discovery that the universe is flying apart in all directions at an ever-increasing rate.

Is the universe bursting at the seams? Or is nature somehow fooling us?

The astronomers whose data revealed this accelerating universe have been awarded the Nobel Prize

for Physics.

And yet, since 1998, when the discovery was first announced, scientists have struggled

to come to grips with a mysterious presence that now appears to control the future of

the cosmos: dark energy.

On remote mountaintops around the world, major astronomical centers hum along, with state

of the art digital sensors, computers, air conditioning, infrastructure, and motors to

turn the giant telescopes.

Deep in Chile's Atacama desert, the Paranal Observatory is an astronomical Mecca.

This facility draws two megawatts of power, enough for around two thousand homes.

What astronomers get for all this is photons, tiny mass-less particles of light. They stream

in from across time and space by the trillions from nearby sources, down to one or two per

second from objects at the edge of the visible universe.

In this age of precision astronomy, observers have been studying the properties of these

particles, to find clues to how stars live and die, how galaxies form, how black holes

grow, and more.

But for all we've learned, we are finding out just how much still eludes our grasp,

how short our efforts to understand the workings of the universe still fall.

Cosmology, the study of the universe as a whole, goes back to the ancient Greeks.

With no telescopes or other optical instruments to probe the stars... observers constructed

models designed to make sense of what they saw.

Their earliest theories stated that all matter in the Universe is composed of some combination

of four elements.

Earth. Water. Fire. And Air.

Each arises from opposing properties of heat and cold, dry and wet, acting upon more primitive

forms of matter.

Aristotle took it a step further. He held that the universe is divided into two parts,

the realm of Earth, in which everything is composed of the four substances, and the realm

of the stars and planets.

These bodies are made up of a fifth substance, unchanging and incorruptible, called aether

or quintessence.

The Greek idea that the universe is a series of concentric circles, with Earth at the center,

yielded to a wealth of new discoveries about the universe.

That Earth is a planet.

In a solar system.

Located in a giant wheel of stars and gas, a galaxy.

Bound by gravity to a local group of 30 galaxies.

Bound, in turn, to a cluster of over a thousand galaxies, and to a supercluster with tens

of thousands of galaxies

This, our cosmic region, takes up a volume about 100 million light years across, set

within a larger pattern of galaxy filaments, superclusters, and enormous empty voids.

Earth is but a speck, within a firmament so vast we can scarcely imagine it.

For all we've learned from snatching photons, the most basic nature of the universe has

only grown more mysterious. Ironically, modern models have recalled the mysterious fifth

element conjured by the Greeks to explain a universe that appears to move in ways not

easily explained.

To understand the predicament now faced by scientists, let's see how they got there in

the first place.

A hundred years ago, most astronomers believed the universe consisted of a grand disk, the

Milky Way. They saw stars, like our own sun, moving around it amid giant regions of dust

and luminous gas.

The overall size and shape of this "island universe" appeared static and unchanging.

That view posed a challenge to Albert Einstein, who sought to explore the role that gravity,

a dynamic force, plays in the universe as a whole.

There is a now legendary story in which Einstein tried to show why the gravity of all the stars

and gas out there didn't simply cause the universe to collapse into a heap.

He reasoned that there must be some repulsive force that countered gravity and held the

Universe up.

He called this force the "cosmological constant." Represented in his equations by the Greek

letter Lambda, it's often referred to as a fudge factor.

In 1916, the idea seemed reasonable. The Dutch physicist Willem de Sitter solved Einstein's

equations with a cosmological constant, lending support to the idea of a static universe.

Now enter the American astronomer, Vesto Slipher.

Working at the Lowell Observatory in Arizona, he examined a series of fuzzy patches in the

sky called spiral nebulae, what we know as galaxies. He found that their light was slightly

shifted in color.

It's similar to the way a siren distorts, as an ambulance races past us.

If an object is moving toward Earth, the wavelength of its light is compressed, making it bluer.

If it's moving away, the light gets stretched out, making it redder.

12 of the 15 nebulae that Slipher examined were red-shifted, a sign they are racing away

from us.

Edwin Hubble, a young astronomer, went in for a closer look. Using the giant new Hooker

telescope in Southern California, he scoured the nebulae for a type of pulsating star,

called a Cepheid. The rate at which their light rises and falls is an indicator of their

intrinsic brightness.

By measuring their apparent brightness, Hubble could calculate the distance to their host

galaxies.

Combining distances with redshifts, he found that the farther away these spirals are, the

faster they are moving away from us. This relationship, called the Hubble Constant,

showed that the universe is not static, but expanding.

Einstein acknowledged the breakthrough, and admitted that his famous fudge factor was

the greatest blunder of his career.

The discovery revolutionized astronomy because it redefined the universe as a dynamic realm.

But if he were around today, Einstein would be surprised to see his own failed idea return.

If the universe is expanding, it must have emerged from a dense and hot primordial state.

A cosmic fireball, we now call the big bang, would have supplied the initial kick.

Even as the universe expanded, gravity began drawing matter together into a web-like structure

that gave rise to galaxies and stars.

If there's enough matter out there, will gravity oneday reign in the big bang, and cause the

universe to collapse in on itself?

To find out, astronomers renewed Hubble's quest to precisely measure the cosmic expansion

rate.

Working with the Hubble space telescope, and giant new observatories on land, they sought

to measure distances far deeper than Hubble ever could.

People are talking about doing precision cosmology for the first time. Because it used to be,

"Cosmology, well we have a rough idea of how big the universe is, maybe to a factor of

two or three." But now with these new measurements we're really getting a handle on the overall

density and structure of the universe. And what they are telling us is not what we expected

to hear.

Hubble's mileage markers were the cepheids. Today, astronomers look for stars like our

sun in their death throes. They spend their lives gradually consuming the hydrogen gas

that makes up their cores. At the end of the line, the dying star swells and sheds its

outer layers, leaving behind a tiny sphere the size of Earth.

It's so dense that if you could scoop out a teaspoonful of matter from its core, it

would weight a thousand metric tons.

If this white dwarf happens to orbit another dying star, it may begin to draw upon the

companion's expanding outer layers. At a critical threshold, it can grow no more, and it explodes.

Scientists at the University of Chicago and Argonne National Lab have been simulating

the thermonuclear reaction that begins deep within the star. A nuclear flame sends hot

ash rising to the surface. It breaks out then begins to wrap around the star. A collision

on the other side of the star triggers the explosion.

Because type 1A supernovae are all thought to explode in the same way, and because they

are extremely bright, they are ideal for measuring extreme distances.

It's like looking at cars with identical headlights approaching on a highway. The dimmer they

appear, the farther away they are.

By documenting explosions through the depth of the universe, two groups of astronomers

had hoped to find out how quickly gravity has been reigning in the cosmos.

Capturing the trickle of photons from events six or eight billion years ago would test

the sensitivity of even the most powerful modern telescopes.

When they spotted a type 1a supernova, astronomers looked at how much its light was shifted to

the red. The larger the shift, the more the universe had expanded since the explosion.

They combined this measurement with its distance, based on the apparent brightness of the supernovae.

Some explosions looked dimmer than expected based on their redshift. That meant their

light had traveled over a greater distance to reach us.

That led the two teams to the same conclusion, that the cosmic expansion rate had been slower

in the deep past. For the universe to reach its current size, the expansion had to actually

accelerate.

Scientists have known since the 1930s that the universe is not necessarily the way it

appears. Back then, astronomer Fritz Zwicky measured the rotation rate of spiral galaxies

and found that their gravitational pull was over 100 times greater than what he expected

based on the amount of matter he could see.

There must be some gravitational presence, Zwicky surmised, that you can't see with a

telescope. He dubbed it "dark matter."

Scientists today have successfully recreated the structure of the universe in computer

simulations by incorporating dark matter in the gravitational sources that sculpted galaxy

clusters and filaments.

Apparently, there's another unseen presence at work in the universe, called "dark energy."

And it's whisper thin.

For comparison's sake, water has a density of 1 gram per cubic centimeter.

Dark energy is a mere 10-29 grams per cubic centimeter. That's a point followed by 28

zeros and a 1, the equivalent of 5 hydrogen atoms in a cubic meter.

In their scan of the early universe using the WMAP satellite, scientists concluded that

matter and dark matter account for only about 26% of the content of the universe. The remainder,

then, is dark energy.

Since 1998, something totally unexpected happened, which is that we discovered not only that

our universe is expanding, this expansion is accelerating. You know, this is a classical

"who ordered that?" type situation. If 70% or so is dark energy in the universe, you

know about 70% of the surface of the Earth is covered with water. Imagine we didn't have

a clue what water was. This is the situation we're in.

So what exactly is it? The simplest answer takes us back to Einstein and his repulsive

force, the cosmological constant. It's the idea that empty space is actually a seething

stew of particles popping in and out of existence. It's a type of energy that is constantly welling

up from the vacuum.

This description is reminiscent of the sudden and violent outpouring of energy that many

scientists believe launched our universe in the first place.

Long after the big bang, vacuum energy exerted enough pressure over extremely large scales

to push the universe out. And, as the universe grew larger, more and more of it came into

existence, causing the expansion to accelerate.

Another explanation takes its name from Aristotle's Quintessence. While similar to vacuum energy,

in theory it can vary over time.

There are still other theories. One unifies dark energy and dark matter into a single

dark fluid that alters the action of gravity on large scales.

Another digs deep into a warren of hidden physics... to suggest that the push of dark

energy may one day turn to a pull. This theory predicts that in about ten billion years,

the universe will begin cascading back together in a big crunch destined to reduce all of

creation to the size of a proton.

Is there a way out of all this cosmic confusion?

Some scientists suggest that the findings derived from type 1A supernovae might be based

on an illusion... that the measurements are due not to cosmic acceleration, but to large-scale

factors we have not yet detected.

Since Nicolaus Copernicus showed that the Earth rotates around the sun, cosmologists

have based their theories on the idea that we exist in no special place.

In that case, our view of the universe is similar to any other vantage point in the

universe. That assumption has allowed us to extrapolate what we see to a vast scale.

We concluded, for example, that the universe has expanded in a uniform manner. That explains

the uniform temperature of light from the early universe, within which we can see a

pattern of variations. And it explains the uniform distribution of galaxies, within which

we see a pattern of filaments and clusters.

Is it possible that we are still only seeing part of a much grander cosmic map? It's like

looking at a desert and assuming the rest of the world is flat, when in fact it's filled

with oceans and mountain ranges.

Perhaps there are non-uniform cosmic structures larger than our field of view, forming bulges

or bubbles.

For argument's sake, if we are located in the center of a giant bubble, then supernovae

out on the fringes might seem to be accelerating away.

Or if we're in a region of higher density, the universe might appear headed for collapse.

For now, it looks like the discovery of the accelerating universe is holding up. Scientists

using NASA's Galaxy Evolution Explorer telescope confirmed the finding by using galaxies in

the distant universe as another kind of mileage marker.

As another check, they calculated the speed that galaxies should collapse into clusters

based on their collective gravity. The data showed that something is holding them back,

and breaking their fall into the clusters.