therelativisticDoppler equation) is
f^0 Dfr
c ̇v
cv; (12.2)
where the sign conventions are the same as for the Doppler effect described earlier. This effect means that
if the source are observer of light waves are movingtowardeach other, the light waves appearbluerthan
they would if the source and observer were not moving relative to each other; this effect is called ablueshift.
Similarly, if the source and observer are moving away from each other, the light appears redder than it would
otherwise, an effect called aredshift.
Astronomers often observe this effect in astronomical bodies. For example, because of the Sun’s rotation,
lines in the Sun’s spectrum are blueshifted on the edge of the Sun moving toward us, and redshifted on the
edge moving away from us.
It was discovered decades ago that all distant galaxies have redshifted light, so they are all moving away
from us. Furthermore, the farther the galaxy, the greater the redshift—meaning that the farther the galaxy,
the faster it’s moving away from us. The American astronomer Edwin Hubble first noted this, and postulated
what is now calledHubble’s law; it relates the speedvwith which a galaxy is moving away from us to its
distanceDfrom us:
vDH 0 D; (12.3)whereH 0 is a proportionality constant called theHubble constant. Observations by several NASA spacecraft
have recently determined the value of the Hubble constant to be aboutH 0 D 71 (km/s)/Mpc. (Aparsec(pc)
is about 3.26 light-years, or about3:09 1016 meters, and so amegaparsec(Mpc) is a million times that.)
Why are all the galaxies moving away from us like this? It’s because the Universe is expanding, which is
causing every distant galaxy to move away from every other one, much like dots drawn on a balloon moving
farther apart as the balloon is inflated. This expansion began 13.7 billion years ago with the Big Bang, the
huge explosion in which the Universe was created, and is continuing to this day.