The Cosmos


Will We Ever Know the True Nature of Dark Energy?

You know that dream where you’re about to take a final exam, only to realize that you have neglected to study and, moreover, to put on pants? I imagine that’s what astronomers must have felt like in 1998, when they found out that most of the cosmos had somehow escaped their notice.

Astronomers knew that the universe had been expanding since the Big Bang, but they assumed that the gravitational pull of all the stuff inside it was gradually slowing that expansion. So they were caught off guard when supernova observations showed that in fact the expansion was speeding up, thanks to a mysterious phenomenon they dubbed “dark energy.”

X-ray image of the remnant of SN 1572, a supernova of the type used to measure the accelerating expansion of the universe. Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.
X-ray image of the remnant of SN 1572, a supernova of the type used to measure the accelerating expansion of the universe. Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.

Nearly 16 years and a trio of Nobel Prizes later, the initial shock has worn off, but the sense of chagrin lingers. “About 70% of the universe is dark energy, so it’s embarrassing not to know what it is,” says Valeria Pettorino, a physicist and cosmologist at the University of Geneva in Switzerland and the University of Heidelberg in Germany.

That uncertainty means physicists can’t predict whether the acceleration will continue at the same pace, speed up, or slow down, leading to no small amount of existential angst over the fate of the cosmos. “We don’t know what’s going to happen,” says Daniel Eisenstein of Harvard University. “The expansion of the universe is really being controlled by this new phenomenon that we don’t [understand].”

So when will we unmask this cosmic meddler? It could take physicists a decade or more to narrow down its hundreds of possible identities, many of which are devilishly hard to distinguish from one another. But researchers are attacking the problem from a number of angles and are already collecting some intriguing hints as to the true nature of dark energy—or at least what it’s not.

The astrophysical observation campaign “is really hitting its stride,” says Eisenstein. The initial finding in 1998 was based on measurements of fewer than 100 supernovae that appeared to be dimmer, and thus farther away, than expected. Since then, researchers have studied about 1,000 supernovae and developed a number of other techniques to gauge how the universe’s expansion rate has changed over time.

So far, all the observations can be accounted for with the simplest explanation: that any cubic meter of space froths with a set amount of repulsive energy, so that as space expands over time, the amount of repulsive energy grows as well.

But this simple solution, known as the cosmological constant, suffers from what may be a fatal flaw. Quantum physics suggests that the vacuum of space does contain energy, thanks to “virtual” particles that constantly pop into and out of existence, but when researchers calculate that energy based on known physics, they come up with a value that is some 10120 times larger than dark energy’s observed strength. “That’s a really embarrassing mismatch,” says cosmologist Michael Mortonson of the University of California, Berkeley, and Lawrence Berkeley National Lab. It’s easier to imagine how to cancel out the vacuum energy completely in the equations than have it retain a small, but non-zero, value, he says.

That has led researchers to ponder alternative explanations, such as the possibility that dark energy is caused by a quantum field that changes strength over time. One such field is the leading candidate to explain inflation, the period of super-fast expansion thought to have taken place just after the Big Bang. Another possibility is that our current understanding of gravity, Einstein’s theory of general relativity, is incomplete. “General relativity is very well tested at the scale of our solar system,” says Pettorino. “What we are trying to understand is if somehow it could be different at scales that are much larger.”

Some of these alternatives already appear to be on shaky ground, including the exotic notion that gravity could be leaking into an extra spatial dimension, making the universe expand faster than it otherwise should, says Mortonson.

But many others are still in the running, and new observations have bolstered the case for certain models. For example, last year researchers analyzing data from Europe’s Planck satellite calculated a value for the expansion rate of the universe today, known as the Hubble constant, that differs from Hubble Space Telescope observations by as much as 10%. The conflicting results can be reconciled by a model called “phantom energy,” a hypothetical dark energy that grows stronger over time. The catch: phantom energy could cause the universe to end in a “big rip,” tearing apart stars, planets, atoms, and their constituents. But don’t panic just yet. Pettorino says the discrepancy can also be explained if there is an extra force in the universe that causes dark energy to interact with another unknown component of the universe: the dark matter, whose presence has only been detected through its gravitational pull on visible matter.

Or the discrepancy, which is small to begin with, may be a flaw in the data analysis. “I think [that's] the more likely explanation,” says Mortonson. “Every team is checking their analysis,” says Pettorino, who is a member of the Planck collaboration.

Future observations should help narrow down the possibilities. Dark energy’s behavior over time, which is measured by the ratio of its pressure and energy density (called its equation of state, or w) is measured to a precision of about 5% today. But in the next five years, new observations, including those with a special camera fitted onto a telescope in Chile called the Dark Energy Survey, will increase the precision to 2% or 3%, says Mortonson. Future ground- and space-based missions, including a planned European space mission called Euclid and a possible US probe called WFIRST, could make even finer measurements.

These missions will not only probe the expansion history of the universe but also chronicle how the distribution of matter has changed over time. One way to do this is with a method called weak gravitational lensing, which looks for distortions in the light from distant galaxies due to any mass that the light passes on its way to a telescope. If dark energy changes over time or if gravity behaves unexpectedly at large scales, we might see evidence of it in the changing “clumpiness” of matter over space and time.

But Eisenstein points out that there are always going to be exotic dark energy models that behave just like the cosmological constant. “The worry is that if we do all these very accurate measurements, and it still looks like a cosmological constant, then we haven’t actually ruled out a lot of the models,” he says. “I think we have a major challenge on the theory side to try to understand what else we can look for.”

“[To understand] dark energy, we will probably require more time, and I would say not less than 10 years,” says Pettorino.

But it’s worth trying to get to the bottom of the mystery, say the researchers. “We thought we had four forces of nature: gravity, electromagnetism, and the weak and strong nuclear forces. Dark energy is either some new force, or some substantial modification to gravity,” says Eisenstein. “It’s a major actor in cosmology and in the history of the universe.”

Go Deeper
Editor’s picks for further reading

The New York Times: The Universe, Dark Energy and Us
In this Op-Ed, Harvard astrophysicist Robert Kirshner reacts to the 2011 physics Nobel award and reflects on the future of dark energy research.

physicsworld: Dark energy: how the paradigm shifted
Explore the history of dark energy and the cosmological constant.

Talks at Google: Dark Energy and the Runaway Universe
In this video, astrophysicist Alex Filippenko, a member of the teams that discovered the accelerating expansion of the universe, discusses the history and implications of dark energy.

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Maggie McKee

    Maggie McKee is a freelance science writer focusing mainly on astronomy and physics. She worked at New Scientist as both a reporter and physical sciences news editor from 2003 to 2012 and in 2012 was one of the winners of the first European Astronomy Journalism prize. She studied physics at Grinnell College and science writing at the University of California, Santa Cruz, and now lives near Boston with her husband and a passel of four-legged friends.

      • billybeer54

        Is it possible,that an ocean of dark matte could have an electro magnetic charge however slight .opposition all forces attract causing expansion.

        • SHREEKANT

          no, dm has no em-charge. the reason of expansion is different

    • Anony

      Saying that the expansion of space creates more dark energy which creates more space is cyclical logic. Also, to say that energy is created is a bit heretical too. On the other hand we know that space is not empty but is “stuff.”. So, in essence, I guess energy IS being created. Crazy ideas, huh? If this is true, then maybe the more fundamental aspect of the problem we should be tackling is this seeming violation of the 2nd law of thermodynamics…where does this energy come from.

      If space is a substance subject to gravity isn’t it possible that the dark energy that exists through space could be more dense in certain pockets of space like near stars and in galaxies and hence be called dark matter?

    • Doctor Who

      Fractal Gemeontry should solve it.

    • sean samis

      Here’s my $0.02 worth: as the universe expands it gets emptier; objects cluster around gravitational attractors and gravitational influence across emptier regions fades as distance increases. Dark energy is just cosmological expansion continuing with a declining “gravitational tension”.

      sean s.

    • Nathan Grant

      I believe that gravity can explain all of this. I also don’t believe we know enough about gravity to state anything about it’s effect’s over inter-stellar distances. Perhaps gravity behaves differently over vast distances – maybe it can even have a repulsive force that we aren’t aware of that is constant where as the attractive force becomes stronger depending on how close mater is to other matter. Really anything is possible at this point.

      Also, something that i think is interesting is when they call it “dark mater.” They make it sound like it’s some mysterious force – when in fact it is just ordinary mater that we can’t see. It makes up more mass than all of the stars and black holes and other bodies in the universe…this really isn’t hard to understand when you consider the VAST distances in the universe. “Dark Energy” has simply arisen as a fancy way of saying WE DONT KNOW what they heck is happening here but we can observe it and messure it and it apears to have some degree (10%) of accuracy…Also isn’t it possible that “dark energy” is just energy continuing out out from the big bang?

    • mpc755

      We know what dark energy and dark matter are.

      Cosmic microwave background

      “With the increasingly precise data provided by WMAP, there have been a number of claims that the CMB exhibits anomalies, such as very large scale anisotropies, anomalous alignments, and non-Gaussian distributions. … A number of groups have suggested that this could be the signature of new physics at the greatest observable scales”

      The new physics is understanding our Universe is a larger version of a polar jet (

      Cosmos may be curved, scientists say

      “Now cosmologists suggest these anomalies occur because the universe is not flat. Instead, these researchers propose the universe may be ever so slightly “open,” curved in such a way that parallel lines, which never converge or diverge when traveling on a flat surface, will eventually diverge from one another, like on a saddle.”

      Our Universe is open because it is a larger version of a polar jet.

      Was the universe born spinning?

      “The universe was born spinning and continues to do so around a preferred axis”

      Our Universe spins around a preferred axis because it is a larger version of a polar jet.

      Mysterious Cosmic ‘Dark Flow’ Tracked Deeper into Universe

      “The clusters appear to be moving along a line extending from our solar system toward Centaurus/Hydra, but the direction of this motion is less certain. Evidence indicates that the clusters are headed outward along this path, away from Earth, but the team cannot yet rule out the opposite flow. “We detect motion along this axis, but right now our data cannot state as strongly as we’d like whether the clusters are coming or going,” Kashlinsky said.”

      The clusters are headed along this path because our Universe is a larger version of a polar jet.

      It’s not the Big Bang; it’s the Big Ongoing.

      Dark energy is the evaporated matter (i.e. aether) which is continually being emitted into the Universal jet.

      There is no such thing as non-baryonic dark matter anchored to matter. Matter moves through and displaces the aether.

      Aether has mass, physically occupies three dimensional space and is physically displaced by the particles of matter which exist in it and move through it.

      Displaced aether pushing back and exerting inward pressure toward matter is gravity.

      The state of displacement of the aether is gravity.

      There is evidence of the aether every time a double slit experiment is performed; it’s what waves.

    • Jokef1000

      Maybe it is just God.

      • Jokef1001

        Couldn’t God be speaking through science?

        • Becky Salibrici

          That’s a question open to speculation. If you believe it, fine. But it’s not a fact, that’s for sure. It’s not even scientific. Religion and politics go together. Now spirituality and science, maybe there’s something there. I believe in the forces of physics; “higher” powers…. I like to feel a sense of connection with the universe, because after all, I am a part of it (a very very small part)….. but it has nothing to do with religion. That’s the problem with conversations about God. First, define God. That’s the milestone.

          • RyanF1

            >”First, define God.”

            To paraphrase St. Augustine, “If you could comprehend (or in this case, define) it, it would not be God.”

      • gatorallin

        ….said no scientist ever….

      • Thinking is good

        It is God waiting for the Scientist to thank him for the Great Brain he gifted them. To honor him for the Intelligence he gave them and to humble him for the Free will in not having to believe in him or trust in him or to need him or to Honor him. That is why Science Law is so finite. God is never changing and all powerful.

    • Wes Hardy

      Wave theory….a wave pushes out till it hits a shoreline and returns with less force….
      A boat is pushed out by the wave but the return of the wave slows it down.

      Where’s my Nobel, I could use a million today more than tomorrow, when it is worth less…inflation theory :)

    • Richard Poleet

      I going with what the man said(Einstein) “some substantial modification to gravity”.

      • gatorallin

        It sure seems we don’t understand gravity that well, or maybe we only understand the parts easiest to see……..or we only understand it on a small scale and still missing the forest for the trees…

    • Tank Carson

      This is where this lay-person (above average intelligence is just a little bit confused)… the “dark force” aside (let me see if I understand this) Inflation is not matter moving by way of kinetic energy imparted by the Big Bang but “space” itself is expanding. Could this ever increasing and exponential curve be reason enough for a “speeding” up for the Universe? I have a headache….

    • TyrellCorp

      Perhaps dark matter and dark energy do not exist and do not need to exist…

    • John Wolfgram

      Sean & Grant, I like your explanations. Now add a known ingredient, temperature. We know that interstellar space has a small temperature above absolute zero. But that temperature can’t exist in a vacuum. So, there must be something there to have a temperature, say ordinary matter in the form of a cosmic atomic dust forming the substance through which light propagates. So, as per your theory, gravity lessons over distance between masses, and distance results from the force of the big bang which gravity is supposed to slow down over time, except that the internal cosmic dust which contains the temperature is an expanding force against gravity pushing the universe into nothingness beyond the universe where there is no cosmic dust (or relatively less cosmic dust) rendering the universe like a pressure cooker that gradually but progressively over comes its container (gravity) and expands forever into the relative nothingness of space beyond the universe.

      In this case the cosmological constant is cosmic dust at a gradually decreasing temperature which decreases over distance caused by the force of the big bang.

      Add to that the multi-universe theory and you have a situation where some universes would expand forever while others will collapse upon themselves into another big gang depending on the amount of cosmic dust/temperature relative to mass producing gravity as a constant. If we suppose that the amount of mass compressed to a single point to cause a big bang is also a constant, we can have multi verses that never end and a necessity that even though the expansion is presently at increasing speed, as internal temperature approaches absolute zero, the pull of gravity can over come the expansive force.

      As to the amount of mass producing gravity, the cosmic dust being everywhere, produces nullifying and thus undetectable gravitational force within the universe, but relative to the absence of gravitational forces outside of the universe, it is a constant gravitational force tending to hold the universe together.

      Just a thought; and as most such thoughts, incomplete.


      • gatorallin

        I like your big gang theory….

    • Ron Henteleff

      Dark energy is simply the reciprocal of dark matter. Once depleted, Dark matter is longer present and so, you have it’s antithesis taking over and expansion results.

      • Ron Henteleff

        The fact 75% is now Dark Energy is evidence expansion has been taking place for some time. Entropy is the final result.

    • D.C. ADAMS

      The universe is not just accelerating, it expands exponentially – You simply calculate relativity in Hyperspaces – The derivatives increase exponentially the further out you are able to observe! –