partially addressed the issue concerning the origin of cosmological redshift, we can now nonetheless turn these results around and ask the opposite question. If there really exists a third mechanism producing a redshift, beyond Doppler and gravity, why do not we see it manifested in the static FRW metrics? After all, FRW space—times with constant curvature also satisfy equation 4 , just like the rest do. And if equation 4 is evidence that z arises from the stretching of light in an expanding space, this process should happen regardless of whether the metric is static or not.

In closely related work, Chodorowski uses a very different technique to arrive at results similar to those reported in this paper. The fact that these two approaches lead to the same conclusions adds significantly to the validity of his and our thesis that cosmological redshift is not due to a new form of wavelength extension, beyond those from kinematic and gravitational effects.

We have sought metrics that can be written in static form, though the transformed coordinates do not necessarily describe a local inertial frame. Yet the velocity of the source may be expressible in terms of these coordinates, as long as we correctly use the proper distance and proper time to evaluate this proper velocity. By the way, this is what we typically do with the Schwarzschild and Kerr metrics.

The decomposition of the cosmological redshift is then based on this proper velocity. If the velocity is zero in this coordinate system, then the time dilation is entirely due to the curvature or gravity , but the cosmological redshift generally includes a second factor that enters because sources are moving with the Hubble flow.

What is interesting, of course, is that because the two sets of coordinates are different one inertial, the other not , the two decompositions are generally not equal, but the final results are the same, as they should be because the underlying physics is identical. Demonstrating that z is a lapse function even for the time-dependent FRW metrics is quite challenging. But given the importance of understanding the origin of cosmological redshift, it is a task worth undertaking.

Following their approach may be a very useful intermediate step in the process of finding the lapse function globally in cases where the space—time curvature is not constant. We will examine this question next and hope to report the results of these efforts in the near future. Abramowicz M. Bajtlik S. Lasota J. Moudens A. Baryshev Y. But in case of Cosmological redshift, the emitted light also gets stetched out thereby increasing its wavelength by the expanding space as Light is also in the space.

This is Doppler redshift. You and your friend are on a compressed carpet. Suddenly your friend started firing spider-man like threads in wave form towards you. Then your other two friends start stretching the carpet from either side assuming you won't fall down.

Now, what would happen to the wavelength of the spiderman waves let's call it that way? What is the difference between a doppler redshift and a cosmological redshift?