why cosmic background radiation is not ether? I mean it's everywhere and it' a radiation then we can measure Doppler effect by moving with a velocity.
Answer
The luminiferous aether was, by definition, a hypothesized medium that was needed for electromagnetic waves to propagate through space. The cosmic microwave background isn't needed for photons to move; indeed, they move through space even if one removes (shields) the cosmic microwave radiation. So that's why CMS isn't luminiferous aether.
On the other hand, the aether also made a particular prediction, the existence of a preferred reference frame. In this sense, the CMB plays the same role as the aether. Cosmologists use the reference frame associated with the CMB as the preferred coordinate system. However, this ability to pick a "preferred" coordinate system depends on the environment – the CMB is just some property of the environment that could possibly be different as well (e.g. if you shield it). In this respect, it still differs from the luminiferous aether that couldn't have been shielded and that guaranteed the existence of a preferred coordinate system in any situation, regardless of details of the environment.
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