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One
might wonder why we can talk about the universe expanding in time given that GR
is based on SR: given relativitys challenge to the idea of the present, how can there be a unique present which
defines the universe and allows it to be described as an object expanding
in time? The answer involves two facts:
1) The mathematical equations of GR can be factored into a space plus time
form, i.e., a 3-geometry whose characteristic size is a function of time, if
the distribution of matter in the 3-geometry is uniform. 2)Hubbles observations show that, at large
enough distances, galaxies at the same distance are uniformly distributed in
space. One can appeal to this idea
theologically by claiming that the present of the universe which God
experiences is defined by its uniform mass distribution and expansion in
time. However, it should be kept in
mind that the underlying equations of GR are even more complex than those of
SR, and the distinctive meaning of space and of time in GR is correspondingly
more problemmatic. See for example Chris J. Isham, "Quantum Theories of
the Creation of the Universe," in Quantum Cosmology and the Laws of
Nature: Scientific Perspectives on Divine Action, ed. Robert J. Russell,
Nancey C. Murphy and Chris J. Isham, Scientific Perspectives on Divine Action
Series (Vatican City State; Berkeley, Calif.: Vatican Observatory Publications;
Center for Theology and the Natural Sciences, 1993), Section 3; Drees,
Beyond the big bang, Appendix 5.
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