The Gradual Realization that Astronomical Information is Bounded, 1965-2020

Following the discovery of the Cosmic microwave background radiation in 1965 several astrophysicists began to realize that the energies of many cosmic messengers could well be bounded through a titanic clash between high energy radiations generated in a supernova explosion and the ubiquitous microwave background photons that would collide with these radiations, confining their energies to finite bounds.

These, however, are not the only bounds limiting information reaching Earth from cosmic domains. A variety of different interstellar and intergalactic processes had, over the decades also been found to deflect and or decelerate messengers reaching us from astronomical sources. Many are due to interactions with distinct gaseous regions along the messengers’ trajectories. Observations with different types of messengers can help us disentangle some of these ambiguities but a fundamental bound must exists, below which these uncertainties will not be overcome. That bound is due to an intrinsic unruliness of the Cosmos — the existence, along any messengers’ trajectories of myriad undetected low-mass bodies. General relativity teaches us that such gravitational attractors delay, deflect and split beams of every kind of messenger identically. Multi-messenger comparisons thus will not disentangle where and how the messengers’ trajectories were distorted. Precision astronomy will have reached its bounds.

I estimate that, at current funding levels world-wide over the next century or two, advances in astronomical instrumentation will lead to angular, temporal and spectral resolving powers comparable to predictable levels of uncertainty in the directions from which messengers are arriving, the relative times at which they were generated at their sources, and the spectral changes associated with those two uncertainties. Although instruments with enhanced capabilities could still be constructed, they would not remove the ambiguities induced by these intrinsic gravitational deflections. Astronomers will then need to determine how else to gain information on cosmic processes that no astronomical messengers will transmit. Such processes could well exist, though we would be unable to observe them.