Sunday, August 01, 2004

The Bleak Universe, part IV

This is part four of a four part essay. In this part, I will discuss two important time limits that will constrain our exploration of the universe as well as bringing the essay to a conclusion by addressing the question of whether or not we live in a bleak universe.

Last week I stated that there was a time limit to our explorations of the universe and that the clock is already ticking. Actually, there are two clocks that we are racing against, one of which is thermodynamic and the other of which is cosmological. I shall first discuss the thermodynamic clock.

We tend to think of the universe as being something that's ancient. Current estimates place it's age at around twelve billion years (give or take a billion). That's certainly old enough in human terms and even in geological terms. In cosmic terms, however, the universe is frighteningly young. Measured against the whole of the universes expected existence, we're practically still in the Big Bang.

The current epoch of the universe has been called the stelliferous era, meaning the era that is filled with stars. Indeed, when we look up at the sky, tend to think of the universe in terms of stars: a vast blackness punctuated by bright points of light. In truth, stars represent a minute fraction of the mass and energy of the universe. Never the less, we are not wrong to think that the stars are special given that they are engines of life.

If order for life to exist, there must be an energy gradient. That means, in simple terms, that heat must flow from a higher point to a low point. Such a system allows for the existence of so-called "heat engines". Life is one such engine. Not only do stars create such a gradient, but they are central to the formation of planets and they allow for the existence of such crucial materials as liquid water.

We tend to think that stars are eternal, like mountains. They are enduring than mountains (which rise and fall live shivers on the skin of the planet) but stars do not live for ever. Like people, stars are born, age, grow old and, ultimately, die (they do not, however, reproduce). Fortunately, when stars die, they return some of their mass back to the universe either by shedding layers of their outer mass as their central reserves of fuel diminish or, more spectacularly, by exploding. Unfortunate, they always take a fair amount of their mass to the gave locking it away in the form of white dwarfs, neutron stars and black holes.

The mass that is returned to the galaxy can be recycled into new stars (with heavier elements than the previous generation, but that's another story). The era of star formation, however, will not last forever. Eventually, the material to make new stars will be used up and no more stars will come into existence to decorate the universe and to provide havens for life. Ultimately, the oldest stars will all burn out leaving the universe a darker, colder place.

The stelliferous era started as early as a million years after the Big Bang and will continue for another hundred trillion, or so. This may seem like a long time (certainly much longer than the current age of the universe), but time is deep.

Once the stars burn out, any chance for us to explore the universe will have to give way to an increasingly challenging effort to survive a universe that becomes progressively more hostile. After the stelliferous age, there is a so-called degenerate era (referring to degenerate matter and not degenerate people) where we may try to scrounge energy from white dwarfs and neutron stars. This era will end at ten to the 39th years. Past that, we may try to salvage a trickle of energy from the evaporation of black holes. If we can manage to do that, we may be able to hold on to some form of existence for a full googol of

Ultimately, even matter itself is likely to betray us since protons are, in the very, very long term, believed to be unstable. Unless we can do something miraculous such as finding a way to reach some other younger, more vital universe, our universe shall become our tomb and the graveyard of any other life that might exist.

Still, even if we are bound to only survive the stelliferous era, a hundred trillion years is not an inconsequential span in terms of human endeavors. As I've noted before, the farthest portion of the universe that we can currently see is only about 10 billion light years away. With a hundred trillion years of leeway, we could get from here to there at a meager 1/10,000th the speed of light (about 67,000 miles per hour). You're probably not surprised that I'm about to tell you that there's a hitch and that it has to do with that second clock, the cosmological one.

The universe is expanding. This was discovered in the early part of the 20th century as the first generation of modern telescopes was brought to bear upon the universe. Spectrographic analysis of the stars showed that distant stars (that is, stars in other galaxies) were shifted towards the red. This is an aspect of a phenomenon known as Doppler shift (actually, it's more complex than that but this will do for our purposes). In a nutshell, this meant that much of the universe was moving away from us. Even more curiously, the more distant stars were receding more rapidly. This was, to say the least, unexpected and puzzling.

The solution to this weird behavior is that space, itself, is expanding. In fact, Albert Einstein almost predicted this. When he developed the Theory of Relativity, he found that seemed to require a universe that was either expanding or contracting. He naturally thought that was ridiculous so he included a fudge factor into his calculations, called the Cosmological Constant, in order to eliminate this counter-intuitive behavior. He would later say that this was his greatest mistake.

Counterbalancing the expansion of the universe is gravity which is "trying" to slow the universe down. For a very long time, cosmologists have wondered whether gravity would eventually win – slowing, stopping and, ultimately, reversing the expansion into a collapse – or whether the universal slow but never quite come to a stop.

In the mid to late nineties, various research groups were making observations of distant supernovae in order to try and pin down a critical value known as Hubble's Constant. In the process of doing this, their data showed that universe not only isn't slowing down enough to stop expansion but that it is, in fact, accelerating.

To say that this was unexpected would have been the understatement of the century. It would be like pushing a bolder down a steep incline only to have it stop midway and start rolling back uphill.

The universe hasn't always been accelerating. Until relatively recently, the universe has been behaving according to theory. The acceleration we're now observing is a relatively new phenomenon (remember, of course, that, to an astronomer, a billion years ago is recent). The reason for this acceleration is still being debated but there is a growing consensus that Einstein wasn't entirely wrong in postulating a cosmological constant. However, instead of holding the universe in a static state, the constant drives it towards expansion and acceleration.

Gravity, on the scale of the other forces that govern the universe, is extremely weak (about 1 over 10 to the 33rd of the strength of the next weakest). It is, however, an important force on the cosmic scale because it acts over an unlimited range and because it only comes in a positive variety (unlike magnetism) so it never cancels itself out. The Cosmological Constant also works over long distance working against gravity by providing a kind of "pressure" that pushes the universe outward. There is one very important difference, though: gravity becomes weaker with distance but the Cosmological Constant exerts a constant force no matter how far apart things are.

In the early universe, gravity was dominant. The universe was expanding but gravity was hitting the brakes fairly hard. Sheer inertia (with a little help from Inflation) kept the universe from immediately imploding. The Constant, by contrast, was fairly feeble. As the universe has grown older and larger, however, gravities grip on the universe has been growing weaker while the Cosmological Constant's influence has remained appropriately constant. At some point, we seem to have crossed over a threshold. Where gravity was ruled, the Constant drives the expansion. The end result of this is that the universe will not only continue to expand forever but that the expansion will go faster and faster, accelerating without end.

What does this mean to us? In a word, it means loneliness. The farthest galaxies we can currently see at the farthest we will ever see. As time moves on, the furthest objects from us will actually disappear over the cosmic horizon forever invisible and inaccessible to us. This process will continue until we left in a universe where only those objects that are gravitationally bound to us will remain with us. It is expected that that will mean the local cluster of galaxies, which is small collection of nearby galaxies numbering about thirty. The rest of the universe will seem vast and empty.

How long this will take is not known, but it is expected to be fairly rapid. It is unlikely that we will have our hundred trillion years of exploration time. Instead, we will be stranded in our small patch of the universe surrounded by an impenetrable and empty darkness.

Do we live in a bleak universe? We must come to terms with the realization that the universe does not have an obligation to be friendly to life. The Copernican Principle may well have instilled an unreasonable sense of optimism in us. Even if there is intelligence out there (and we have cause to suppose that it's out there somewhere), there is no reason to expect that it will be close enough for us to ever meet it or even to signal it. We may well be alone in the only sense that matters.

Shall we despair? Although we may be denied our fondest dreams, I think that we need not despair. If there are no alien intelligences available to us, we always have the possibility of creating new intelligences, either cybernetically or biologically (The science fiction author David Brin suggests that we might decide to "uplift" certain animals to sapience via genetic engineering and selective breeding). We might even decide to be cultivators of intelligence in our galaxy by going out and seeding promising worlds with life and sheparding them through the hazards posed by a dangerous universe. If the galaxy is now a hostile jungle, we might turn it into a garden.

Even if the economics and physics of interstellar travel consign us to live and die in the neighborhood of our own star, I think that we have cause for happiness. If life is uncommon and intelligence is rare that only means that it is precious and special. We can look out at the universe and rejoice that we are, in fact, privileged for the very reason that we can look out at the universe in wonderment and awe. The Copernican Principle to the contrary, we are not mediocre and any universe that can contain beings such as ourselves is not truly bleak.

This concludes this essay. I would like to acknowledge Fred Adams and Greg Laughlin for coining the term stelliferous as well as for providing a detailed description of the ages of the universe in their book The Five Ages of the Universe which I heartily recommend.

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