We know that some galaxies are moving away from us faster than the speed of light and we know it by measuring the redshift, but how's that possible? If they're moving away say at 2c, how would the light of the galaxy even reach us? How do we measure "redshift" for something faster than light?
Answer
We know that some galaxies are moving away from us faster than the speed of light and we know it by measuring the redshift, but how's that possible?
The following papers give good explanations:
http://users.etown.edu/s/stuckeym/AJP1992a.pdf
http://arxiv.org/pdf/astro-ph/0011070v2.pdf
In summary, Hubble Law: $v = H(t)D$, where $v$ is recession velocity, $D$ is distance, and $H(t)$ is the Hubble "constant" at a given time, requires that beyond a certain distance velocity is greater than the speed of light. If recession velocity at the location of a traveling photon were greater than the speed of light the entire time the photon from a distance galaxy were traveling, we would never observe the photon. A photon emitted from a galaxy moving away from us faster than light, initially is also receding from us. However, the photon may eventually get to a region of spacetime where recession from us is $ If they're moving away say at 2c, how would the light of the galaxy? even reach us? Only if the photons from the galaxy reach a region of spacetime where recession velocity is $ How do we measure "redshift" for something faster than light? Red-shift is measured as the change in wavelength of the light, but rather than interpreting the results using special relativity (which would result in $v
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