The "tired light" hypothesis, mainstay of a dwindling band of contrarians who deny the big bang and its corollary, the expanding universe, has suffered a one-two punch. Observations of supernovae and of galaxies provide the best direct evidence that the universe is truly expanding.
It's a conclusion that most astronomers reached long ago. In 1929, Edwin Hubble announced that light from distant galaxies is redder than light from nearby ones. Hubble and others took the redshifts as evidence that the universe is expanding, causing distant galaxies to speed away faster than nearby ones. However, an alternative explanation, the "tired-light" hypothesis, claimed that galaxies' light reddens because it loses energy as it passes through space. In this scenario, distant galaxies are red not because they are moving, but because their light has traveled farther and gotten pooped along the way. Measurements of the cosmic microwave background put the theory firmly on the fringe of physics 30 years ago; still, scientists sought more direct proofs of the expansion of the cosmos.
Two new papers provide the best direct evidence yet. The first, slated to appear in Astrophysical Journal, measures the brightening and dimming of a certain type of supernova. Thanks to Einstein's theory of relativity, if distant supernovae are speeding away from us, they will appear to flare and fade at a more leisurely pace than close-by ones. A team of scientists led by Gerson Goldhaber of the Lawrence Berkeley National Laboratory in Berkeley, California, has shown that this is, indeed, the case with 42 recently analyzed supernovae.
In the second study, Allan Sandage, an astrophysicist at the Carnegie Observatories in Pasadena, California, and Lori Lubin of Johns Hopkins University in Baltimore analyzed space-based measurements of the surface brightness of galaxies. Both the standard expanding-universe and the tired-light theory, they realized, agree that redshifted light should make distant galaxies look dimmer than they really are. In an expanding universe, however, time dilation and other relativistic distortions will also dim distant galaxies, making them appear much fainter than tired-light theory dictates. What's more, young stars--and thus young galaxies--tend to be considerably brighter than old ones. When that extra brightness is taken into account, the observations match expanding-universe predictions, as Lubin and Sandage will report in Astronomical Journal. For the tired-light theory to be correct, young galaxies would have to be dimmer, rather than brighter, than old ones. "The expansion is real. It's not due to an unknown physical process. That is the conclusion," says Sandage.
Although not surprising in themselves, the results are useful for "tidying things up in our cosmology," says Michael Pahre, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Even so, "I don't think it's possible to convince people who are holding on to tired light," says Ned Wright, an astrophysicist at the University of California, Los Angeles. "I would say it is more a problem for a psychological journal than for Science."