Does Dark Energy Really Exist?

COSMOLOGY Does DARK ENERGY Maybe not. Really Exist? The observations that led astronomers to deduce its existence could have another explanation: that our galaxy lies at the center of a giant cosmic void By Timothy Clifton and Pedro G. Ferreira n science, the grandest revolutions are often of a universe populated by billions of galaxies triggered by the smallest discrepancies. In the that stretch out to our cosmic horizon, we are led I16th century, based on what struck many of to believe that there is nothing special or unique his contemporaries as the esoteric minutiae of ce- about our location. But what is the evidence for KEY CONCEPTS lestial motions, Copernicus suggested that Earth this cosmic humility? And how would we be able ■ The universe appears to be was not, in fact, at the center of the universe. In to tell if we were in a special place? Astronomers expanding at an accelerat- our own era, another revolution began to unfold typically gloss over these questions, assuming ing rate, implying the exis- 11 years ago with the discovery of the accelerat- our own typicality sufficiently obvious to war- tence of a strange new ing universe. A tiny deviation in the brightness of rant no further discussion. To entertain the no- form of energy—dark ener- exploding stars led astronomers to conclude that tion that we may, in fact, have a special location gy. The problem: no one is they had no idea what 70 percent of the cosmos in the universe is, for many, unthinkable. Never- sure what dark energy is. consists of. All they could tell was that space is theless, that is exactly what some small groups of ■ Cosmologists may not ac- filled with a substance unlike any other—one physicists around the world have recently been tually need to invoke exotic that pushes along the expansion of the universe considering. forms of energy. If we live rather than holding it back. This substance be- Ironically, assuming ourselves to be insignifi- in an emptier-than-average came known as dark energy. cant has granted cosmologists great explanatory region of space, then the It is now over a decade later, and the existence power. It has allowed us to extrapolate from cosmic expansion rate varies with position, of dark energy is still so puzzling that some cos- what we see in our own cosmic neighborhood to which could be mistaken mologists are revisiting the fundamental postu- the universe at large. Huge efforts have been for a variation in time, lates that led them to deduce its existence in the made in constructing state-of-the-art models of or acceleration. first place. One of these is the product of that the universe based on the cosmological princi- earlier revolution: the Copernican principle, that ple—a generalization of the Copernican principle ■ A giant void strikes most cosmologists as highly un- Earth is not in a central or otherwise special po- that states that at any moment in time all points likely but so for that matter sition in the universe. If we discard this basic and directions in space look the same. Combined does dark energy. Observa- principle, a surprisingly different picture of what with our modern understanding of space, time tions over the coming years could account for the observations emerges. and matter, the cosmological principle implies will differentiate between Most of us are very familiar with the idea that that space is expanding, that the universe is get- the two possibilities. our planet is nothing more than a tiny speck or- ting cooler and that it is populated by relics from —The Editors biting a typical star, somewhere near the edge of its hot beginning—predictions that are all borne an otherwise unnoteworthy galaxy. In the midst out by observations. DIXON DON 48 SCIENTIFIC AMERICAN © 2009 SCIENTIFIC AMERICAN, INC. April 2009 UNEVEN EXPANSION OF SPACE, caused by variations in the density of matter on an epic scale, could produce the effects that astronomers conventionally attribute to dark energy. You are here © 2009 SCIENTIFIC AMERICAN, INC. www.SciAm.com SCIENTIFIC AMERICAN 49 Astronomers find, for example, that the light where, we are led to the conclusion that the uni- from distant galaxies is redder than that of near- verse must be permeated by an exotic form of en- by galaxies. This phenomenon, known as red- ergy, dark energy, that exerts a repulsive force. shift, is neatly explained as a stretching of light Nothing meeting the description of dark en- waves by the expansion of space. Also, micro- ergy appears in physicists’ Standard Model of wave detectors reveal an almost perfectly smooth fundamental particles and forces. It is a sub- curtain of radiation emanating from very early stance that has not as yet been measured directly, times: the cosmic microwave background, a relic has properties unlike anything we have ever seen of the primordial fireball. It is fair to say that and has an energy density some 10120 times less these successes are in part a result of our own hu- than we may have naively expected. Physicists mility—the less we assume about our own signif- have ideas for what it might be, but they remain icance, the more we can say about the universe. speculative [see “The Quintessential Universe,” by Jeremiah P. Ostriker and Paul J. Steinhardt; Darkness Closes In Scientific American, January 2001]. In short, So why rock the boat? If the cosmological prin- we are very much in the dark about dark energy. ciple is so successful, why should we question it? Researchers are working on a number of ambi- The trouble is that recent astronomical observa- tious and expensive ground- and space-based tions have been producing some very strange missions to find and characterize dark energy, results. Over the past decade astronomers have whatever it may be. To many, it is the greatest found that for a given redshift, distant superno- challenge facing modern cosmology. va explosions look dimmer than expected. Red- shift measures the amount that space has A Lighter Alternative expanded. By measuring how much the light Confronted with something so strange and from distant supernovae has redshifted, cosmol- seemingly so improbable, some researchers are ogists can then infer how much smaller the uni- revisiting the reasoning that led them to it. One verse was at the time of the explosion as com- of the primary assumptions they are questioning pared with its size today. The larger the redshift, is whether we live in a representative part of the the smaller the universe was when the supernova universe. Could the evidence for dark energy be COPERNICUS’S occurred and hence the more the universe has accounted for in other ways if we were to do LEGACY expanded between then and now. away with the cosmological principle? The observed brightness of a supernova pro- In the conventional picture, we talk about the The Copernican principle holds that vides a measure of its distance from us, which in expansion of the universe on the whole. It is very Earth does not occupy a special place in the universe. The universe has a turn reveals how much time has elapsed since it much like when we talk about a balloon blowing uniform density (homogeneity) and occurred. If a supernova with a given redshift up: we discuss how big the entire balloon gets, looks the same in every direction looks dimmer than expected, then that superno- not how much each individual patch of the bal- (isotropy). va must be farther away than astronomers loon inflates. But we all have had experience with Though powerful, the principle thought. Its light has taken longer to reach us, those annoying party balloons that inflate un- applies only on scales much larger than a galaxy. After all, if the cosmos and hence the universe must have taken longer to evenly. One ring stretches quickly, and the end were completely uniform, it would be grow to its current size [see box on opposite takes a while to catch up. In an alternative view a thin gruel of atoms rather than a page]. Consequently, the expansion rate of the of the universe, one that jettisons the cosmologi- constellation of galaxies. Also, the universe must have been slower in the past than cal principle, space, too, expands unevenly. A principle applies in space but not in previously expected. In fact, the distant superno- more complex picture of the cosmos emerges. ) time. We live in a special era—long enough after the big bang that vae are dim enough that the expansion of the uni- Consider the following scenario, first suggest- complex life can form but not so long verse must have accelerated to have caught up ed by George Ellis, Charles Hellaby and Nazeem Copernicus that stars have all died off. with its current expansion rate [see “Surveying Mustapha, all at the University of Cape Town in MAGES ( MAGES Copernicus is commonly associat- Spacetime with Supernovae,” by Craig J. Hogan, South Africa, and subsequently followed up by I ed with a dethroning of humanity ETTY Robert P. Kirshner and Nicholas B. Suntzeff; Sci- Marie-Noëlle Célérier of the Paris-Meudon Ob- G from any position of importance. But entific American, January 1999]. servatory in France. Suppose that the expansion RARY/ as historian Dennis Danielson of the B I This accelerating expansion is the big sur- rate is decelerating everywhere, as matter tugs on L University of British Columbia argues, RT although pre-Copernican Europeans prise that fired the current revolution in cosmol- spacetime and slows it down. Suppose, further, A placed Earth at the center of the ogy.

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