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What about blueshifts?

I know about redshifts in the universe. What about blueshifts?

Hello James Watt,
Because the universe is expanding rather than contracting at the largest distance scales (scales larger than galaxy groups of about 2 Megaparsecs), nearly every object (such as galaxies or groups of galaxies or quasars) is receding away from every other object once one gets far enough away, and this mutual recession of objects (and the space between them) results in the observation of redshifts rather than blueshifts. Were the universe contracting (as is allowed by an alternative but unrealized solution to Einstein's equations of General Relativity) instead of expanding, one would see blueshifts. The shifts are Doppler shifts of spectral features in the spectrum of the objects, such as the wavelengths of hydrogen lines, and are larger with increasing recessional velocity. The further an object is from us, the faster the recessional velocity, and the larger the doppler redshift. The most distant known quasar has a redshift of about 5.01. The most distant known galaxy has a redshift of 5.6 or so.

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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