Avi Loeb and Freeman Dyson on the future of the universe

This post originally appeared on Astrobites.org on February 03, 2011 at https://astrobites.org/2011/02/03/avi-loeb-and-freeman-dyson-on-the-future-of-the-universe/.

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 measuring the distance to many galaxies. By the Sachs-Wolfe effect, if some advanced civilization pulled together an cluster of galaxies far bigger than “normal” superclusters, then we could detect it from the energy lost by CMB photons as they climb out of the cluster’s enormous gravitational potential well. Signatures of such anomalous overdensities are not actually observed in either type of survey, but Freeman notes that given the trillion-year timescale of the problem, “we have plenty of time to start doing it ourselves.”

Their wonderful senses of humor make the full correspondence a pleasure to read. The conversation is reproduced in its entirety below with permission from Freeman and Avi - we hope you will find the ideas they discuss as fascinating as we did.

Avi Loeb: Dear Freeman,

In 2002 I wrote a paper about the long-term future of our Universe (Phys. Rev. D65, 047301, 2002). Back then, you asked me in an e-mail to let you know if I have any interesting follow-up thoughts on the subject.

Attached is a short paper that I had just submitted for publication on the same subject. I would love to hear any comments you might have about it.

With best wishes, Avi

Freeman Dyson: Dear Avi,

Thank you for sending the paper. I found nothing wrong with it. It presents a dismal picture of the future awaiting our descendants. Since I am an incurable optimist, I raise the question, how much this future could be changed by a large-scale intervention of intelligent life. A very rough estimate indicates that large-scale ``cosmic engineering’’ could be feasible. Using available gravitational energy as the motive power, roughly ten percent of the mass from one percent of the observable universe could be collected within a volume small enough to remain permanently bound together gravitationally. The collection could be done in a single Hubble time and could then be maintained with small active adjustments. So our descendants would stay in communication with a hundred million galaxies instead of only one. It is also possible that some of our more advanced colleagues elsewhere in the universe already began this process. We should look out for evidence of large-scale coordination of gamma-ray bursts or other phenomena indicating high velocity movement of large masses. It would be interesting to examine such possibilities in detail. Thankyou for the suggestion. Yours ever, Freeman Dyson.

Avi Loeb: Freeman,

Your underlying assumption is that intelligent beings prefer as much company as possible. I can only say that as I get older I prefer to stay away from other people as much as possible, since I have the feeling that I heard it all. Extrapolating to the distant future, I am entirely complacent with us being surrounded by vacuum and protected by an event horizon. Aside from the benefit of not having any distractions, this will reduce the risk of a hostile invasion by another civilization.

Yours, Avi

Freeman Dyson: Each to his own taste. History without hostile invasions would be very boring. FD.

Avi Loeb: Putting prejudice aside, we already have relevant data for testing your proposition about “cosmic engineering” on very large scales. SDSS provided us with a map of the distribution of galaxies out to a redshift of z=0.3 (and SDSS III is now reaching farther out). The biggest bound systems in the survey are clusters of galaxies, containing at most ~10^{15} solar masses or ~1000 galaxies each. The existence of clusters is fully consistent with the initial conditions we detect in the microwave background at redshift z=1000. Since the initial conditions are Gaussian, it should be easy to identify “cosmic engineering” in the form of a rare overdense region (supercluster) containing many more than ~1000 galaxies. A region compact enough to bind a million galaxies against cosmic acceleration would have imprinted an anomalously large Sachs-Wolfe effect on the microwave background or would have induced unusually high peculiar velocities. We do not see anomalies of this magnitude. The biggest supercluster in our vicinity is the Shapley supercluster, but it is expected to be desolved in the future by the cosmic acceleration according to the calculation in

Munoz, J, & Loeb, A. “The Density Contrast of the Shapley Supercluster”, MNRAS, 391, 1341

Of course, it is possible that the Shapley supercluster is still “under construction”, or that “cosmic engineering” operates on much smaller scales.

Avi

Freeman Dyson: That is disappointing. On the other hand, if our colleagues have been too lazy to do the job, we have plenty of time to start doing it ourselves. FD.