Key Concepts
The universe appears to be expanding at an accelerating rate, implying the existence of a strange new form of energy dark energy. The problem: no one is sure what dark energy is.
Cosmologists may not actually need to invoke exotic forms of energy. If we live in an emptier-than-average region of space, then the cosmic expansion rate varies with position, which could be mistaken for a variation in time, or acceleration.
A giant void strikes most cosmologists as highly unlikely but so for that matter does dark energy. Observations over the coming years will differentiate between the two possibilities.
In science, the grandest revolutions are often triggered by the smallest discrepancies. In the 16th century, based on what struck many of his contemporaries as the esoteric minutiae of celestial motions, Copernicus suggested that Earth was not, in fact, at the center of the universe. In our own era, another revolution began to unfold 11 years ago with the discovery of the accelerating universe. A tiny deviation in the brightness of exploding stars led astronomers to conclude that they had no idea what 70 percent of the cosmos consists of. All they could tell was that space is filled with a substance unlike any other one that pushes along the expansion of the universe rather than holding it back. This substance became known as dark energy.
It is now over a decade later, and the existence of dark energy is still so puzzling that some cosmologists are revisiting the fundamental postulates that led them to deduce its existence in the first place. One of these is the product of that earlier revolution: the Copernican principle, that Earth is not in a central or otherwise special position in the universe. If we discard this basic principle, a surprisingly different picture of what could account for the observations emerges.
Most of us are very familiar with the idea that our planet is nothing more than a tiny speck orbiting a typical star, somewhere near the edge of an otherwise unnoteworthy galaxy. In the midst of a universe populated by billions of galaxies that stretch out to our cosmic horizon, we are led to believe that there is nothing special or unique about our location. But what is the evidence for this cosmic humility? And how would we be able to tell if we were in a special place? Astronomers typically gloss over these questions, assuming our own typicality sufficiently obvious to warrant no further discussion. To entertain the notion that we may, in fact, have a special location in the universe is, for many, unthinkable. Nevertheless, that is exactly what some small groups of physicists around the world have recently been considering.
Ironically, assuming ourselves to be insignificant has granted cosmologists great explanatory power. It has allowed us to extrapolate from what we see in our own cosmic neighborhood to the universe at large. Huge efforts have been made in constructing state-of-the-art models of the universe based on the cosmological principle a generalization of the Copernican principle that states that at any moment in time all points and directions in space look the same. Combined with our modern understanding of space, time and matter, the cosmological principle implies that space is expanding, that the universe is getting cooler and that it is populated by relics from its hot beginning predictions that are all borne out by observations.
Astronomers find, for example, that the light from distant galaxies is redder than that of nearby galaxies. This phenomenon, known as redshift, is neatly explained as a stretching of light waves by the expansion of space. Also, microwave detectors reveal an almost perfectly smooth curtain of radiation emanating from very early times: the cosmic microwave background, a relic of the primordial fireball. It is fair to say that these successes are in part a result of our own humility the less we assume about our own significance, the more we can say about the universe.
Darkness Closes in
So why rock the boat? If the cosmological principle is so successful, why should we question it? The trouble is that recent astronomical observations have been producing some very strange results. Over the past decade astronomers have found that for a given redshift, distant supernova explosions look dimmer than expected. Redshift measures the amount that space has expanded. By measuring how much the light from distant supernovae has redshifted, cosmologists can then infer how much smaller the universe was at the time of the explosion as compared with its size today. The larger the redshift, the smaller the universe was when the supernova occurred and hence the more the universe has expanded between then and now.
The observed brightness of a supernova provides a measure of its distance from us, which in turn reveals how much time has elapsed since it occurred. If a supernova with a given redshift looks dimmer than expected, then that supernova must be farther away than astronomers thought. Its light has taken longer to reach us, and hence the universe must have taken longer to grow to its current size. Consequently, the expansion rate of the universe must have been slower in the past than previously expected. In fact, the distant supernovae are dim enough that the expansion of the universe must have accelerated to have caught up with its current expansion rate [see "Surveying Spacetime with Supernovae," by Craig J. Hogan, Robert P. Kirshner and Nicholas B. Suntzeff; Scientific American, January 1999].
This accelerating expansion is the big surprise that fired the current revolution in cosmology. Matter in the universe should tug at the fabric of spacetime, slowing down the expansion, but the supernova data suggest otherwise. If cosmologists accept the cosmological principle and assume that this acceleration happens everywhere, we are led to the conclusion that the universe must be permeated by an exotic form of energy, dark energy, that exerts a repulsive force.
Nothing meeting the description of dark energy appears in physicists' Standard Model of fundamental particles and forces. It is a substance that has not as yet been measured directly, has properties unlike anything we have ever seen and has an energy density some 10120 times less than we may have naively expected. Physicists have ideas for what it might be, but they remain speculative [see "The Quintessential Universe," by Jeremiah P. Ostriker and Paul J. Steinhardt; Scientific American, January 2001]. In short, we are very much in the dark about dark energy. Researchers are working on a number of ambitious and expensive ground- and space-based missions to find and characterize dark energy, whatever it may be. To many, it is the greatest challenge facing modern cosmology.
A Lighter Alternative
Confronted with something so strange and seemingly so improbable, some researchers are revisiting the reasoning that led them to it. One of the primary assumptions they are questioning is whether we live in a representative part of the universe. Could the evidence for dark energy be accounted for in other ways if we were to do away with the cosmological principle?
In the conventional picture, we talk about the expansion of the universe on the whole. It is very much like when we talk about a balloon blowing up: we discuss how big the entire balloon gets, not how much each individual patch of the balloon inflates. But we all have had experience with those annoying party balloons that inflate unevenly. One ring stretches quickly, and the end takes a while to catch up. In an alternative view of the universe, one that jettisons the cosmological principle, space, too, expands unevenly. A more complex picture of the cosmos emerges.
Consider the following scenario, first suggested by George Ellis, Charles Hellaby and Nazeem Mustapha, all at the University of Cape Town in South Africa, and subsequently followed up by Marie-No lle C l rier of the Paris-Meudon Observatory in France. Suppose that the expansion rate is decelerating everywhere, as matter tugs on spacetime and slows it down. Suppose, further, that we live in a gargantuan cosmic void not a completely empty region, but one in which the average density of matter is only a half or maybe a third of the density elsewhere. The emptier a patch of space is, the less matter it contains to slow down the expansion of space; accordingly, the local expansion rate is faster within the void than it is elsewhere. The expansion rate is fastest at the very center of the void and diminishes toward the edge, where the higher-density exterior begins to make itself felt. At any given time different parts of space will expand at different rates, like the unevenly inflated party balloon.
Now imagine supernovae exploding in different parts of this inhomogeneous universe, some close to the center of the void, others nearer the edge and some outside the void. If we are near the center of the void and a supernova is farther out, space expands faster in our vicinity than it does at the location of the supernova. As light from the supernova travels toward us, it passes through regions that are expanding at ever faster rates. Each region stretches the light by a certain amount as it passes though, and the cumulative effect produces the redshift we observe. Light traveling a given distance is redshifted by less than it would be if the whole universe expanded at our local rate. Conversely, to achieve a certain redshift in such a universe, the light has to travel a greater distance than it would in a uniformly expanding universe, in which case the supernova has to be farther away and therefore appear dimmer.
Another way to put it is that a variation of expansion rate with position mimics a variation in time. In this way, cosmologists can explain the unexpected supernova observations without invoking dark energy. For such an alternative explanation to work, we would have to live in a void of truly cosmic proportions. The supernova observations extend out to billions of light-years, a significant fraction of the entire observable universe. A void would have to be of similar size. Enormous by (almost) anyone's standards.
From Scientific American
