National Geographic News
Nobel in physics picture: A nearby galaxy that contains a Type 1A supernova.

A Hubble picture of the galaxy NGC 5584 featuring a Type Ia supernova.

Image courtesy STScI/JHU/TA&M/ESA/NASA

Victoria Jaggard

National Geographic News

Updated October 4, 2011

What goes up must come down. Few on Earth would argue with the fundamental law of gravity. But today the 2011 Nobel Prize in Physics was awarded to three scientists who uncovered a dark side of the force.

New Nobel laureates Saul Perlmutter and Adam Riess of the U.S. and Brian Schmidt of Australia contributed to the discovery that the universe is not only expanding but also speeding up.

The finding led to the now widely accepted theory of dark energy, a mysterious force that repels gravity. Measurements show that dark energy accounts for about 74 percent of the substance of the universe.

But more than a decade after the Nobel-worthy find, scientists are still trying to pin down exactly what dark energy is and and thus solve what some experts call "the most profound problem" in modern physics.

(Also see "New Galaxy Maps to Help Find Dark Energy Proof?")

Does Gravity Work Differently?

Until dark energy, physicists were convinced that gravity should be causing the expansion rate of the universe to slow.

"When I throw my keys up in the air, the gravity of the Earth makes them slow down and return to me," said Mario Livio, a theoretical physicist at the Space Telescope Science Institute (STScI) in Maryland, said during the Decade of Dark Energy Symposium, held in 2008.

But by studying the light from distant supernovae, astronomers saw that the supernovae's host galaxies are flying away from each other at increasing speed.

The observation that the universe's expansion rate is actually speeding up, Livio said, is as if "the keys suddenly went straight up toward the ceiling."

So far, one of the biggest challenges for dark energy researchers is marrying observations to theory.

"We have two known, totally unsatisfactory explanations," said Michael Turner, a cosmologist at the University of Chicago.

One possibility is there is no dark energy, and gravity works differently than scientists think.

(See "Dark Energy's Demise? New Theory Doesn't Use the Force.")

But "physicists are conservative. We don't want to throw away our theory of gravity when we might be able to patch it up," Nobel co-winner Riess, an STScI cosmologist, told National Geographic News.

"Basically it all comes down to the fact that there's one relatively simple equation we work with to describe the universe," Riess said.

"Because we see this extra effect, we can either blame it on the left-hand side of the equation and say we don't understand gravity, or we can blame it on the right-hand side and say there's this extra stuff."

Dark Energy a Product of Quantum Vacuum?

The extra stuff—and a leading contender for explaining dark energy—is quantum vacuum energy.

The idea is tied to quantum mechanics, which predicts that even in the vacuum of space, particles are constantly winking in and out of existence, generating energy.

(Related: "Dark Matter Is an Illusion, New Antigravity Theory Says.")

The trick is that no one has been able to unify the math used in quantum mechanics, which describes the physics of the very small, with the equations in general relativity, which deal with large-scale interactions.

"The two theories use two different sets of rule books, [and] we've always known that these two books are incompatible," Riess said.

Unfortunately, "dark energy is one of the few cases in nature that really requires us to [somehow] use both sets of rules."

(Related: "Einstein's Gravity Confirmed on a Cosmic Scale.")

Measuring Supernovae's Stretched Light

To help solve the riddle, NASA and the U.S. Department of Energy had planned to conduct the Joint Dark Energy Mission (JDEM), the first program specifically designed to study dark energy.

But in the National Research Council's 2010 Decadal Survey, JDEM wasn't recommended for funding. Instead the NRC ranked the Wide Field Infrared Survey Telescope (WFIRST)—slated to launch in 2020—as the best mission to settle essential questions in both dark energy and exoplanet research.

In the meantime, current NASA missions have already played a key role in measuring dark energy, said Michael Salamon, program scientist for NASA's Physics of the Cosmos program.

"For one, the Hubble Space Telescope has weighed in on dark energy by virtue of the measurements of supernovae," Salamon said.

Researchers first observed accelerated expansion by studying Type Ia supernovae—the explosive deaths of white dwarf stars. (Related: "'Death Dance' Stars Found—May Help Prove Einstein Right.")

Astronomers know that each Type Ia explosion has about the same brightness.

As light from the most distant explosions travels toward Earth, it is stretched by the universe's expansion so that it appears red, a phenomenon known as redshift. The higher the redshift, the longer light has been traveling and the farther back in time the supernova occurred.

Examining as many supernovae as possible can help researchers measure how fast galaxies are moving away from one another.

Supernovae studies have allowed scientists to see that dark energy has been impacting galaxies since as far back as nine billion years ago. (See "Mysterious Dark Energy Has Existed For Most of Time, Scientists Say.")

Other groups are looking for even earlier clues in the cosmic microwave background, the leftover radiation from the big bang, believed to have occurred about 13.7 billion years ago.

In 2003 NASA's Wilkinson Microwave Anisotropy Probe produced the first full map of the early microwave sky in unprecedented detail.

Essentially looking back in time, WMAP revealed tiny ripples in density that were the seeds of today's galaxies, Licia Verde, an astrophysicist at the Institute of Space Sciences in Bellaterra, Spain, said during the 2008 symposium.

"This is a cosmic symphony. You are really seeing sound, [and] the sound can help you understand how the instrument was made," Verde said. (Related: "Is This What the Big Bang Sounded Like?")

And in 2005 astronomers found that sound waves rippling through the primordial plasma 400,000 years after the big bang had left imprints in modern nearby galaxies.

These so-called baryon acoustic oscillations offer another yardstick for measuring the expansion rate of the universe over time and putting limits on the value of dark energy.

Dark Energy Mystery Persists

Ultimately it will take data from a combination of methods to help unravel the mystery, the experts said.

"The name of the game is to take more measurements over the expansion history of the universe, make each of them more precise, and tighten the model for understanding how dark energy works," STScI's Riess said.

A key goal of experiments is to measure the ratio of energy density to pressure in the universe, denoted by the letter w.

This value tells physicists "what kind of gravity a material has—whether it's repulsive or attractive—and how strong it is," Riess said.

"If [dark energy] is vacuum energy, then w will be -1 always and precisely," a find that would match quantum predictions with general relativity.

Otherwise, it might be time to rewrite the rules.

Lawrence Krauss, a theoretical physicist at Arizona State University, noted at the STScI symposium that most observations currently show the value for w as pretty close to -1.

For theorists, he quipped, "measuring w … is therefore not going to tell us anything we don't know already."

But "new windows show us new surprises. You have to do what you can do, because you don't know where the answer's going to come from."

A. Chandrasekaran
A. Chandrasekaran

"physicists are conservative" Nobel co-winner Riess. 

Does proposing the existence of 1900% unknown stuff of abnormal matter really conservative? 5% normal matter and 95% unknown abnormal matter/energy. It is introducing 1900% new things to keep the equations/theory solvent. I don't think it ever happened in Science.

Simply put, older/farther history shows more red-shift than recent/nearer history!

Older history means earlier time. So, the speed was higher in the distant past than recent past!

That is called Deceleration.

Hap Howlin
Hap Howlin

Perhaps the equations for a singularity could incorporate a cosmic version of diffusion to explain the increase in speed for the particles of the universe. What if we looked more at the singularity as a lump of sugar in a cup of coffee. Rather than a violent explosion, a cosmic chemical process.


Acceleration always is not connected to expansion .Our fellow cosmologists must search to that direction aswell.An acceleration for example towards a massive black hole will make the dark energy concept useless and open new horizons to cosmology

Ayoub Chami
Ayoub Chami

In my point of  view , I think  the dark energy has a big force that force vector is the opposite of the gravity vector

the intensity of the force of gravity is smaller than the forceof dark energy.

but I'm not sientist

Brian Wills
Brian Wills

If the Nobel laureates were correct everything would be heating up not cooling down! Contracting not expanding..

Brian Wills
Brian Wills

So totally wrong but who would listen? The universe is NOT expanding. This is the part that can bring about the Grand Unification... Everything, evey piece of matter is CONTRACTING from the moment it is created so the illusion is that it is expanding but the fact is it is contracting. Hear goes to Cold, not the other way around! Out from the sun and into the black holes....

Lars K.
Lars K.

I can tell.

Universe is a limited fix sphere with its inside surface area receiving more and more matter sent by Big Bang. The whole inside surface of the sphere is therefore a kind of black hole, that's why we don't see the matter, and that's why we observe acceleration of the universe expansion.

At some point a critical mass will cause a massive explosion on the inside surface, making our universe implode. And then Big Bang again.

If the rules of the universe offers a deterministic future of the matter, and if those rules are not dependent on any parameter from previous Big Bang iterations, then we will all have a new life again. And our lives will be identical to all our previous incarnations.

The universe started filled witha homogenous floating mass, and gravity did not exist. But then something changed the rules of universe, and a slow pressure from outside the sphere started to move matter towards the center, which eventually caused the first Big Bang. It is a slow pressure since we don't see any impact on the velocity of matter travelling towards the inner surface.

Extremely small particles have extremely little interaction with the pressure from outside the universe(and possibly other rules from the same origin), just like very small objects have very little gravitation. And that allows other close range matter rules to take control over the behaviour of the extremely small particles. We call that quantum physics.

The parameters involved is pretty much beyond our reach to measure, yet.

Trying to model quantum physics using only the known set of parameters is why we have failed to make a deterministic model of it.

There you go.

Danielle Nidea
Danielle Nidea

Or maybe time is also a form of matter with a state different from all those proven and theorized, therefore it has no volume, mass, etc. but exists not as an abstract object. Therefore, when the Big Bang occurred, it was distributed to the universe the same way other forms of matter were. What if time is repellent to all gravity (since it has special properties) therefore more "time particles" are found in other parts of the universe not belonging to galaxies and at the edge of the universe than the amount of "time particles" found in galaxies? What if because of this, distant objects travel "faster" away from the center of the universe from our point of view, but from that location in which the object is found, the speed is either slower or constant? I don't know.

Benjamin Walden
Benjamin Walden

The reason is simple why the universe is expanding faster today then it was in the past it is because it is being measured.Look up doctor quantum on you tube. There is a video that shows atoms doing 1 thing and when the scientist measure what exactly what the atoms are doing they do something totally different from when there not being measured.

and compared to the whole universe and our little planet earth, you could say we are the size of an atom. 

Jeremy Peel
Jeremy Peel

Since reading the explanation of "dark energy". I have wondered If the farther you look out into the universe the further back in time you see and the faster galaxies are "falling" away from each other and the closer to us the slower the galaxies are 'falling" away from each other. My logic tells me that as time went by the universe is slowing down. For example 13.7 billion light years away or 13.7 billion years ago they are traveling faster than 13.7 million light years away or 13.7 million  years ago the are traveling slower. Is my logic wrong or am i missing something fundamental??

Kamran Naqvi
Kamran Naqvi

Is is possible that the emptiness of space is what is causing the universe to expand and accelerate. Like the process of osmosis. Means matter and energy are moving ever faster towards space with is more empty, like a suction effect. I do also believe that there will be other universes since the space is unlimited. It would be another bigger mass out there that could be exerting the force on our universe. But if this was true our universe would only expand in that direction. 

I thing i will stick to the first explanation that emptiness of space around us is causing a suction effect just like in osmosis.

Kamran Naqvi (Houston TX)

Kamran Naqvi
Kamran Naqvi

Is is possible that the emptiness of space is what is causing the universe to expand and accelerate. Like the process of osmosis. Means matter and energy are moving ever faster towards space with is more empty, like a suction effect. I do also believe that there will be other universes since the space is unlimited. It would be another bigger mass out there that could be exerting the force on our universe. But if this was true our universe would only expand in that direction. 

I thing i will stick to the first explanation that emptiness of space around us is causing a suction effect just like in osmosis.

Kamran Naqvi (Houston TX)

Donald Long
Donald Long

Is it possible that, at the Big Bang, matter was thrown in every direction in the emptiness of space? Let’s assume that there is no up-down, left-right in the Universe. At the very start, there was an extreme force of gravity because of the massive amount of matter (whatever the kind of matter at that moment in time). Consequently, matter was «falling in every directions». The only resistance was rather a «retaining power» of gravity. With time and galaxies getting farther away from each other, gravity is losing its «grip». 

So, things would be falling even though they are falling in opposite directions. The Universe could better be seen as a ball instead of a disc. 

Then, is there a need to postulate «Dark matter» to explain something that seems contradictory? Gravity alone would explain the expansion of the Universe and the progressive gain in its speed of expansion.  

William Kus
William Kus

Could it be that...  as matter gets farther away from other matter, there isn't anything to hold it back.  Like, leaving the gravitational field of everything and going into an area of space with no gravity at all, but pure energy?

Well that doesnt make sense.

But, I think a good explanation is that maybe there is something really far away that is attracting the stuff.  Maybe there is some enormous thing far out there that is attracting our universe, maybe some other Universes that are super duper massive.

Pete Digons
Pete Digons

@William Kus  That's actually very interesting. The acceleration could be due to the weakening of gravitational forces as the distance between objects continues to increase. 


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