A Dark Force in the Universe

Three years ago, observations of distant, exploding stars blew to smithereens some of astronomers’ most cherished ideas about the universe. To piece together an updated theory, they’re now thinking dark thoughts about what sort of mystery force may be contorting the cosmos.

According to the standard view of cosmology, the once infinitesimal universe has ballooned in volume ever since its fiery birth in the Big Bang, but the mutual gravitational tug of all the matter in the cosmos has gradually slowed that expansion.

In 1998, however, scientists reported that a group of distant supernovas were dimmer, and therefore farther from Earth, than the standard theory indicated. It was as if, in the billion or so years it took for the light from these exploded stars to arrive at Earth, the space between the stars and our planet had stretched out more than expected. That would mean that cosmic expansion has somehow sped up, not slowed down. Recent evidence has only firmed up that bizarre result (SN: 3/31/01, p. 196).

In 1929, Edwin P. Hubble discovered that distant galaxies are fleeing from one another as if the entire universe is swelling in size. Ever since, astronomers have been hoping to answer a key question: Will the expansion of the universe, slowed by gravity, go on forever, or will the cosmos eventually collapse into a Big Crunch?

Despite decades of effort and countless studies devoted to the ballooning of the universe, the recent findings stunned astronomers. Few suspected that all along they were asking the wrong question.

“For 70 years, we’ve been trying to measure the rate at which the universe slows down. We finally do it, and we find out it’s speeding up,” says Michael S. Turner of the University of Chicago.

An accelerated expansion would seem to contradict all common sense, says Andreas J. Albrecht of the University of California, Davis. Throw a ball into the sky, and after it reaches a certain height, it will come back down, he notes. Now imagine throwing another ball upward and finding that instead of it falling back down, it somehow keeps moving up faster and faster. For that to happen, there would have to be some force pushing upward on the ball strongly enough to overcome gravity’s downward tug.

Astronomers have come to believe that just such a force is stretching the very fabric of space.

What is this mystery force?

Cosmologists have proposed that it derives from dark energy — a substance whose properties and origin scientists have only begun to explore. At stake is more than just a better understanding of the fate of the universe: The very presence of dark energy may enable scientists to explain the fundamental forces of the universe and tease out the hidden connections among them.

Says Albrecht: “This is the most exciting endeavor going on in physics right now.”

Dark matter

Astronomers have dark imaginations. They’re already obsessed with another phenomenon that they call dark matter, which is entirely separate from dark energy. Dark matter is the invisible material that theorists say makes up 95 percent of the mass of the universe. It gathers into vast clumps and exerts an ordinary gravitational tug on its surroundings. The proposed dark energy, in contrast, would inhabit empty space and would be evenly distributed throughout the universe.

Moreover, dark energy would exert a repulsive force — what might be called antigravity. More accurately, dark energy would be the flip side of ordinary gravity because it would possess a strange property called negative pressure. Something with negative pressure resists being stretched, as a coiled spring does: Pull on the spring and it pulls back.

To understand what pressure — negative or positive — has to do with gravity, take a look at Einstein’s general theory of relativity. According to that theory, matter isn’t the only source of gravity. There are two other sources: energy, which is interchangeable with mass according to Einstein’s famous equation E = mc², and pressure.

A familiar example of pressure is an inflated balloon. In this everyday experience, pressure within the balloon has a negligible effect on its gravity. At physical extremes, however, pressure can dominate. When that occurs, some strange things can happen, such as the formation of black holes.

Pressure prevents a star as massive as the sun from imploding under its own gravity. That’s because the radiation emitted by the star exerts a gaslike pressure outward.

Stars more massive than the sun must exert an even stronger pressure to counterbalance their gravity. For a star greater than about four times the sun’s mass, the counterbalancing pressure becomes as strong as the density of the star. When this happens, pressure contributes as much as mass does to the gravitational force, Einstein’s theory says. In effect, the gravitational pull inward drastically increases.

The more the star contracts, the greater its pressure and density, and thus the stronger the gravity. Unable to resist, the star undergoes a runaway collapse, and its gravity becomes so strong that not even light can escape its grasp. A black hole is born.

The contribution of pressure is “an aspect of gravity that was there all along,” notes Turner. He says that anyone who accepts the reality of black holes has implicitly accepted the notion that pressure can be a key source of gravity.

According to Einstein’s theory, pressure has another mind-bending property: It can be negative. An object having negative pressure resists being stretched. “Think of negative pressure as silly putty or a rubber sheet. The atoms don’t want to be drawn apart; there’s a force that pulls them together,” says Turner. Negative pressure, he notes, would impart a springiness or elasticity to space.

It’s counterintuitive to think that a material such as rubber, which draws itself inward when stretched, could push objects outward. Yet if dark energy’s antigravity effect — its ability to exert negative pressure — were strong enough, it could swing the gravity meter from the plus side to the minus side, Einstein’s theory dictates.

Gravity normally pulls matter together. Instead of pulling, dark energy would cause gravity to push. Instead of tugging and slowing the expansion of the universe, dark energy would rev it up.

As bizarre as dark energy may seem, it’s the only theory to explain the accelerating cosmos that is compatible with Einstein’s general theory of relativity, says Turner.

Dark energy

In its simplest version, dark energy would be a true constant, equally distributed throughout the universe and continuously exerting the same amount of force as the universe expands. In 1917, Einstein posited a version of this energy, which he called the cosmological constant. Physicists have sporadically been returning to that idea ever since. Because the cosmological constant would exist even in the absence of matter or radiation, its origins might lie within empty space itself.

This property could tie dark energy to one of the stranger properties of quantum mechanics. Quantum theory dictates that empty space — what physicists call the vacuum — seethes with energy as pairs of particles and antiparticles pop in and out of existence.

This vacuum energy has some subtle but measurable effects. For example, it shifts the energy levels of atoms slightly and exerts a force between closely spaced metal plates (SN: 2/10/01, p. 86). In 1967, the Russian astrophysicist Yakov B. Zeldovich showed that vacuum energy has an intriguing property. The energy associated with this nothingness has negative pressure.

That means vacuum energy could push galaxies apart at ever-increasing speeds, making it an ideal candidate for being the dark energy.

Alas, there appears to be a huge problem. Calculations reveal that the energy stored in the vacuum is 120 orders of magnitude larger than the dark energy that cosmologists are positing.

“If the vacuum energy density really is so enormous, it would cause an exponentially rapid expansion of the universe that would rip apart all the electrostatic and nuclear bonds that hold atoms and molecules together,” note Paul J. Steinhardt of the University of Pennsylvania in Philadelphia and Robert R. Caldwell of Dartmouth College in Hanover, N.H., in a recent review article. “There would be no galaxies, stars, or life.”

It’s likely, physicists admit, that they don’t really know how to calculate vacuum energy. That complication may have to do with their limited knowledge about the nature of gravity. Einstein’s theory holds that gravity curves empty space — the vacuum — but scientists don’t yet know how gravity does so on a quantum mechanical scale.

Thus, scientists have yet to unify quantum theory with gravity. Some hold out the hope that when they do, they’ll miraculously find that the 120 orders of magnitude drop to zero — almost. There might be just enough vacuum energy left over to account for the amount harbored by dark energy.

Many researchers think that’s a forlorn hope, however. They believe that a better understanding of the vacuum energy will reveal it to be exactly zero.

In that case, dark energy would have to be something else. Several theorists believe this something else blankets the universe and varies with time and place. Steinhardt, his University of Pennsylvania colleague Rahul Dave, and their collaborators call this variable form of dark energy “quintessence.”

Quintessence takes on a different form and strength depending on what time it is in the universe. Scientists have established that just after the Big Bang, high-energy radiation filled the universe and was the dominant form of energy. Matter contributed very little to the cosmic-energy budget. In that era, quintessence would have mimicked the properties of radiation, Steinhardt says. Like radiation, it would have exerted positive pressure.

As the universe cooled and particles slowed, the energy balance shifted in favor of matter. Material started to clump together to form larger structures. Steinhardt proposes that at the onset of that era, some 50,000 years after the Big Bang, quintessence changed. As he and his colleagues see it, quintessence — dark energy — settled down to a fixed value and began exerting a negative pressure throughout the cosmos.

In this vision, the dark-energy density initially paled in comparison with the density of matter. Gravity thus acted in its familiar fashion, slowing the expansion of the universe. But as the volume of the universe continued to expand, its matter density decreased. As matter density dwindled, the energy density associated with quintessence remained constant — or nearly so. Consequently, quintessence became gravity’s new boss. The expansion of the cosmos would then have gone into overdrive.

It’s no coincidence that humans are living at a time when it’s possible to observe cosmic acceleration, says Steinhardt. The same shift in the mass-energy balance that gave rise to stars, galaxies, planets, and life also transformed quintessence into a cosmic accelerator.

Steinhardt admits he hasn’t come up with any fundamental explanation of why the quintessence field would change in this way. The answer, he says, could lie in new physics, perhaps in a new elementary particle implied by quintessence. The explanation could also provide a hint about how physicists might tackle one of their thorniest and most intriguing challenges — explaining the existence of the fundamental forces and how they intertwine. Quintessence, or dark energy, could be a linchpin that holds together both old and new physics.

In a version of quintessence proposed by Albrecht and his University of California, Davis colleague Constantinos Skordis, the repulsive force may come from other, unseen dimensions or even from other universes beyond our own. That dovetails with a theory from elementary particle physics, which posits that our three dimensions plus time are but a tiny part of a much broader, multidimensional canvas.

The extra dimensions wouldn’t have a direct influence on our own four-dimensional space-time. But because gravity exerts itself by distorting space, the gravitational field associated with the extra dimensions might affect our own. Albrecht suggests that gravity’s ability to repel as well as attract could stem from the existence of those other dimensions. Those dimensions in turn could provide additional hints about another deep puzzle of physics — the quantum nature of gravity, he notes.

Albrecht says his theory offers another advantage. It describes quintessence by using only simple constants of nature, such as the speed of light, the gravitational constant, and Planck’s constant of quantum mechanics. The quintessence field that he and Skordis construct from these constants could indeed have become dominant long after the Big Bang, prompting the current phase of accelerated expansion.

Albrecht acknowledges the ad hoc nature of quintessence theories, which are still in their infancy. “We each have our own angles,” he notes. “They all have a lot of weaknesses.”

Cosmic expansion

Several studies now in the works may enable astronomers to confirm whether or not cosmic expansion is accelerating. Moreover, the studies could also reveal which of the two proposed forms of dark energy — quintessence or vacuum energy — is driving that acceleration. Astronomers think they can distinguish the two types of dark energy because quintessence would give the universe a smaller push.

If vacuum energy really is the dark energy, then the universe will expand forever at an accelerating rate.

If quintessence proves correct, then the amount by which space has stretched over the past few billion years is less than if dark energy is the vacuum energy. Because the volume of the cosmos is smaller in a quintessential universe, supernovas up to a few billion light-years from Earth would appear somewhat brighter and fewer galaxies would exist within a given span of cosmic time. Under the quintessence theory, the dark energy varies in time and space, so determining the fate of the cosmos isn’t so straightforward.

Indeed, dark energy might even be a fleeting phenomenon that gives the universe an extra kick for several billion years and then disappears. In that case, it could resemble an extended replay of inflation — the brief, mysterious epoch of hyperexpansion that is thought to have occurred during the earliest moments of the universe (SN: 12/19 & 26/98, p. 392).

Dark energy “is involved in very fundamental issues,” says Turner. “This could be a key to understanding the forces of nature, including the quantum theory of gravity.”

Strange as dark energy seems, Turner notes, “I guarantee you it’s not going away.”