Recent observations have led to the idea that the Universe is filled with a mysterious new substance, a substance so overwhelming that it accounts for three quarters of all the mass and energy found in space. Astronomers have called it “dark energy” for want of any idea about what it really is. There is no natural explanation for it in any current theory in physics.
“A long time ago, in a galaxy far, far away . . . .” This may not be the most original way to open a chapter (any self-respecting Star Wars fan will be sure to recognize it), but for astronomers it has a special resonance. A long time ago, in a galaxy far, far away, a star became a supernova. The light from this explosion traveled for billions of years through space before a tiny fraction of it fell into telescopes on Earth during the late 1990s, and changed the way we think of the Universe. At first, the observation just seemed a little odd because the supernova was somewhat dimmer than astronomers expected. They paid it little attention, reasoning that perhaps something was wrong with their measurements. But as more very distant supernovae were observed, so the discrepancy repeated itself, and what started as a scientific curio turned into one of the greatest challenges to our modern understanding of the Universe.
The supernovae that revealed dark energy were being studied by two independent teams of astronomers who were using them to measure the expansion rate of the early Universe. They then planned to compare those rates with the well-measured expansion rate for today’s Universe, and calculate by how much this expansion had slowed down. All astronomers believed that the expansion rate was greater in the past, because the gravity of the celestial objects would be slowing it down. But, when the teams completed their calculations, they had a great surprise: the rate of expansion in the past was lower than it is today. Far from slowing down, the Universe’s expansion is accelerating. As the researchers investigated further, their observations led to the conclusion that some form of “antigravity force” had switched on throughout space, and was now driving the celestial objects apart. The prestigious academic journal Science named the discovery its “Breakthrough of the Year” in 1998. Once astronomers became accustomed to the new idea, they welcomed this accelerating Universe because it offered a potential solution to an accumulating body of nonsensical observations.
The 1990s were dichotomous years for cosmology. On the one hand, the theory of the Big Bang was no longer seriously doubted, yet, on the other hand, puzzling observations were beginning to strike at the very fundamentals of our understanding of the cosmos. There was the age crisis (see How Old is the Universe?), in that some celestial objects had been measured to be older than the Universe itself—clearly an impossible situation. Then there were the inventories of matter: studies of the microwave background radiation were telling astronomers that the distribution of matter and energy, just 380,000 years after the Big Bang, was a finely balanced mix that produced no overall curvature of space–time, a situation known as a “flat Universe” (see How Did the Universe Form?).
The mass density for a flat Universe corresponds to an average of just 10–26 kilograms per cubic meter; although this might sound minuscule, astronomers consistently failed to reach this figure when they added up everything they could see in the Universe. Detailed analysis of the microwave background readings, this time taken from spacecraft, confirmed the mass deficiency. The abundance of mass was just four percent of that needed for a flat Universe. Even if astronomers included all of the hypothetical dark matter proposed to hold galaxy clusters together and to make galaxies rotate correctly (see What is Dark Matter?), they still only reached 26 percent of the required density.
THE COMPONENTS OF THE UNIVERSE: CALCULATIONS SUGGEST THAT ALMOST THREE QUARTERS OF THE UNIVERSE IS DARK ENERGY, MOST OF THE REST IS DARK MATTER AND JUST A SMALL PERCENTAGE IS MATTER AS WE KNOW IT
The discovery of the accelerating expansion not only solved the age crisis, by making the Universe appear younger than it actually is, it also gave cosmologists a way to “flatten” the Universe.
One of the earliest conclusions that Einstein drew from his Special Theory of Relativity was that mass and energy are interchangeable. He investigated, mathematically, the effects of placing energy into space and found that it causes a curvature of the space–time continuum, just as mass does (see Was Einstein Right?). He called the amount of energy naturally occurring in space–time the “cosmological constant.” It exists rather like the energy in a glass of water at room temperature: this energy may not be immediately obvious, but it must be removed before the water can freeze.
Working before Edwin Hubble’s discovery of the expanding Universe, Einstein calculated the amount of energy—the value of his cosmological constant—necessary in space–time to resist all the gravity generated by the various celestial objects and prevent the Universe from collapsing. When Einstein learned of Hubble’s discovery of expanding space, he considered that his cosmological constant was superfluous and famously called it his biggest blunder. The realization of the accelerating Universe, however, has made astronomers think there may be unseen energy in space–time after all.
If the “vacuum” of space contains enough energy, it will overcome the force of gravity between the celestial objects and drive the Universe to expand at an ever-accelerating rate. Physicists tend to refer to this as “vacuum energy,” whereas astronomers have taken to calling it dark energy, to underline its mysterious nature. While it may sound similar to dark matter, and indeed a few researchers are trying to find links between the two, most believe they are nothing to do with each other because they work on different scales. Dark matter was introduced to solve the movement of galaxies; dark energy was invoked to solve the accelerated expansion of the whole Universe.
“The Universe is made mostly of dark matter and dark energy, and we don’t know what either of them is.”
SAUL PERLMUTTER CONTEMPORARY COSMOLOGIST
To account for the observed acceleration, the density of dark energy needs to be only very low, which is probably why we do not see its effects at small cosmological scales, such as within the Solar System or even our whole Galaxy, the Milky Way. Only when a vast swathe of space lies between a distant object and us does the cumulative effect of dark energy become evident. When the amount of it is totaled up across the entire Universe, it becomes overwhelming, accounting for around three-quarters of the mass and energy in the cosmos, and rendering space almost perfectly flat.
Einstein’s theory incorporating a cosmological constant as dark energy does not say where this energy comes from; it just computes its effects once it is there. In the years since the discovery of the accelerating Universe, all attempts to explain a positive cosmological constant have run into severe problems. To start with, it is difficult to conjure a cosmological constant from present physics, some of which has even been designed specifically to remove it. While quantum theory actually predicts a colossal cosmological constant: 10120 times (one followed by 120 zeros) larger than that inferred by the acceleration of the Universe, supersymmetry theory (see What is Dark Matter?) was formulated in order to get this down to zero. With its proposed raft of mirror-image particles it succeeds in canceling out the cosmological constant altogether. But supersymmetry was developed before the existence of dark energy was proposed; if the dark energy is Einstein’s cosmological constant then we will have to either scrap supersymmetry or revise it in a way that no one can currently conceive. Faced with such a choice, scientists have been searching for other dark energy solutions. Their next proposal was “quintessence,” different from the cosmological constant because it is not a vacuum energy but a new long-range force.
Physicists have spent a lot of time trying to understand the disparate forces of nature, and have settled on the belief that just four govern behavior in the Universe: gravity, electromagnetism, the strong nuclear force and the weak nuclear force (see Was Einstein Right?). Occasionally it has been suggested a fifth force of nature must also be present, but no conclusive evidence has been found. Now, however, the accelerating expansion gives them ample justification in saying that some new force could be driving the cosmos. Named in honor of the classical fifth element thought by the Greeks to compose the celestial objects, the quintessence force has one property that the cosmological constant does not: variability. Whereas the cosmological constant is the same everywhere, quintessence can in principle vary with time, and from place to place, making it far more versatile. A number of different versions of quintessence theory have been proposed, each depending upon how fast the force varies with time. One version, known as “tracker quintessence,” closely follows the density of matter and energy in the Universe to produce a gradual acceleration with time. The most extreme version is termed “phantom energy,” which builds up inexorably so that the expansion moves faster and faster until eventually the Universe rips itself to pieces (see What Will Be the Fate of the Universe?).
Being a force, quintessence is expected to be generated by the celestial objects themselves, just as electromagnetism or indeed gravity is generated. This produces an overall force field that accelerates the Universe on its largest scales. On smaller scales, it will also generate a force between individual celestial objects and hence move them around, just as gravity does. In other words, if quintessence exists, we should see motions that cannot be explained by gravity.
Some anomalous motions have been explained by invoking dark matter or “modified Newtonian dynamics” (see What is Dark Matter?), and a small number of researchers persist in attempts to see whether these motions are what might be expected from quintessence, thus explaining both dark matter effects and dark energy effects as different manifestations of the quintessence force. Most, however, believe that the two phenomena are entirely unrelated, but finding a form of quintessence that can accelerate the Universe while leaving individual celestial objects untouched is proving tricky. So some physicists are thinking that, instead of adding a new force of nature, the solution might be to modify an old one. Perhaps there are unexpected properties of gravity that appear over gargantuan distances, which Einstein’s general relativity did not predict.
It turns out that modifying gravity is just as difficult as making quintessence work. Modifications to give a large-scale acceleration usually introduce changes on the smaller scale too, which would affect the movements of the planets by noticeable amounts. With the best of our technical expertise, we do not see such effects; planetary movements are accurately explained by Newton’s gravity or Einstein’s general relativity. However, an American theoretician, Gia Dvali, has developed an imaginative modified theory of gravity called DGP, in which the particles that carry gravity, the hypothetical gravitons, have a small mass and so impart an underlying shape to space–time.
To produce the observed acceleration of the Universe, DGP gravity allows the gravitons to decay with a half-life of 15 billion years (this means that in 15 billion years their number diminishes to half). As the gravitons steadily disappear—ending up in parallel universes (see Are There Alternative Universes?)—so the strength of gravity between objects decreases and the Universe’s expansion speeds up. This behavior would alter the Moon’s orbit by about a millimeter a year, and the detection of such an amount is just within the capability of current lunar laser ranging experiments. If they show nothing on this scale then DGP gravity cannot be right.
Perhaps the most outrageous and yet paradoxically the most conservative explanation for dark energy is to overturn an assumption so ingrained in cosmology that most people have forgotten it is an assumption. It is the “cosmological principle,” which essentially states that viewed on a sufficiently large scale there are no preferred directions or preferred places in the Universe. In astronomical parlance, it is said that the Universe is homogeneous and isotropic, that is possessed of the same composition and properties, and uniform, in all directions. Alexander Friedman introduced the cosmological principle in 1923 to make it possible to solve the equations of general relativity. Using this assumption meant that Friedman could think of matter as a uniform fluid filling space, and as such it allowed the early cosmologists to investigate relativity and to predict the Big Bang. Since that time, cosmologists have clung to the idea, despite finding ever-larger density variations across the Universe.
The awareness of larger structures in the Universe began around 1937 when Clyde Tombaugh, who found Pluto, studied 30,000 galaxies and discovered that many were found in clusters. He also saw giant voids that appeared to be empty of galaxies. Taken together, the clusters and the voids provide density variations that are not compatible with the cosmological principle, unless one looks at larger scales so that these density variations can be averaged out. The further astronomers looked, however, the more structure they seemed to find, and we now know that clusters of galaxies are gathered together to form superclusters that spread across space for hundreds of millions of light years.
The first example of such a gigantic structure was seen in 1989. The so-called “Great Wall” of galaxies is some 500 million light years long and 200 million light years wide, yet a mere 15 million light years in depth. This spurred a tremendous period of surveying in the 1990s, when astronomers used ever more powerful instruments to simultaneously record the light from dozens and even hundreds of galaxies. They made hundreds of thousands of observations, enough to give a reliable picture of how galaxies are distributed across the Universe, and found that only on scales greater than 300 million light years could the Universe be said to obey the cosmological principle. And newer discoveries cast doubt even on this. In 2003, another great wall of galaxies was found, a much greater wall, in fact, because it stretches over 1.37 billion light years in space. Even more recently, there was the hint of a gigantic void in space, seen in microwave background data as a cold region about a billion light years in diameter. Radio telescope observations subsequently confirmed the dearth of galaxies in this vast tract of space, some 40 times larger than any previously known void.
There is currently no explanation for these observations. If we abandon the cosmological principle entirely and admit that the Universe cannot be thought of as more-or-less the same everywhere, then some effects suggested by general relativity that are usually considered negligible would become increasingly important. At the very least, we should expect the speed of the expansion of galaxies to be different in different places. It is thought that we may be located in a low-density region of the Universe; if so, then the Universe might indeed appear to be accelerating because our local bubble would be expanding slightly faster than the surrounding regions.
The trouble is that if the assumption of uniformity is abandoned and we accept that the distribution of matter is more complex than we thought, the mathematics of general relativity becomes impossible to solve. So how do we make progress? The only way to choose between the four options: cosmological constant, quintessence, modified gravity or complexity, is to conduct bigger and better surveys of galaxies.
Surveys are not usually thought of as being the most exciting science that can be performed. Mostly, they are necessary preparation in the quest to find interesting objects for more detailed study. But in the search for dark energy, surveys are essential because the galaxies are like leaves afloat on the cosmic ocean. They are moved around both by gravitational forces and by the expansion of the Universe. The further we look into space, the more the expansion overwhelms the individual movements—and it is this movement of the cosmic “ocean” we are interested in seeing.
Systematic observations of the furthest reaches of space will allow astronomers to measure the acceleration of the expansion at different times in the history of the Universe. Comparing these accelerations will tell them whether the acceleration has speeded up, or remained constant, or was maybe suddenly turned on at some critical moment. This will hopefully reveal the nature of the Universe’s mysterious dominant component that today we call dark energy.