Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-04-21
Am.J.Phys. 71 (2003) 358-364
Astronomy and Astrophysics
Astrophysics
8 pages including 6 figures, to appear in Am. J. Phys., 2003. Reference added in postscript
Scientific paper
We use the dynamics of a galaxy, set up initially at a constant proper distance from an observer, to derive and illustrate two counter-intuitive general relativistic results. Although the galaxy does gradually join the expansion of the universe (Hubble flow), it does not necessarily recede from us. In particular, in the currently favored cosmological model, which includes a cosmological constant, the galaxy recedes from the observer as it joins the Hubble flow, but in the previously favored cold dark matter model, the galaxy approaches, passes through the observer, and joins the Hubble flow on the opposite side of the sky. We show that this behavior is consistent with the general relativistic idea that space is expanding and is determined by the acceleration of the expansion of the universe -- not a force or drag associated with the expansion itself. We also show that objects at a constant proper distance will have a nonzero redshift; receding galaxies can be blueshifted and approaching galaxies can be redshifted.
Davis Tamara M.
Lineweaver Charles H.
Webb John K.
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