The Gradual Realization that Astronomical Information is Bounded, 1965-2021

The discovery of the Cosmic Microwave Background in 1965 indicated that cosmic messenger energies must be bounded. Gould \& Schreder noted that collision with microwave background photons would quickly damp high gamma-ray energies down to $_{~}$ 10$^{13}$ eV. Greisen and Zatsepin \& Kuz’min, showed that ionic cosmic rays would similarly be bounded at $_{~}$ 10$^{22}$ eV. Both limits are by now well established. I will show that gravitational deflection by myriad low-mass bodies, like interstellar planets and comets or dark-matter blobs, impose ultimate bounds also on angular, temporal and spectral resolution.\footnote{Martin Harwit,\textbf{Cosmic Messengers, The Limits of Astronomy in an Unruly Universe” Cambridge University Press} 2021} I shall provide examples of resolving powers currently approaching those bounds. Multi-messenger observations will not overcome these because, as general relativity shows, gravitational deflections and time delays affect all messengers identically. At current funding rates world-wide, we will within a century or two attain instrumental capabilities, improvements on which will no longer lead to further astronomical advances. Some astronomical phenomena could remain forever unobservable, showing a need to consider how our knowledge of the Universe then might nevertheless advance.