When stars fade out… a disturbing prediction of the future of the universe but a consoling thought about our present.

Not entirely sure why, but I find an article in this months Scientific American somewhat disturbing. Written by Lawrence M. Krauss and Robert J. Scherrer (Scherrer is a cosmologist and science fiction writer – something of a busman’s holiday, I suppose, for him) it posits a logical outcome of the expansion of the universe.

Essentially the deal is as follows. In the last ten years astronomers discovered that the expansion of the universe is speeding up. This has very real effects. Individual galaxies of which our Milky Way is one will be pulled apart from each other to the point where they are so far from each other that they are impossible to see. This means that hypothetical astronomers of the future will have no reference points by which to judge the previous history of the universe. They simply may not be able to find evidence that the universe started with the big bang and expanded outwards.

The discovery of a speeding expansion necessitated a rethink of previous models.

The acceleration of the universe implies that empty space contains almost three times as much energy as all the cosmic structures we observe today: galaxies, clusters and superclusters of galaxies. Ironically, Albert Einstein first postulated such a form of energy to keep the universe static. He called it the cosmological constant’

This has profound implications.

With cosmologist Glenn Starkman of Case Western Reserve University, Krauss explored the implications for the fate of life in a universe with a cosmological constant. The prognosis: not good. Such a universe becomes a very inhospitable place. The cosmological constant produces a fixed “event horizon,” an imaginary surface beyond which no matter or radiation can reach us. The universe comes to resemble an inside-out black hole, with matter and radiation trapped outside the horizon rather than inside it. This finding means that the observable universe contains only a finite amount of information, so information processing (and life) cannot endure forever [see “The Fate of Life in the Universe,” by Lawrence M. Krauss and Glenn D. Starkman; Scientific American, November 1999].

Long before this information limit becomes a problem, all the expanding matter in the universe will be driven outside the event horizon. This process has been studied by Abraham Loeb and Kentaro Nagamine, both then at Harvard University, who found that our so-called Local Group of galaxies (the Milky Way, Andromeda and a host of orbiting dwarf galaxies) will collapse into a single enormous supercluster of stars. All the other galaxies will disappear into the oblivion beyond the event horizon. This process takes about 100 billion years, which may seem long but is fairly short compared to the wilderness of eternity.

In other words it will be impossible to see other galaxies. Nor will it be possible to trace the prior history of the universe from their movement. And the odd thing about this is that it almost seems to deliver us back to a viewpoint which was extant at the beginning of the 20th century when it was believed that our own galaxy was an ‘island universe’ in an infinite space.

General relativity gave a first pillar (as Krauss and Scherrer put it) of our contemporary cosmological models by demonstrating that space could not be unchanging, that it had to expand or contract. Hence this delivered us the conceptual underpinning of the ‘big bang’.

Next measurements by American astronomers Vesto Slipher and Edward Hubble, working individually, provided the information that galaxies tended to be moving away from us (not all it must be noted – which is why our Local Group will collapse into a supercluster) and that their velocity was increasing. This provided the second pillar.

Cosmic microwave radiation gave us a third pillar… a legacy of the early expansion of the universe, indicating that the big bang was both hot and matter was dense within the growing universe.

Finally, nuclear fusion itself, the processes of which used certain elements, specifically the lightest ones producing helium and deuterium. These processes match models of the big bang and better still account for contemporary conditions.

The problem is that each one of these pillars is subject to change as the universe ages. Relativity grew out of observations of the universe as it is now. But this universe is not constant. The retreat into the darkness of the galaxies is only the most obvious problem. Cosmic radiation will dissipate to levels that are impossible to record. Analysis of chemical elements will be obscured by later production of helium in stars.

And this leads to problems. As the article notes:

Although the observational abundance of light elements will not provide any direct evidence for a fiery big bang, it will nonetheless make one aspect of future cosmology different from the illusory cosmology of a century ago. Astronomers and physicists who develop an understanding of nuclear physics will correctly conclude that stars burn nuclear fuel. If they then conclude (incorrectly) that all the helium they observe was produced in earlier generations of stars, they will be able to place an upper limit on the age of the universe. These scientists will thus correctly infer that their galactic universe is not eternal but has a finite age. Yet the origin of the matter they observe will remain shrouded in mystery.

Nor is this unlikely. Let’s consider the timescales. 5 billion years from now we can expect the Andromeda galaxy to be filling the sky as the local group collapses in on itself. 100 billion years from now we will be part of a globe like supercluster and all other galaxies will be invisible. 100 trillion years form now the last stars will burn out. Now, granted, let’s put this in perspective, the universe is, according to our best models about 13.7 billion years old, so we are talking about unimaginable depths of time. Empires may well rise and fall. But that’s the problem. Our universe appears to be infinite both in terms of space and time. Knowledge will almost inevitably be lost… assuming we, or others, survive to pass it on.

And that infinity only really refers to its compass, not to the matter within it. For that we can only expect that…
It will consist of an island of stars embedded in a vast emptiness… The ultimate future of the observable universe is to collapse into a black hole, precisely what will in fact occur to our galaxy in the distant future.

It’s a remarkable thought, that the universe will in effect be dotted with black holes marking the sites of not merely galaxies, but collections of galaxies, each of those holes become ever more distant from each other as the inexorable expansion continues. Forever.

And Krauss and Scherrer point to an oddity of the present period (let’s use the term ‘period’ loosely, we’re talking billions of years here).

The window during which intelligent observers can deduce the true nature of our expanding universe might be very short indeed. Some civilizations might hold on to deep historical archives, and this very article might appear in one—if it can survive billions of years of wars, supernovae, black holes and countless other perils. Whether they will believe it is another question. Civilizations that lack such archives might be doomed to remain forever ignorant of the big bang.

So, we, are privileged to see past and future of the universe. But wait. Lest you start cheering the Anthropic Principle (which argues that the universe is of its nature – whether deliberately or not – conducive to life) our two authors take a different line.

They argue that…

First, this would quite likely not be the first time that information about the universe would be lost because of an accelerating expansion. If a period of inflation occurred in the very early universe, then the rapid expansion during this era drove away almost all details of the preexisting matter and energy out of what is now our observable universe. Indeed, one of the original motivations for inflationary models was to rid the universe of pesky cosmological objects such as magnetic monopoles that may once have existed in profusion.

More important, although we are certainly fortunate to live at a time when the observational pillars of the big bang are all detectable, we can easily envisage that other fundamental aspects of the universe are unobservable today. What have we already lost? Rather than being self-satisfied, we should feel humble. Perhaps someday we will find that our current careful and apparently complete understanding of the universe is seriously wanting. 

Who is to know what artifacts of the earliest phase of the universe have already vanished, what echoes of their existence we have missed or simply ignored because they are scattered in fragments impervious to analysis? It reminds me a bit of the arguments used to counter ‘intelligent’ design which point out the myriad flaws in our physical makeup. To reify an idea that we are more than simply coincidentally fortunate to be able to see this unfolding history and future of our universe over the reality of our probable ignorance is of dubious utility. But that’s not to ignore the fact that we are fortunate and that in an immensity of time we get ringside seats. Somehow, despite the end of it all, and I admit that philosophically I find the idea of an empty void somewhat less than great, that is in itself cheering. Perhaps this is the price we pay for living in an entropic universe. But then again, we also get to see the big lights and the show looks set to continue for quite a while.