What Does Dark Energy Mean for the Fate of the Universe?

A mysterious dark energy dating back to the dawn of the universe could be poised to rip it apart.

By Zeeya Merali|Wednesday, April 17, 2013
dark-energy-1
dark-energy-1
European Space Agency/NASA

[This article originally appeared in print as "Confronting the Dark."]

Astronomer Brian Schmidt vividly recalls his first inkling of the astonishing discovery that would win him a share of the 2011 Nobel Prize in Physics. It was a moment back in 1997 filled not with euphoria, but trepidation. 

Based at the Australian National University in Canberra, Schmidt was attempting to pinpoint the positions of supernovae — exploding stars that, at their apex, can outshine 5 billion suns. These bright, celestial objects serve as beacons across the sky, helping astronomers peer deep into space and calculate the size, shape and mass of the universe. 

Since most of Schmidt’s colleagues were scattered across the globe — in Europe, South America and the United States — the group had developed a 24-hour relay approach to analyzing their telescope data: Schmidt would work all day in the East before emailing the baton over to colleague Adam Riess, then at the University of California, Berkeley, who continued the study during daylight in the West. 

The morning in question, Schmidt received a graph from Riess, mapping the latest estimates for supernovae distances — but they were nothing like he expected. “I could see what was going on just by eye,” Schmidt says. “I remember thinking, ‘Oh Adam! Oh Adam! What have you done?’ ” 

Schmidt’s incredulity could be forgiven. He thought he would see an upward curving diagonal line, rising from the bottom left of the graph to the top right. Instead, the line veered downward, like the tail of a frightened dog. The surprising curl, frowning back at Schmidt, told him that astronomers might have to rethink the way the universe worked.

At the time, Schmidt thought he had a pretty good handle on the evolution of the cosmos: It began in a tiny fireball of energy — the Big Bang — and had expanded outward ever since, carrying galaxies and supernovae along for the ride. Yet these cosmic bodies exerted a gravitational pull, tugging back on each other just like the sun reined in the Earth. As far as Schmidt knew, the laws of physics kept the galloping cosmos in check; the universe was expanding, yes, but the force of gravity was slowing down the rate of expansion.

Yet Riess’ results told another story. Bizarrely, the supernovae appeared to be farther away from Earth than anybody had anticipated, implying that the cosmos was altogether bigger than astronomers had bargained for, as though gravity’s pulling power was somehow being overwhelmed. 

The best explanation was seemingly nonsensical: The universe’s expansion must be speeding up. Schmidt immediately deemed that conclusion “absurd.” No one had ever observed a force capable of driving acceleration like this; he dismissed the finding as a mistake. 

As the months passed, however, the disturbing notion persisted. What’s more, an independent team, led by Saul Perlmutter at the Lawrence Berkeley National Laboratory, California, had arrived at the same result. In 2011, Schmidt, Riess and Perlmutter shared the Nobel Prize in Physics for groundbreaking measurements revealing that the expansion of the universe is accelerating. Yet, despite having more than a decade to mull over the result, cosmologists are still struggling to understand how this could be happening. 

Putting a name to the root of their frustration, physicists somewhat whimsically chalked the speed up to an unknown “dark energy” that mysteriously pushes space apart, combatting gravity’s inward pull. If dark energy were to drive a galloping kind of expansion, the universe itself might one day be torn apart in a Big Rip. The deep mystery enshrouding this anti-gravity effect is perhaps the biggest puzzle of modern physics, with little consensus over where dark energy comes from, how it works, or if it exists at all. 

Evidence of Expansion

The first hints that the universe is expanding date back almost a century. Before that point, physicists still held dear the picture of the universe laid out by Isaac Newton more than 200 years earlier, in which space and time were immutable and could be measured accurately by rigid rulers and clocks. According to Newton, gravity was a force that could reach across empty space, pulling objects together by invisible threads. 

That view was challenged by Albert Einstein, who in 1915 laid out an alternative theory of gravity: general relativity. In his framework, the three dimensions of space and time are woven together to create a four-dimensional fabric, which acts as the source of gravity because it bends and warps around massive objects, like stars. Smaller objects, such as planets, roll into these space-time dips, as though pulled toward the heavier structures by a force. 

At first Einstein imagined the universe to be spherical and static — neither expanding nor contracting. To his surprise, however, the equations of general relativity presented an unstable cosmos: A slight variation in the delicate balance between radiation (or light) and matter could set the universe either expanding outward or shrinking inward. Determined to maintain his static picture, Einstein threw in an extra stabilizing element — named the “cosmological constant” — that counteracted any tendency for the cosmos to shrink under the pull of gravity by providing an outward push. The cosmological constant was little more than a fudge factor, to hold the universe still. 

But the static picture was wrong.

Roen Kelly/DISCOVER

By the 1930s, American astronomers Vesto Melvin Slipher and Edwin Hubble had measured the movement of distant galaxies, convincing everyone — even Einstein — that the universe was expanding, despite it all. Slipher and Hubble had opened a new window on the cosmos, one astronomers still peer through today. 

A critical find was based on the Doppler effect — the same phenomenon that makes a police siren change in pitch as the car approaches and then roars away. Both sound and light are made up of waves, and the pitch you hear or the color you see depends on the wavelength — the distance between successive peaks in the wave as it reaches you. 

Nineteenth-century Austrian physicist Christian Doppler realized that the wavelength that you measure will change if the source of the waves is moving relative to you. Waves that emanate from a source moving away from you are stretched by the time they reach you — lowering the pitch of sound waves and shifting the color of light waves toward the longer wavelength, or red, end of the spectrum. Waves coming from a source traveling toward you will be squished together — sounding higher in pitch or appearing blue-tinted. 

In 1912, Slipher found that the light from all galaxies he could see was redder than expected, indicating that the waves had been stretched. This “redshift” meant that those galaxies are moving away from Earth, and the amount of redshifting revealed their speeds. 

Calculating how far away a galaxy lies is tough, Schmidt notes, because “it’s not like you can just lay down rulers between us and them.” Hubble made an educated guess based on the reasoning that the brightest stars in each galaxy all shine with the same luminosity, like light bulbs of equal wattage, so the fainter they appear, the farther away they lie. 

It was a crude assumption, since not all stars have the same luminosity, but it roughly worked. Hubble found that the redder the light from distant galaxies, the faster those galaxies were speeding away. In 1929, he famously announced this proved the universe is expanding. 

“You see the same thing if you take a balloon and draw little stars on it,” explains Schmidt. “Blow the balloon up, and every star moves away from every other star — and the further away the stars are, the faster they move apart.” In the same way, Hubble’s discovery fit a picture of a universe that initially started out compact but was now expanding outward like an inflating balloon. 

Lighting Cosmic Candles

I meet with Schmidt on one of his rare visits to England, where he is being inducted as a fellow of London’s Royal Society, the world’s oldest scientific academy. He appears strikingly young — almost cherubic, with a mop of blond hair, blue eyes and full cheeks. By contrast, most Nobel winners are recognized toward the end of their careers, allowing time to establish the impact of their work. But Schmidt is only 46, Riess is a little younger, and Perlmutter a few years older. To be commended so soon after making their discovery is a mark of how significant their peers find it. 

Schmidt’s fascination with the night sky blossomed in high school, when his family moved to Alaska — a challenging place to do astronomy, he notes, because “during the summer it never gets dark, and during the winter it’s colder than hell.” But Alaska had the aurora borealis, the natural display of colored lights created at high latitudes as charged particles hit the atmosphere. 

His imagination fired, Schmidt combined stargazing with his other childhood passion: computing. In 1981, his father, a biologist, bought one of the first IBM PCs, and the 14-year-old Schmidt spent two years programming it to calculate when eclipses would happen. 

BIG-BANG-3
BIG-BANG-3
Roen Kelley/DISCOVER

That skill with computer coding came in handy a few years later when, as an undergraduate at the University of Arizona in Tucson, he wrote software to sift through the myriad celestial dots of light picked up by telescopes to identify supernovae, which are more luminous than regular stars and so short-lived they last only a few weeks. 

At the time, astronomers were still struggling to pin down the universe’s expansion rate, and Schmidt’s student project to spot supernovae was a key. Since Hubble’s guess that every star has the same luminosity is not strictly true, to chart the universe’s expansion astronomers needed more reliable cosmic candles — celestial objects that they could trust to burn with the same luminosity no matter how far from Earth. 

They turned to a type of supernova created by the death of stars of about the same mass as our sun. During their lifetime, such stars burn hydrogen and helium, giving them the energy to resist the incessant pull of gravity trying to draw their atoms inward. Once this fuel is used up, however, the remaining matter is crushed into the center of the star, which becomes a white dwarf.

These heavyweights are so dense — a teaspoon of white dwarf matter weighs several tons — that their ferocious gravity can strip matter from the outer layers of neighboring stars, adding to their bulk. When a white dwarf’s mass hits a critical value, 1.38 times the mass of the sun, it explodes like a giant thermonuclear bomb. 

Crucially, because these supernovae, called type 1a, all ignite when they hit the same mass, the brightness of their explosions is similar enough to guide astronomers. Just by measuring how bright the explosion appears, astronomers can estimate a supernova’s distance from Earth. And because light waves are stretched as they travel through expanding space, the redshift allows astronomers to directly measure that expansion.

In 1989, by then a Ph.D. candidate at Harvard, Schmidt used supernovae distance markers to work out how fast the universe was expanding in real time. There he met Riess, who was a graduate student three years below him with the same adviser, Robert Kirshner. 

Riess, too, had become hooked on science as boy. To his parents’ chagrin, he had a penchant for gruesome experiments: At age 6, he cut worms in half to see if they kept wiggling. (They did.) Later, wondering about electricity, he stuck a piece of metal across the two openings of a household socket. “I blew out the circuits in my home, but I learned about short-circuiting,” he laughs.

The idea to use a type 1a supernova to track the universe’s expansion further back in time came from Schmidt and Riess’ acquaintance — and soon-to-be rival — Perlmutter. Perlmutter had already identified seven type 1a supernovae 10 times farther away than any that had been seen before. Since light from distant objects takes time to reach Earth, the deeper you look into the sky, the further back into the history of the universe you see. 

And pinpointing the position of these extremely far-flung supernovae could reveal how fast the universe had been expanding in the past. “If the cosmos had been expanding rapidly, the redshifts of distant supernovae would be more pronounced when compared to the light emanating from nearby supernovae. 

On the other hand, if the cosmos had been expanding slowly, the redshifts of the distant supernovae would be less extreme.” By relating the redshifts of these very old stellar corpses to those of recent supernovae, it would be possible to check whether the rate of expansion was changing or not. “It was such a straightforward measurement, I was surprised everybody wasn’t doing it,” Perlmutter reflects.

Universe on a Knife-Edge

Perlmutter was driven by a desire to work out the ultimate fate of the universe. Decades earlier, cosmologists looking at Einstein’s equations determined three possible destinies lying in wait for the universe, depending on how much stuff — galaxies, stars, humans — it contained. If the density of matter in the visible universe was large enough, the expansion would not just slow; it would, thanks to gravity, eventually go into reverse, contracting the visible universe into an infinitesimally small point — a Big Crunch. 

By contrast, if the universe contained less material than this critical value, the expansion, though slowing, would never stop; if especially dramatic, acceleration could sunder the universe in a Big Rip. The third possibility was that the universe was poised at a critical knife-edge between these two options, in a perpetual steady state.

These deep philosophical questions led Perlmutter into astronomy in the first place. “As a kid, I wanted to understand the owner’s manual of the universe,” he says. Astronomy enabled experiments through which the answers could be found. “Does the universe go on forever in time and space, or does it eventually end? That’s a question every schoolchild asks,” he says. And presumably it could be answered, because the history of the universe’s expansion could be experimentally derived.

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dark-energy-4
Roen Kelly/DISCOVER

Some of the first to try and solve the problem were cosmologists Alan Guth at MIT and Andrei Linde, then at the Lebedev Physical Institute in Moscow. Independently, while puzzling out other astronomical mysteries, they arrived at the same tantalizing predication that the universe was exactly balanced at critical density, the knife’s edge. 

In particular, cosmologists had been struggling to explain why the cosmos looks amazingly similar, no matter which direction they trained their telescopes or how far out they looked. The puzzle emerged after astronomers measured the cosmic microwave background — a bath of radiation, left over from the Big Bang — and found only slight variations in its temperature across the entire sky. 

Two points at the opposite extreme limits of the sky — 14 billion light-years to the north horizon and 14 billion light-years to the south horizon — are as far as we can see. Yet the background temperature differs by only one part in 10,000 between them. The question was, how could two parts of the sky some 28 billion light-years apart have essentially the same temperature?

Guth and Linde’s answer was an elegant one: Our universe went through an incredibly rapid growth spurt, known as inflation, that stretched the infant cosmos at a rate faster than the speed of light, just 10-30 second after the Big Bang. If this was true, then before inflation occurred, two patches of the universe that started out as neighbors would be close enough together and have enough time to even out their temperatures. Then inflation would seize hold, ripping apart these now almost-identical patches and flinging them to the opposite ends of the sky, solving the mystery of why the universe looks the same in every direction. 

Crucially, the math showed that ironing out temperature wrinkles in the cosmos would also leave the universe at critical density — delicately balanced between continuous growth and eventual collapse to a Big Crunch. But so far, astronomers had discovered enough matter to make up only 30 percent of this critical density. It meant that 70 percent of the universe was playing hide-and-seek with astronomers. 

Riess wanted to be the one to find it.

NASA/ESA/Hubble Heritage Team (STScI/AURA)

The Search for Dark Matter

Failing to spot some 70 percent of the matter in the visible universe may seem like a glaring oversight, but astronomers were aware that telescopes simply could not capture all the objects that must be lurking in space. The first confirmation that much of the matter in the universe is invisible came when they noticed that the outer portions of a significant number of galaxies were rotating inexplicably fast. 

Those outer stars seemed to be pulled by far more gravity than could be accounted for by adding up the contributions of the visible stars. The most plausible answer was the galaxies also contained clouds of what they dubbed “dark matter” that could not be seen by conventional means, but which exerts a gravitational tug. 

Riess wondered if there might be enough dark matter in the universe to comprise the missing 70 percent predicted by cosmologists. And he thought he could prove it by measuring the rate at which the expansion of the universe was slowing down. If the universe’s expansion was slowing significantly, it would be fair to conclude there was unaccounted gravity pulling it back on itself, with a huge amount of dark matter at the root. Just how much there was would, in turn, dictate whether the cosmos would grow forever or end with a crunch. 

The race was on to measure the expansion rate of the universe, involving a long slog of telescope observations and data analysis by both groups. In early 1997, Perlmutter saw the first hints of something unexpected. Measuring brightness, he found that supernovae for a given redshift were much farther from Earth than anyone had ever dreamed. If correct, the implication was shocking: Instead of the gradually slowing cosmic expansion predicted by cosmologists, the universe was speeding up. But before publishing such a claim, Perlmutter would need to check and recheck the finding.

Riess, meanwhile, had created a computer program that spit out calculations on cosmic density, and he was getting the same strange results. Instead of pointing to a universe in which matter comprised some 30 percent — the figure needed for the expected scenario of a universe expanding forever — his program seemed to mock him with a seemingly nonsensical result: negative 30 percent. The value did not appear to correspond to anything physically reasonable at all. 

At first Riess thought his program was malfunctioning. But eventually he realized there might be an explanation, something totally unrecognized until now: Perhaps dark matter wasn’t the only “stuff” contributing to the overall density of the universe by exerting either a gravitational push or pull. Maybe something else was lurking as well.

You've Got Mail

This is when Riess emailed the crucial graph to Schmidt in Australia, not daring to tell him that he believed that it showed the universe was not only expanding, but that this growth was ramping up faster and faster with time. With the bizarre results in hand, Riess and Schmidt, too, now stood on a ledge, and they had to intensely examine their results. 

The timing could not have been worse for Riess, who not only was getting married within weeks, but was holed up in his lab while his fiancee made last-minute arrangements. “Christmas vacation occurred, and I just stayed at work,” Riess laughs. 

By early January 1998, Schmidt and Riess agreed that the result was real and told their team, allowing Riess to take a break to get married and go on his honeymoon. 

Finally it was time to present the results to the world. At a talk at the American Astronomical Society meeting later that month, Perlmutter’s team presented data on accelerating expansion that Schmidt’s colleagues immediately realized fit with their own. Both teams were now in accord. The universe was flying apart faster than anyone had thought, propelled by an unknown force unaccounted for by any theory of physics to date. 

The term dark energy began to be bandied about to describe the mechanism that drove the expansion. But in truth, it was little more than a label highlighting physicists’ ignorance about what it was, where it came from, and how and why it was acting the way that it apparently did. 

In 2000, the observation moved beyond a reasonable doubt, thanks to measurements of microwave radiation that rippled out from the original Big Bang. The measurements taken by the BOOMERanG Experiment (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics) and MAXIMA (Millimeter Anisotropy eXperiment IMaging Array) represented “cosmic stretch marks” that were proof positive of expansion at ever-greater rates.

With each astronomical observation over the past decade, evidence that the universe is composed of 30 percent matter — made up from adding both visible and dark matter components together — and 70 percent dark energy has grown stronger. Riess has since hunted down supernovae that exploded more than 7 billion years ago, filling in gaps: The universe first slowed down as the inward pull of matter dominated over the relatively mild outward push of dark energy. 

As the cosmos expanded, matter gradually spread out, and its gravitational grip weakened, hitting a balance with dark energy about 5 billion years ago, causing the expansion to coast at a steady rate for a while, neither accelerating nor slowing down. In more recent times, because the universe has continued to grow but no new matter has formed within it, matter has been diluted further, spreading across an ever-growing space. As the density of matter in the universe has steadily dropped, the universe’s expansion has sped up. 

Dark Energy's Origin Story

Despite such insights, physicists remain in the dark about dark energy’s origin. In one model, cosmologists propose that dark energy emerges from the fuzzy laws of quantum physics, which govern the subatomic realm. Quantum mechanics is famed for its weirdness because it states that before you look at a particle, it does not have any set properties; instead, it exists in multiple places simultaneously. 

This built-in caprice means that you can never say for sure that a particle is not there; even the supposedly empty vacuum is bubbling with particles that pop in and out of existence for fleeting moments. This roiling froth of “virtual” particles adds energy to empty space itself, though so far the theory predicts far more energy than we actually see. 

Could quantum effects be creating dark energy? And could some kind of constant resembling Einstein’s cosmological constant predict the phenomenon overall? “Because of this, cosmologists have been flailing around trying to find some explanation for dark energy that sort of looks like a cosmological constant, but isn’t a cosmological constant,” says Schmidt.

An alternative model is “quintessence,” the idea that the cosmos is pervaded by a field that lay dormant for much of its early history, but then kicked into gear, driving expansion only in the recent past. It’s possible to pit the two models against each other because in quintessence, the strength of dark energy varies while the cosmological constant (as the name suggests) is always the same.

Working with NASA on its Wide-Field Infrared Survey Telescope (WFIRST) mission, due to launch sometime after 2020 (see “Mapping the Dark,” page 47), Perlmutter will help choose between the different models by studying groups of supernovae that lie farther out in space than any yet studied, following the universe’s expansion history back in time. By peering into this distant past, he should be able to tell whether dark energy has remained the same or has varied over time as quintessence predicts. 

And quintessence is only one possible alternative to the cosmological constant. Another explanation holds that our universe is housed inside a black hole — the superdense stellar corpse left behind in the aftermath of certain supernovae explosions. Calculations from cosmologist Stephon Alexander at Dartmouth College in New Hampshire show that when squashed together by an attractive force, subatomic neutrinos can form a cosmos-spanning superfluid with an anti-gravity effect matching the strength of dark energy. 

The catch? Squeezing the neutrinos into a superfluid requires the kind of pressure generated inside superdense cosmic objects, meaning that in this model, our universe would need to be contained within something like a black hole. “It sounds crazy, but I think it is minimalist,” Alexander says. 

Looking Beyond

Riess greets these theories and their many rivals with bemusement. “Over the past decade, there has been greater desperation. It’s understandable because this is a really hard problem.” Perlmutter adds: “There’s been around one paper about dark energy every day by a theorist for the past 12 years.” 

But rather than pick a front-runner, Riess plans to search for evidence with an unbiased mind. “Like a baseball umpire, I’ll stay impartial and call the strikes as I see them,” he says. 

Beyond choosing among the multitude of possibilities for the origin of dark energy, experiments may also help answer the question that fascinated Perlmutter, Schmidt and Riess in 1994: What is the ultimate fate of the universe? Where it was once thought that cosmic destiny would be governed by the density of matter, it now seems to be at the mercy of dark energy’s whims. 

If dark energy continues on its current course unchecked, some versions of the theory suggest it could cleave the cosmos in a Big Rip, tearing apart stars, planets and atoms. If dark energy slows and then flips, eventually pulling in tandem with, rather than against, gravity, the Big Crunch — in which our local universe is crushed down into an infinitely small speck — will be back on the cards, although this seems less likely now. 

One upcoming chance to tease out the answer could come from Euclid, a European Space Agency telescope designed to study the dark universe, and poised to launch into orbit around Earth by 2020 (See “Mapping the Dark,” at right). Euclid should begin generating data eight to 10 years from now, but, Perlmutter warns, it may not serve up the answers we anticipate. “If experience has shown me anything, it’s that experiments can lead somewhere completely different than what you thought.” 

Schmidt notes the long wait following Isaac Newton’s 17th-century theory of gravity for Einstein’s general relativity. “To explain why the cosmological constant is there, we’ll need another Einstein — and we just don’t know when that brilliant insight will come,” he says. “It could happen tomorrow, but it could take another 150 years.”


Mapping the Dark

A group of new satellites and observatories will help map the dark side, shedding real light on the nature of the universe and its eventual fate.

euclid
euclid
ESA/C

Euclid

Equipment: A visible-wavelength camera and near-infrared camera/spectrometer.
Expected Launch: By 2020
Mission: Piece together the past 10 billion years of cosmic history to determine how dark energy has sped up the expansion of the universe.
Method: Euclid will measure gravitational lensing, where light from far-off objects bends around a massive body (a regular star or cluster of dark matter), to determine dark matter’s distribution. The observatory will also measure patterns in the distribution of galaxies left by acoustic waves in the early universe. Because these patterns have a fixed size, they can be used to determine the distance of objects and probe the expansion of the universe for the effects of dark energy.

wfirst
wfirst
NASA/GSFC/SDT

Wide-Field Infrared Survey Telescope (WFIRST) 

Equipment: Infrared telescope with a field of view nearly twice the size of the moon.
Expected launch: After 2020
Mission: Determine what dark energy is, and whether it has changed over time. 
Method: In addition to the methods used by Euclid, WFIRST will precisely measure distances and redshifts for many more supernovae than can be monitored by Earth-based telescopes, thus determining the history of the universe’s expansion and behavior of dark energy with unprecedented accuracy.

Balloon Observations of Millimetric Extragalactic Radiation and Geophysics (BOOMERanG)

Equipment: Telescope-equipped balloon.
Launched: From Antarctica in 1998 and 2003, spending about two weeks in the air each time. 
Mission: Map radiation from the early universe.
Method: The balloon’s mirrors focused cosmic microwave background radiation onto supersensitive thermometers. Mapping this data allowed cosmologists to confirm that the universe is flat, and it gave them an idea of the density of its constituent parts: normal and dark matter, as well as dark energy.

–Breanna Draxler

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