Expert: Courtney Seligman - 8/22/2008

Question
Hi Courtney,

I have a Question to ask about light from distant galaxies. If We look at far off galaxies(12 bill. light years) the light is red shifted as the galaxies are moving away from us. But closer galaxies give off blue shifted light as they are moving towards us(or blueshifted for another reason). Is this blue shifted light faster than the speed of light?

Robert

Answer
In any given part of the Universe, galaxies are moving more or less randomly relative to each other. So in the region close to us, some are moving toward us, and have (very small) "blueshifts" in their spectra, while others are moving away from us and have (very small) "redshifts" in their spectra.

The same thing happens for galaxies that are far away; but in their case, the overall expansion of the Universe adds a net "redshift", due to their distance, which is much larger than their random motions, so they all appear to be "redshifted", by the amount due to the Universal expansion at that distance, plus or minus the (small) redshifts or blueshifts due to their own motions.

In other words, for nearby galaxies, we can see small blueshifts, if they happen to moving toward us, because the local Universal expansion is too small to overwhelm their individual motions; while for faraway galaxies, the Universal expansion always overwhelms their individual motions, so we only see redshifts.

However, these redshifts and blueshifts only refer to the change in the wavelength of the light, as we observe it, due to the Doppler effect. The actual speed of the light is not affected at all. Whether things are moving toward us, or away from us, the speed of their light, as it reaches us, is exactly the same as the "speed of light". So no matter what galaxies we observe, and no matter what redshifts or blueshifts they happen to have because of their own or Universal expansion motions, the speed of their light, all the way from them to us, and as it reaches us, is exactly the same.

Courtney Seligman, Professor of Astronomy, Long Beach City College