Long-term future of extragalactic astronomy

If the current energy density of the universe is indeed dominated by acosmological constant, then high-redshift sources will remain visible to usonly until they reach some finite age in their rest-frame. The radiationemitted beyond that age will never reach us due to the acceleration of thecosmic expansion rate, and so we will never know what these sources look likeas they become older. As a source image freezes on a particular time framealong its evolution, its luminosity distance and redshift continue to increaseexponentially with observation time. The higher the current redshift of asource is, the younger it will appear as it fades out of sight. For the popularset of cosmological parameters, I show that a source at a redshift z=5-10 willonly be visible up to an age of 4-6 billion years. Arguments relating theproperties of high-redshift sources to present-day counterparts will remainindirect even if we continue to monitor these sources for an infinite amount oftime. These sources will not be visible to us when they reach the current ageof the universe.Comment: Phys. Rev. D, in press (2001