by Nathan Sanders | Feb 3, 2011 | Quick Notes
This morning I was delighted to receive an email from Avi Loeb, who’s paper on the (far) future of astronomy we discussed yesterday. Avi shared with me a conversation he had by email with another noted theorist, Freeman Dyson. The premise of Avi’s paper is that about a trillion years from now, all extragalactic light sources will cease to be visible due to the accelerating expansion of the universe. Future astronomers would therefore be stuck looking only within their own galaxy. Not only would the absence of extragalactic sources make for a (apparently) lonely universe, but it would deprive future astronomers of the tools that we have used to arrive at our current understanding of cosmology, such as extragalactic supernovae and the cosmic microwave background (CMB) radiation. Despite this, Avi suggests an observational signature by which these astronomers could still derive the standard cosmological model: hypervelocity stars.In their correspondence, Freeman, in his role as an “incurable optimist,” suggests a means by which future civilizations could avoid this fate. He proposes that a civilization could harness the gravitational energy of a group of galaxies in some way as to pull together colossal collections of galaxies before they were thrown apart from each other by cosmological expansion. Perhaps this will remind you of another one of his well-known ideas for civilization-scale engineering, the Dyson Sphere. Freeman suggests searching for such anomalous overdensities of galaxies to detect such “cosmic engineering.”Avi notes two possible observational signatures of such engineering: redshift surveys and the Sachs-Wolfe effect. Redshift surveys such as SDSS make 3D maps of the large scale structure of the local universe by...
by Nathan Sanders | Feb 2, 2011 | Daily Paper Summaries, Quick Notes
Update: you can read Avi Loeb and Freeman Dyson’s discussion of this issue in our latest post.Imagine a civilization in our galaxy a trillion years in the future. Astronomers in this society may not know of a universe beyond their own galaxy. At approximately the year 100 billion, all galaxies outside the Local Group of gravitationally bound galaxies will have sped out beyond the event horizon of the observable universe and the Local Group will have long since conglomerated into a single galaxy (“Milkomeda”). The Sun will have died out long ago, but the lowest mass stars in the present-day Milky way (0.1~1 solar masses) may still be living.What’s more, the accelerating expansion of the universe predicted by the standard ΛCDM cosmology has redshifted the photons of the cosmic microwave background beyond the event horizon of the entire future universe, so future BOOMERANG or WMAP experiments will not work. Clearly, these future observers could not use the same tools we have to understand the origins of the universe. Fortunately, Harvard Professor Avi Loeb has a very interesting short paper on the arXiv today that speculates on the tools astronomers living a trillion years in the future could use to infer the standard model of cosmology we have derived from present-day observations of the universe.Loeb suggests that these future observers look for the precious few stars that have had a velocity sufficient to escape from Milkomeda (hypervelocity stars, HVSs). HVSs can be ejected from a galaxy by gravitational interactions with the supermassive black hole at the galaxy’s nucleus. After about two billion years of travel, Loeb estimates, the acceleration of...
