This paper examines an observable consequence for the diffuse extra-galactic background radiation (EGBR) of the hypothesis that if closed, our universe possesses time symmetric boundary conditions. By reason of theoretical and observational clarity, attention is focused on optical wavelengths. The universe is modeled as closed Friedmann Robertson Walker. It is shown that, over a wide range of frequencies, electromagnetic radiation can propagate largely unabsorbed from the present epoch into the recollapsing phase, confirming and demonstrating the generality of results of Davies and Twamley (1993). As a consequence, time symmetric boundary conditions imply that the optical EGBR is at least twice that due to the galaxies on our past light cone, and possibly considerably more. It is therefore possible to test experimentally the notion that if our universe is closed, it may be in a certain sense time symmetric. The lower bound on the ``excess'' EGBR in a time symmetric universe is consistent with present observations. Nevertheless, better observations and modeling may soon rule it out entirely. In addition, many physical complications arise in attempting to reconcile a transparent future light cone with time symmetric boundary conditions, thereby providing further arguments against the possibility that our universe is time symmetric. This is therefore a demonstration by example that physics today can be sensitive to the presence of a boundary condition in the arbitrarily distant future.
1996 Academic Press, Inc.
I. INTRODUCTION
Quantum cosmology studies the relation between the observed universe and its boundary conditions in the hope that a natural theory of the boundary condition might emerge (see [2] for an outstanding review of this enterprise.) Assessment of a particular theory requires an understanding of its implications for the present day. To that end, this paper elaborates on work of Gell-Mann and Hartle [3] and Davies and Twamley [1] by examining the observable consequences for the diffuse extragalactic background radiation (EGBR) of one possible class of boundary conditions, those that are imposed time symmetrically at the beginning and end of a closed universe [3], and sketches some of the considerable difficulties in rendering article no.