What about blueshifts?
I know about redshifts in the universe. What about blueshifts?
Hello James Watt,
You have heard that distant galaxies do appear to be more 'blue in color' than nearby galaxies, and this is the case, but this is not related to any blueshift, but rather due to the fact that distant galaxies, because one is looking back in time to see them at great distances, are younger (a galaxy at a distance of 5 billion light years has stars on average 5 billion years younger than the stars in nearby galaxies). Light from young galaxies is dominated by hot, blue stars, much younger than yellow and red stars such as our own sun. Thus the distant galaxies appear blue, even as their light is redshifted.
There is an exception to the 'no blueshift' rule for 'nearby' objects (less than about 5 megaparsecs ~ 15 million light years from the Milky Way). Objects within our own local group of galaxies, such as the Andromeda Galaxy, are close enough that the expansion of the universe does not overwhelm the local pull of gravity. Such local objects can have blueshifts, and in fact the blueshift of the Andromeda galaxy is z = -0.00042 (the negative sign indicates blueshift). This indicates that our Galaxy and Andromeda are pulling towards each other via gravity at 125 km/s.
No one has ever discovered an extragalactic object with a blueshift of more than a very small (in absolute value) negative number (such as z= -0.5). If one were to be discovered, this would be of enormous importance and would force a rethinking of our cosmology, but it is not currently expected.
Brian Yanny, Experimental Astrophysics Group Fermilab
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