Presentation #209.04 in the session Cosmology I.
As astronomers, we generally think about the Universe in terms of phenomena observations reveal — cosmic expansion, supernova explosions, evolution of different types of stars or planetary systems, changes in the abundances of different chemical elements, etc. An alternative perspective on the Cosmos, however, can yield new insights not covered by this conventional approach. It concentrates on how astronomical information reaches us by means of photons, gravitons, neutrinos, and gravitational waves, atoms and ions, dust grains, comets or asteroids from beyond the Solar System, shock waves from distant exploding magnetars, etc. Each type of messenger has to traverse vast regions of space to reach us, and seldom reaches us intact, potentially having suffered energy losses, deflection by gravitational or magnetic fields, or never even arriving.1, 2, 3, 4 Studied in this way, we discover intrinsic limits to the information we ultimately will be able to extract from the Cosmos as it exists now, and as it did in the past. Multimessenger approaches will not overcome many of these limits. Untraceable gravitational deflections which, as general relativity teaches us affect all messengers equally, will ultimately prevent our unscrambling precisely where any arriving messengers originated, how energetic they were when radiated by their source, how their polarization may have been altered, etc.5 . I will provide estimates of the ultimate bounds on the transmission of information from this viewpoint. It shows that instrumental advances continuing at current rates will have us reaching these bounds within a century or two. We will then realize that some astronomical phenomena may well exist, though we shall be unable to directly observe them because no messengers will faithfully transmit the information they generate. To my knowledge, this is the first attempt to exhaustive approach the question of “Where will the ultimate limits to our understanding of the Universe lie?” I have tried to answer this question in my recent book “Cosmic Messengers: The Limits of Astronomy in an Unruly Universe”, Martin Harwit, Cambridge University Press, 2021.6
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3. K. Greisen, Phys. Rev. Lett. 16, 748, 1966; G.T. Zatsepin & V.A.Kuz’min, JETPL 4, 78, 1966
4. E. Waxman & J. Bahcall, Phys. Rev. Lett. 78, 2292, 1997; M. Murase & E. Waxman, Phys. Rev. D, 94, 103006, 2016.
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6. M. Harwit, “Cosmic Messengers,” Camb. Univ. Pr., 2021; R. C. Smith, The Observatory, 141, 256, 2021.