Observation of the Final Boundary Condition: Extragalactic Background Radiation and the Time Symmetry of the Universe

This paper examines an observable consequence for the diffuse extragalactic 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 [1]. 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.