Lately, cosmology has gotten strange. That's saying a lot, considering how strange it was to begin with.

The universe is thought to have begun with a Big Bang, an explosion of existence so profound that the energy from it still permeates the sky, now cooled to a few degrees above absolute zero, a microwave echo of that stupendous event.

The Cosmic Background Explorer satellite has created a map of the microwave background, and one of the more staggering implications of this map is that the universe is not infinite: it may be shaped like a doughnut. This would mean that if you were able to start traveling in a certain direction at a very high velocity, eventually you might wind up right back where you started, approaching it from the opposite side.

That's strange.

Stranger yet is the apparent existence of dark matter and dark energy. A feature of certain cosmological theories called supersymmetry implies (don't ask me how, I'm assuming the cosmologists know what they're talking about) that most of the matter and energy created in the Big Bang aren't even observable. They exist in a cold and dark form that interacts with normal matter only through gravitation, and perhaps through the weak nuclear force. Electromagnetism, the force behind chemistry and the formation of molecules and which predominates our perception of the Universe, does not apply. As we ride Spaceship Earth around the Sun, and as the Sun rotates around the galactic core at a rate of 200 miles a second, we sweep through a headwind of a dark matter gas that does not have the power to stir the hair on our heads--yet is massive enough to keep all the stars in our galaxy from flying off in all directions at once.

Now that is really strange.

Even stranger is the brand new theory of the Big Rip.

Edwin Hubble discovered in the early twentieth century that the Universe is expanding. Light from distant objects is shifted toward the red end of the spectrum. The Doppler effect explains this change in color.

It's easier to conceive of the Doppler effect if you employ the oft-used example of sound waves from an approaching train. If you are standing at the train station listening to the whistle of a fast-approaching train, the pitch of the whistle is higher than it would be if you were on the train. The sound cannot travel faster than the speed of sound, obviously, but the distance between successive waves of sound is compressed as the whistle moves toward you. This is perceived as an increase in pitch. As the train passes you, the pitch drops. As it leaves the station without you getting a chance to jump aboard, the pitch of the whistle drops below the sound as it is heard by an observer on the train itself.

According to relativity, the speed limit of the Universe is the speed of light, which travels through space at a constant speed, regardless of the difference in speed between its source and the speed of its observer. Though the speed of light is itself invariant, the distance between adjacent wavefronts is increased if the source is moving away from the observer. The distance between crests in the light wave grows, and its frequency drops, and we observe this change as a shift in the color of light toward the red end of the spectrum. (Conversely, if a light source is moving rapidly toward us, we perceive its light to become bluer.)

Astronomers have known since Hubble's day that distant galaxies are moving away from us. It is almost as if we were at the center of the Universe when it began to expand. But this effect is an illusion. Space itself is expanding. It's difficult to imagine this in three dimensions, so consider a two dimensional example. If we were shrunk down to the size of bacteria and were standing on the surface of a balloon, looking over at our friends standing a few millimeters away, and the balloon's inflation were increased, it would seem as though they were getting further away.

In fact, if we were surrounded by other friends, they would all look as if they were moving away from us. From their perspective, of course, it would be we and all of the rest of their friends would would be moving away. It looks as though we're at the center of expansion, but it only looks that way. So too for the expansion of the Universe: the multidimensional manifold of spacetime itself is growing larger, and it only looks as if we are at the "center" of that expansion. Everything is getting further away from everything else.

The question inevitably comes to mind as to what the ultimate fate of the universe will be, if it's continually growing larger and larger. Will it keep expanding forever? Will it slow down and stabilize at some point? Could the expansion actually reverse itself, with all the matter in the Universe coming back together in a Big Crunch at some unimaginably distant point in the future? It depends on many factors, not least of which is the amount of matter in the Universe. If there is enough matter, the collective gravitational pull of everything on everything else might be enough to slow the expansion and eventually stop it... and perhaps cause the collapse of spacetime back in on itself.

Or so we thought, until astronomers determined that the expansion of the Universe is apparently accelerating.

Astronomers have been observing extremely distant objects such as supernovas and quasars with the Hubble Space Telescope, and with other powerful telescopes based on the ground. Their observations indicate that the light emitted from those objects is red-shifted even more than would be expected from the passive expansion of the Universe. The more distant the object, the greater its red shift, and the greater the increase in its red shift from expected values. The simple and inescapable conclusion is that those galaxies are moving away from us faster now than they used to.

There is an antigravitational force that permeates the Universe, and which appears to exert itself more strongly the more distant one is from its source. This flies in the face of intuition and experience, as well as the physics of all the kinds of energy and force that we know and understand. All other forms of energy appear to weaken as a function of distance.

Albert Einstein introduced the idea of an antigravitational force in his theory of General Relativity in 1917. It was more of a "fudge factor" than an explicit prediction, but it was required to balance his equations properly. He named this factor the cosmological constant. Later he came to regard it as the worst mistake he had ever made, but it seems as though he was correct after all. (As if any more proof were needed of Einstein's staggering genius as a theoretician.) The problem was that during his day, there was no theoretical framework to explain the presence of the cosmological constant... and no experimental evidence of its existence. Now there seems to be just such evidence. And the theory which provides for the existence of an antigravitational force whose power increases with distance is that of dark energy.

Imagine this, if you will: Supersymmetry is true and unobservable dark matter and dark energy predominate in our universe. Galaxies are speeding up as they get further away from each other. Eventually, the acceleration will grow to such an extent that the most distant galaxies will Doppler-shift below the frequency of visible light and will become invisible. Infrared telescopes in orbit have already revealed such objects. As the acceleration grows, every other galaxy in the observable universe will eventually be so far from every other galaxy that they will no longer be visible. The only objects in the sky will be the stars in our own galaxy. We will seem to be totally alone in the universe in our local island of stars.

And as I read in Science News this week, a trio of researchers at CalTech in Pasadena have come up with the idea of the Big Rip.

What if the reason for the acceleration of the distant galaxies is that the density of the dark energy in the universe is actually increasing? If that happens, the strength of the antigravitational force will continue to increase without limit. And if that happens, it won't be merely the galaxies that fly away from one another.

Eventually, thirty six billion years from now, according to their calculations, the antigravitational force of dark energy will grow so large that the stars will start to separate from one another, slowly at first, then more rapidly, until every star system is isolated from every other by a steadily increasing gulf of emptiness. At that point, it won't take long for the sky to empty of stars as they accelerate away, shifting down into dull red and then into invisibility. The sky will grow dark... lit only by the sun.

For a time.

The intensity of the dark energy will become so great that it will begin to affect objects on a smaller and smaller scale.

Not long after the stars vanish, the stars and their planets will be pushed away from each other, fleeing outward into the void, leaving the sun far behind.

And then the planets themselves will be pulverized by the relentless power of dark energy. The void will be filled with an increasingly diffuse pall of dissociated matter.

And eventually, just before the very end, when the intensity of dark energy begins its asymptotic spike towards infinity, atoms themselves will be sundered. Femtoseconds later, the universe of normal time, space, energy and matter will cease to be.

What will be left behind?

What will come after?

Jeff Kirk

March 15, 2003