Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991acaps..11..197k&link_type=abstract
Acta Astrophys. Sin., Vol. 11, No. 3, p. 197 - 212
Astronomy and Astrophysics
Astrophysics
1
Cosmology: Inflationary Universe, Cosmology: Friedmann Universe
Scientific paper
In the r-t diagram, where t is the cosmic time and r is the radial coordinate defined as the proper radial distance divided by the scale factor, the locus of the incoming photon emitted at the big bang and reaching us now, r-(t), is a mirror image of the locus of the outgoing photon emitted at the big bang with r = 0, r+(t). The former graphs our present world picture while the latter traces the growth of the (particle) horizon since the big bang. The equation r-(t) = r+(t)/2 determines an epoch tD with the property that, for all events on the world picture before tD, the horizon problem exists to varying degrees of severity, while for events after tD, there is no horizon problem. For the flat Friedmann universe tD turns out to be quite recent, long after the epoch of the MBR (microwave background radiation z ≍ 103); for the inflationary model of Guth, tD is near the beginning of the inflationary era, long before the MBR epoch. For a curved Friedmann universe the time axis of the Penrose diagram can be identified with the parameter η in the usual parametric representation of the scale factor and the space axis, with the angular radial coordinate χ. Thus interpreted, the diagram readily gives zD = 0.86 (3.66) for a closed (open) universe corresponding to H0-1 = 20·109yr and q0 = 1.65 (Ω0 = 0.05).
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