What Is Dark Energy? Next
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So what is dark energy? Well, the simple answer is that we don't know. It seems to contradict many of our understandings about the way the universe works.

We all know that light waves, also called radiation, carry energy. You feel that energy the moment you step outside on a hot summer day.

Einstein's famous equation, E = mc2, teaches us that matter and energy are interchangeable, merely different forms of the same thing. We have a giant example of that in our sky: the Sun. The Sun is powered by the conversion of mass to energy.

Something from Nothing
Could dark energy show a link between the physics of the very small and the physics of the large? But energy is supposed to have a source — either matter or radiation. The notion here is that space, even when devoid of all matter and radiation, has a residual energy. That "energy of space," when considered on a cosmic scale, leads to a force that increases the expansion of the universe.

Perhaps dark energy results from weird behavior on scales smaller than atoms. The physics of the very small, called quantum mechanics, allows energy and matter to appear out of nothingness, although only for the tiniest instant. The constant brief appearance and disappearance of matter could be giving energy to otherwise empty space.

It could be that dark energy creates a new, fundamental force in the universe, something that only starts to show an effect when the universe reaches a certain size. Scientific theories allow for the possibility of such forces. The force might even be temporary, causing the universe to accelerate for some billions of years before it weakens and essentially disappears.

Or perhaps the answer lies within another long-standing unsolved problem, how to reconcile the physics of the large with the physics of the very small. Einstein's theory of gravity, called general relativity, can explain everything from the movements of planets to the physics of black holes, but it simply doesn't seem to apply on the scale of the particles that make up atoms. To predict how particles will behave, we need the theory of quantum mechanics. Quantum mechanics explains the way particles function, but it simply doesn't apply on any scale larger than an atom. The elusive solution for combining the two theories might yield a natural explanation for dark energy.

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Stranger and Stranger
Most of the universe seems to consist of nothing we can see. Dark energy and dark matter, detectable only because of their effect on the visible matter around them, make up most of the universe. We do know this: Since space is everywhere, this dark energy force is everywhere, and its effects increase as space expands. In contrast, gravity's force is stronger when things are close together and weaker when they are far apart. Because gravity is weakening with the expansion of space, dark energy now makes up over 2/3 of all the energy in the universe.

It sounds rather strange that we have no firm idea about what makes up 74% of the universe. It's as though we had explored all the land on the planet Earth and never in all our travels encountered an ocean. But now that we've caught sight of the waves, we want to know what this huge, strange, powerful entity really is.

The strangeness of dark energy is thrilling.

It shows scientists that there is a gap in our knowledge that needs to be filled, beckoning the way toward an unexplored realm of physics. We have before us the evidence that the cosmos may be configured vastly differently than we imagine. Dark energy both signals that we still have a great deal to learn, and shows us that we stand poised for another great leap in our understanding of the universe.

Fate of the Universe
Fate of the Universe Next
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Competing Models
Does the Matter Matter?
The Big Rip
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Gravity always pulls matter together. The baseball moves too slowly and gets pulled back to Earth. The spaceship moves fast enough to escape Earth, but it never escapes gravity entirely. When the word first got out that the expansion of the universe was accelerating, many astronomers questioned the results. They felt that the observations must be wrong, or the interpretation must be flawed. The whole concept was so difficult to believe because it requires significant changes in our understanding of the way the universe works.

Say you step outside and throw a baseball up into the air. The gravity of Earth begins immediately to act on the baseball, slowing it down even as it rises into the air. The upward speed of the baseball slows until it stops at its peak, then gravity's pull causes it to drop down at an ever-increasing speed. What you can't see is that the baseball also has a tiny gravitational pull that acts upon Earth. Gravity always acts to pull matter together.

Now consider a spaceship. If launched with enough speed, a spaceship will escape Earth's gravity to the extent that it will not fall back to the planet. However, it hasn't escaped the pull of Earth entirely. Though it travels away, the spaceship will be continuously slowed — just not to the point where it stops.

Competing ModelsThese same concepts apply to the expansion of space. That expansion was launched in the Big Bang, and ever since then, each bit of matter in the universe has been attracted to every other bit by the force of gravity. This should have been slowing down the expansion.

Before the discovery of dark energy, scientists had two models of how the universe's expansion would work. In one scenario, there would be enough matter in the universe to slow the expansion to the point where, like the baseball, it would come to a halt and start to retract, everything crashing back together in a "Big Crunch."

In the other scenario, there would be too little matter to stop the expansion and everything would drift on forever, always slowing and slowing but never stopping — like the spaceship. The galaxies would drift apart from each other until they were out of view. The universe would continue growing larger as countless generations of stars faded and died out. It would end in a vast, dark, and cold state: a "Big Chill," if you will.

The Big CrunchThe Big ChillThough space expands from the energy of the Big Bang, the universe's mass generates enough gravity to eventually stop and reverse the expansion in the Big Crunch scenario (left). In the Big Chill scenario (right), the universe has too little mass and drifts on forever, slowing but never stopping.
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Does the Matter Matter?By the early 1990s, astronomers had calculated how much mass was in the universe, and decided on the Big Chill as the most likely end of the universe. But then dark energy showed up in our observations.

According to the Big Chill, the universe should be expanding more slowly today than it did in the past, because gravity has had time to work on slowing the universe down over all these billions of years. But astronomers found that the universe is moving faster today than it was a billion years ago, meaning something must be working to speed it up.

This result seems crazy because gravity always pulls and slows — it never pushes. Yet some force appears to be pushing the universe apart. Astronomers, concluding that we just don't know what this force is, have attributed it to a mysterious dark energy.

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The Big RipThe Big Rip
The universe expands faster and faster, until galaxies and even atoms are eventually torn apart in the Big Rip scenario. With dark energy, the fate of the universe might go well beyond the Big Chill. In the strangest and most speculative scenario, as the universe expands ever faster, all of gravity's work will be undone. Clusters of galaxies will disband and separate. Then galaxies themselves will be torn apart. The solar system, stars, planets, and even molecules and atoms could be shredded by the ever-faster expansion. The universe that was born in a violent expansion could end with an even more violent expansion called the Big Rip.

So out of the three scenarios for the fate of the universe — re-collapse to a Big Crunch, expand ever more slowly to a Big Chill, or expand ever faster to a Big Rip — we have managed to narrow the possibilities down somewhat.

Evidence has ruled out the Big Crunch. The Big Chill is probably the least that will happen. Whether or not the universe goes all the way to a Big Rip depends on what dark energy really is, and whether it will stay constant forever or fade away as suddenly as it appears to have arisen. And that we do not yet know.

No matter which scenario is right, the universe still has at least a few tens of billions of years left — which leaves us plenty of time to look for the answers.

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Explore More IndexWhat Is Dark Energy?
Does it give us answers — or just reveal more questions? Fate of the Universe
Will dark energy eventually tear the universe's atoms apart? Type Ia Supernovae
How exploding stars help measure the cosmos Out of Space, Back in Time
How can we see what happened in the early universe? Did Einstein Predict Dark Energy?
He called it his "biggest blunder." But was it? Related Links
Find more topical dark energy information Credits
Special thanks to these contributors

Type Ia Supernovae

To find distances in space, astronomers use objects called "standard candles." Standard candles are objects that give a certain, known amount of light. Because astronomers know how bright these objects truly are, they can measure their distance from us by analyzing how dim they appear.

For example, say you're standing on a street evenly lined with lampposts. According to a formula known as the inverse square law, the second streetlamp will look one-fourth as bright as the first streetlamp, and the third streetlamp will look one-ninth as bright as the first streetlamp, and so on. By judging the dimness of their light, you can easily guess how far away the streetlamps are as they stretch into the distance.

For short distances in space — within our galaxy or within our local group of nearby galaxies — astronomers use a type of star called a Cepheid variable as a standard candle. These young stars pulse with a brightness that tightly relates to the time between pulses. By observing the way the star pulses, astronomers can calculate its actual brightness.

But beyond the local group of galaxies, telescopes can't make out individual stars. They can only discern large groups of stars. To measure distances to far-flung galaxies, therefore, astronomers need to find incredibly bright objects.


Written in the StarsType Ia Supernova Formation

Scientists can find distances to galaxies by studying the dimness of Type Ia supernovae, which give off a standard amount of light. So astronomers turn to exploding stars, called supernovae. Supernovae, which occur within a galaxy about every 100 years, are among the brightest events in the sky. When a star explodes, it releases so much energy that it can briefly outshine all the stars in its galaxy. In fact, we can sometimes see a supernova occur even if we can't see its home galaxy.

To determine distances, astronomers use a certain type of exploding star called a Type Ia supernova. Type Ia supernovae occur in a binary system — two stars orbiting one another. One of the stars in the system must be a white dwarf star, the dense, carbon remains of a star that was about the size of our Sun. The other can be a giant star or even a smaller white dwarf.

White dwarf stars are one of the densest forms of matter, second only to neutron stars and black holes. Just a teaspoon of matter from a white dwarf would weigh five tons. Because white dwarf stars are so dense, their gravity is particularly intense. The white dwarf will begin to pull material off its companion star, adding that matter to itself.

When the white dwarf reaches 1.4 solar masses, or about 40 percent more massive than our Sun, a nuclear chain reaction occurs, causing the white dwarf to explode. The resulting light is 5 billion times brighter than the Sun.

Because the chain reaction always happens in the same way, and at the same mass, the brightness of these Type Ia supernovae are also always the same. The explosion point is known as the Chandrasekhar limit, after Subrahmanyan Chandrasekhar, the astronomer who discovered it.

To find the distance to the galaxy that contains the supernova, scientists just have to compare how bright they know the explosion should be with how bright the explosion appears. Using the inverse square law, they can compute the distance to the supernova and thus to the supernova's home galaxy.

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Explore More IndexWhat Is Dark Energy?
Does it give us answers — or just reveal more questions? Fate of the Universe
Will dark energy eventually tear the universe's atoms apart? Type Ia Supernovae
How exploding stars help measure the cosmos Out of Space, Back in Time
How can we see what happened in the early universe? Did Einstein Predict Dark Energy?
He called it his "biggest blunder." But was it? Related Links
Find more topical dark energy information Credits
Special thanks to these contributors

Out of Space, Back in Time

To examine the way the universe behaved in the past, astronomers look at extremely distant objects, such as supernovae in galaxies billions of light-years away. But how does that work? How can astronomers look out into space and see the universe back in time?

The answer lies in the speed of light. Light waves move very fast, about 186,000 miles per second (300,000 km/s). Light moves so fast that as you go about your daily life, it appears to travel instantaneously from one place to another. For example, it takes only a few billionths of a second for light to travel across your bedroom when you turn on a lamp.

In space, however, the distances are so immense that the time that light takes to travel is noticeable.

Going the Distance
It takes light 1.3 seconds to travel between the Moon and Earth.

Light from the Sun takes 8 minutes to travel to Earth. The Moon is Earth's closest companion, at about 239,000 miles (390,000 km) away. Light takes around 1.3 seconds to travel that distance.

The Sun is 93 million miles (150 million km) away, far enough that the light it emits needs about 500 seconds to travel to Earth. We call the distance light takes to travel in a second a light-second, the distance it takes to travel in a minute a light-minute, and so on. So the Sun is about eight light-minutes away from Earth. The light shining on you right now first left the Sun eight minutes earlier.

Across our Milky Way galaxy, distances are measured in terms of how many years it takes light to travel. The nearest star is over four light-years away. So when we look at that nearest star, we see it not as it is today, but as it was four years ago. We are seeing the light that left that star four years previously and is just reaching us now.

The diameter of our galaxy is 100 thousand light-years. So when we look at even more distant stars, we see them as they were thousands to tens of thousands of years ago, depending on how far away they are and thus the distance their light has had to travel.

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A Galaxy Far, Far Away
The light we see today on Earth from the Andromeda galaxy (top) began traveling toward us 2.5 million years ago. The light we see shining from the Virgo Cluster is even older — 60 million years. Galaxies are yet farther away in both space and time. Our nearest large neighbor galaxy, Andromeda, is about two and a half million light-years away. The Virgo Cluster of galaxies is the largest nearby collection of galaxies, at about 60 million light-years from the Milky Way. The light we see today from galaxies in the Virgo Cluster started on its path toward us at the same time as the age of the dinosaurs was ending on Earth. If you were in a Virgo Cluster galaxy today, and you had a telescope powerful enough to study Earth, you would be able to see the prehistoric reptiles.

Very distant galaxies are billions of light-years away. At that distance, their light tells what the universe was like billions of years ago. Since the age of the universe is about 14 billion years, these distant observations allow astronomers to measure changes over the lifetime of the universe. So when astronomers look out into space, they are essentially also looking back into time.

This fact was vital to the teams studying the expansion of space, because their goal was to compare the speed of the universe's expansion in the past with the speed of the universe today. By studying extremely distant supernovae in faraway galaxies, they were able to judge the speed of the universe's expansion in the early universe.

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Explore More IndexWhat Is Dark Energy?
Does it give us answers — or just reveal more questions? Fate of the Universe
Will dark energy eventually tear the universe's atoms apart? Type Ia Supernovae
How exploding stars help measure the cosmos Out of Space, Back in Time
How can we see what happened in the early universe? Did Einstein Predict Dark Energy?
He called it his "biggest blunder." But was it? Related Links
Find more topical dark energy information Credits
Special thanks to these contributors

Did Einstein Predict Dark Energy?

Oddly enough, dark energy — for all the surprise around its discovery — is not an entirely new concept in physics. There is historical background for this idea, and it comes from the preeminent astronomer of the 20th century, Albert Einstein.

In 1917, Einstein was applying his new theory of general relativity to the structure of space and time. General relativity says that mass affects the shape of space and the flow of time. Gravity results because space is warped by mass. The greater the mass, the greater the warp.

But Einstein, like all scientists at that time, did not know that the universe was expanding. He found that his equations didn't quite work for a static universe, so he threw in a hypothetical repulsive force that would fix the problem by balancing things out, an extra part that he called the "cosmological constant."

Then, in the 1920s, astronomer Edwin Hubble, using a type of star called a Cepheid variable as a "standard candle" to measure distances to other galaxies, discovered that the universe was expanding. The idea of the expanding universe revolutionized astronomy. If the universe was expanding, it must at one time have been smaller. That concept led to the Big Bang theory, that the universe began as a tiny point that suddenly and swiftly expanded to create everything we know today.

Once Einstein knew the universe was expanding, he discarded the cosmological constant as an unnecessary fudge factor. He later called it the "biggest blunder of his life," according to his fellow physicist George Gamow.

Today astronomers refer to one theory of dark energy as Einstein's cosmological constant. The theory says that dark energy has been steady and constant throughout time and will remain that way.

A second theory, called quintessence, says that dark energy is a new force and will eventually fade away just as it arose.

If the cosmological constant is correct, Einstein will once again have been proven right — about something even he thought was a mistake.