Journal
AMERICAN JOURNAL OF PHYSICS
Volume 77, Issue 8, Pages 688-694Publisher
AMER ASSOC PHYSICS TEACHERS AMER INST PHYSICS
DOI: 10.1119/1.3129103
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Funding
- NSF [AST-0507395]
- Alexander von Humboldt Foundation of Germany (DWH).
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A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity) but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon's path. In the context of the expanding universe, the kinematic interpretation corresponds to a family of comoving observers and hence is more natural. (C) 2009 American Association of Physics Teachers. [DOI: 10.1119/1.3129103]
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