The Cosmos


Who’s Afraid of the Dark? Alternatives to Dark Energy

Last week, we asked whether astronomers could be wrong about dark matter, the invisible stuff that seems to help hold galaxies together. Is it possible that dark matter doesn’t really exist?

This week, we’ll investigate whether there are viable alternatives to the idea of dark energy, the mysterious stuff that astrophysicists believe is pushing our universe apart.

In every direction we look, galaxies are hurtling away from us. That isn’t surprising in itself—after all, the Big Bang sent space and everything in it flying apart. One would expect that the gravitational pull of all the “stuff” in the cosmos would gradually slow down this expansion, bringing it to a dead stop or even collapsing everything back together in a “Big Crunch.” Yet instead, astronomers see that the galaxies in our universe are rushing apart faster and faster.

What could be causing this acceleration? Physicists call it dark energy, and it could make up more than 70 percent of the cosmos. But so much remains unknown about dark energy that some scientists are asking whether it exists at all.

What if, instead of a mysterious unseen energy, “there is something wrong with gravity?” asks Sean Carroll, a theoretical physicist at the California Institute of Technology.

Einstein’s theory of general relativity represents gravity as the curvature of space and time. Perhaps this idea “is still right, but we’re not solving the equations correctly,” suggests Carroll. “We’re used to thinking of the universe expanding perfectly smoothly, and we know it isn’t, and maybe these deviations are important.” If we accounted for how the universe is clumpy instead of smooth, it might turn out that the gravitational pull of clusters of galaxies and other large agglomerations of matter alter spacetime more than previously appreciated. Distant objects would thus appear to be farther away than they actually are, leading to the false conclusion that the universe’s expansion is accelerating.

The problem with this kind of model, Carroll says, is that while it suggests that these clumped-up astronomical bodies might distort our view of the universe more than suspected, gravity still remains the weakest of the known fundamental forces of nature. Also, these astronomical clumps would evolve in size and gravitational strength over time. In contrast, the mysteries that dark energy was invoked to solve require something with a lot of energy that changes less over time.

Another approach is to modify the laws of gravity to do away with dark energy. This tack suggests that “the laws of gravity as we know them work better on relatively small scales such as our solar system,” says Carroll, but perhaps they need “tweaks” to work on cosmic scales. Carroll and other theorists have developed alternative descriptions of gravity that could explain why the universe evolved as it did. One set of scenarios suggests that the strength of gravity increases over time and has different values depending on the distances involved. But critics argue that, to avoid contradicting well-established features of general relativity, these models are unacceptably contrived.

Another family of alternative gravity models analyzes how gravity behaves if there are extra dimensions of reality, as suggested by string theory. But this approach has problems of its own: It leads to empty space “decaying” into particles in potentially detectable ways, Carroll says.

To avoid modifying gravity, some theorists have suggested that our galaxy and its neighborhood might lie within a giant void, an emptier-than-average region of space roughly 8 billion light years across. With so little matter to slow down its expansion, the void would expand faster than the rest of the universe. If we lived near the heart of this void, our observations of accelerating cosmic expansion would be an illusion.

“The advantage of giant void models is that they don’t require any new physics to explain the apparent acceleration of the universe, like the existence of some weird dark energy or a modified theory of gravity,” says theoretical cosmologist Phil Bull at the University of Oxford.

Still, “there are lots and lots of problems with void scenarios,” says theoretical physicist Malcolm Fairbairn at King’s College London. “It’s very difficult to get them to fit existing data—for instance, the cosmic microwave background (CMB) radiation usually gets distorted in these models compared to what we actually see.” For the void model to match observations of CMB radiation, we would need to be very close to the center of the void, to within one part in 100 million. That “seems like an unacceptable ‘fine-tuning’ to some people,” says Bull. “Why should we find ourselves so close to the center?”

In addition, astronomers using NASA’s Hubble Space Telescope recently found evidence against the existence of such a void. After refining their measurements of the rate at which the universe is expanding, they all but ruled out the possibility that the accelerating expansion is an illusion created by a void. In addition, if we are living inside a void, Bull and his colleagues argue, we should see very strong fluctuations of cosmic microwave background radiation reflected off hot gas in the clusters of galaxies surrounding the void. Yet we do not see any reflections that strong. “This was pretty much the final nail in the coffin for void models,” Bull says.

To support the existence of dark energy—or vindicate one of these alternatives—we need giant sky surveys which will clock the speeds of even more galaxies, Fairbairn says. The colorful scenarios that theorists are dreaming up “ultimately show what an interesting and weird universe we live in,” Carroll says. “It’s one where we must keep an open mind as to what the answers may be.”

Go Deeper
Editor’s picks for further reading

COSMOS: Doubts Over Dark Energy
Reexamining the evidence for dark energy.

New York Times Magazine: Out There
In this article, Richard Panek explores the evidence for dark energy.

NPR’s 13.7: Dark Energy and the Joy of Being Wrong
In this blog post, Adam Frank recounts the history of the discovery of dark energy.

Tell us what you think on Twitter, Facebook, or email.

Charles Choi

    Charles Q. Choi has written for Scientific American, The New York Times, Wired, Science and Nature, among others. In his spare time, he has traveled to all seven continents, including scaling the side of an iceberg in Antarctica, investigating mummies from Siberia, snorkeling in the Galapagos, climbing Mt. Kilimanjaro, camping in the Outback, avoiding thieves near Shaolin Temple and hunting for mammoth DNA in Yukon.

    • Anonymous

      The universe is colapsing, we are in a very tiny region of a very large universe. The increase in acceleration is due to a increase in the curviture of space due to a very large mass way over there—->. The expansion is an illusion due to our limited view of the universe. Gravity is responsible for both dark energy and dark flow.

      • Da Rabb

        Yeah, right. You have chosen one of the “billions and billions” times infinity…that might have a chance of being even close. You have a better chance with the lottery.

    • Monalucky

      How to wrap my mind around,make sense out of it.I wonder how it is effecting& impacted our world,Human mind&body and our perceptions of reality.

    • I’m curious about everything, especially the mysterious. Dark matter is a very interesting curiosity and just look at its velocity! I enjoy the subject and pretty much all quantum mechanics and various physics theories it’s all as uncertain as the quarks that are theoretically flinging around and constantly jiggling us into an energetic and lively fuss that makes the universe a constant?

      • Kshulrob

        PERCHANCE………The entire Universe is fleeing……together……to keep the flow of gravity going……..and perchance the Universe is going in a circle…..(that we cannot see or comprehend)( to keep gravity where it needs to be) All has been well for centuries and all will be well for centuries………perchance we are making a problem where there is no problem……perchance this is the way it is supposed to be.
        and we should just accept all that we can see, all we cannot see and all that is an earthly….we cannot change anything. The Universe is so beyond man’s mental & physical abilities……

    • Pingback: Exposure! « Lumps 'n' Bumps()

    • Gemini

      The problem with the void theory is there would be an measurable red/blue shift with relation to the direction of the sky we are looking in and our position from the center of the universe because void theory according to the article is our local group moving faster or slower in relation to the universes expansion rate…but we haven’t heard anything about that. Frankly void theory assaults everything we know about the universe and everything we have discovered to date. The theory simply doesn’t fit with the data being collected.

      • There is only a direction-dependent redshift of the type you described for observers who are not in the centre of the void. If we are at the centre, the model is perfectly spherically symmetric about us, so there can be no such direction dependence. That we need to be very close to the centre of the void, to avoid such direction-dependence, gives rise to the fine tuning problem that was mentioned in the article.

        While it’s true that giant void models now seem to be inconsistent with the data, as discussed in the article, it’s worth remembering that, for quite some time, they *were* consistent with observations. This is despite their being a massive departure from the standard cosmological model!

    • Christopher p berg

      The alternative that never gets any attention is to reexamine the relative size of the Universe. If you run the math with with a larger system, dark matter and dark energy both drop out of the equations because we still live in a universe that is expanding.

    • Rdshoupe

      I am only an interested layman and this comment may seem absurd to the experts but, according to my limited understanding, when we observe the furthest known edge of our universe we are viewing ancient events. Attempting to draw conclusions from them applicable to present conditions seems futile to me. We simply do not know, cannot know, the current state of the universe through optical observations.

      • That’s a very interesting question. You’re quite correct – when we look at distant objects, we’re actually seeing them as they were a long time ago. A corollary to this is that we can’t see distant objects as they are *today*, because the light hasn’t had enough time to travel from them to us yet. There are a number of ways of getting around this problem, though.

        One is to make observations of objects close to us, and then to assume that we live in a “typical” region of the Universe, so that every other region looks the same as our local region (on average at least). With this assumption, looking at nearby objects as they are today is broadly the same as looking at anywhere else in the universe as it is today, up to some small differences due to statistical fluctuations (which we call “cosmic variance”). This is called the Cosmological Principle. For most work in cosmology, it’s simply accepted as a basic assumption. But some cosmologists, including myself, have been working on ways of testing the Cosmological Principle to make sure that it’s valid (e.g.

        Another way of getting around the problem is to come up with a theory that you think describes how the universe evolves. You then take distant observations as “initial conditions”. The theory tells you how things change over time, and thus how things should look at present, given your observed initial conditions. Of course, you need to test your theory to make sure it works (e.g. by doing experiments using things that you *can* directly observe), but if it does work, then using it to predict how things should look in unobservable regions is perfectly valid.

    • Pep

      Check out “The Electric Sky – A Challenge to the Myths of Modern Astronomy” by Dr. Donald E. Scott. This book, by a electrical engineer/astrophysicist, looks at the cosmos from an electrical/plasma point of view and presents some very logical explanations that belie dark energy, black holes, the big bang and the expanding universe. You can find a description of this book here:

    • Cfo

      I think that the rate of expansion of space is not universally constant, but is also a fabric like space itself, which is warped by the presence of matter/energy such that the expansion is slower in dense regions and greater in empty regions. Hence objects with more space between them are expanding away from each other at a faster rate.

    • T. Cator

      This was an informative and interesting article, but to those of you who want to dismiss dark matter and energy so lightly, I’d like to point out that there is valid scientific reasoning behind it. Cosmologists, and astronomers did pull the idea out of a hat. Certainly the technical details are beyond the scope of this article, but if you’re interested you should read peer reviewed journal articles. As an undergrad working on my B. S. in physics I don’t yet posses the answers (in detail) as to why dark matter and energy are so widely accepted, but they are widely accepted, and the ideas are accepted by professors that I know and respect.

    • I have been wondering for quite a few years now if dark matter, dark energy, and inflation are all merely ad hoc corrections of a flawed combination of big bang + General Relativity, and whether or not we should be reexamining alternatives to our theories of gravity and cosmic evolution. I suppose MOND (and the GR generalizations) would count towards that.

      As more dark matter experiments have failed I’ve become increasingly concerned that dark matter may be a 21st century aether. My main concern is that cosmology headlines have been terribly about all these ideas, and if it turns out it’s something else many people will get the silly idea that science is equally likely to be wrong about anything else—which is clearly not true.

      It could be a serious blow to public perception of science which is already far below where it should be. And scientific comprehension seems to be of utmost importance to our species’ success or failure.

    • sassy

      Love the topic but I’ve been searching the web site for some time now to find out when this show is on. Beats me!

    • Ujjalgoraya

      I am of the old school of thought. I think the Universe is expanding because of “Big Bang” push and according to Isaac Newton the matter is “clamped -to gather ” through Gravity. Dark matter and Dark Energy are concept and their existence still has to be proved!

    • Zapher134 on twitter

      If higgs field causes Mass and curves space or emits Gravitons to create Gravity, then as the Universe expands & no new higgs bosons are created it stands to reason that the field weakens, creating the acceleration we blame on dark energy and potentially allowing speeds higher than light speed, as we experienced right after the Big Bang during inflation.

    • Pbarnes

      This is the way it works – Dark Energy and all. This is a 20 yr old idea that still seems likely to me. I call it The Crystal Paperweight Theory. As you read it keep in mind what we know about Ma Nature and her methods – how the complex emerges from the simple. “Emerges”, “emergent”, popular words these days, kind of a fad, so I plan to use them several times to impress you with the profundity of this theory.

      Assume 3 spatial dimensions with a definite and rigid coordinate system and time. Not time as we experience it, but an underlying property from which our time arises. Populate this space with very small particlles that carry the basic forces – electromagnetic, strong& weak nuclear forces. There is no gravity at this level; you will see how it emerges later. These particles have no rest mass as in E=MC^2, but they do have inertia with respect to our assumed coordinate system. I don’t know if there is more than one flavor of these particles, but in any case they form a crystal structure where all of the forces match up exactly in bonds and the forces are not readily apparent. The electromagnetic and strong nuclear forces attract everything together, but at very close distances the weak nuclear force is overpoweringly repulsive (just like some people).

      I suppose the above could corrospond to thr Higgs particle and the Higgs condensate. In any case what we think of as “empty” space is really a frictionless gas crystal with mind boggling internal pressure.

      At the Big Bang everything was tightly compressed and the weak nuclear force was overpowering and started forcing the particles apart causing space to expand. At present then particles are still close enough together that the weak force still predominates causing the expansion to accelerate. Sometime in the future, as the particles separate further, the forces will balance out and the expansion will cease to accelerate. Later on as the particles inertia carries them further apart, the attractive forces will predominate and the expansion will first slow and then accelerate inward, leading to the Big Crunch.

      The crystal structure must be like BB’s poured into a bottle, it has to look the same from any direction. Faraday’s view that magnetic lines of force are a strain in the fabric of space induced by a magnetic field was correct. A magnetic field twists the crystal bonds of space and exposes some unbalanced electromagnetic forces. As a charged particle of matter passes through this region of space it is deflected by these unbalanced forces, giving us the Right Hand Rule. So we see that the Right Hand Rule is an emergent property.

      Now suppose we whip up a great turbulence in some region of space and create some bubbles. Certain sizes of these bubbles will be more or less stable. The stable sizes will have a shell where the space particles match up and bond fairly completely. The more completely the bonds match up (less exposed forces to be detected) the more stable the bubble. Solid knots of particles are also possible, but these will have a rest mass of 0.

      For peeling this layer of the onion lets assume the bubbles are empty – they have nothing in their interior. These bubbles have surface properties determined by the fluid (space) they are in and by their size – analogous to the bubbles in a flotation cell used in the mining industry. These varying surface properties give us electrons, protons, etc. Once again we see how something with many properties ( matter) emerges from something less complex (space particles). Notice that I have already used emerges three times. I suspect you are becoming convert already.

      E=MC^2. OK, but how do it do dat? physics does not address this question, but with our ontology the answer is straight forward and obvious – the bubbles (mass particles) collapse. With the whole universe hammering down on that collapsing bubble it is easy to see how tremendous energy is released. Note that C (the speed that vibrations travel through space) is going to be proportional to the internal pressure of our space crystal, as is the released energy.

      Consider a bubble, let’s say a neutron, that is sitting at rest with respect to our space crystal. To get it to move we must apply a force to one side. On the other side space must separate and the space particles flow around the neutron. The amount of space particles displaced will be proportional to the size (mass) of the neutron. Since the space particles have inertia with respect to our assumed coordinate system they will flare out past neutron ever so slightly and then impinge on the backside. Since space is frictionless all the energy we expended is now applied (after a very small but finite time delay) to the backside of our neutron causing it to keep moving ahead. I believe the ancient Greeks had a word for this idea, but I can’t recall it. At higher velocities this flare displacement is larger and one would expect the neutron to expand to fill the void created. This is why higher velocities give greater mass. At close to the speed of light a shock wave starts to form, to speed up you would have to move the whole universe ahead of you, thus the limit on velocity to the speed of light.

      These space particles are the best thing since sliced bread, aren’t they? Such a simple concept and we can already see our universe taking shape. Let’s see how gravity emerges.

      Consider a lump of matter floating around in space. It is composed of bazillions of particles (electrons, protons, neutrons). Each of these bubbles disrupt and spread apart the space crystal bonding, this means less internal ppressure in the space crystal in and around the lump of matter. Get two or more of these low pressure regions close enough together and the crystal pressure will force them together. this also explains time dilation in a gravitational field. Because of less internal pressure and rigidity things happen more slowly and we measure time by change. This time dilation causes gravitational lensing. As anyone who has played around with polarized sunglasses knows, light waves have a physical dimension. As a light wave ripples through space suppose it passes a massive object to its right. Even though a light wave is relatively narrow, things will happen more slowly on the side closest to the massive object. This is analogous to braking the right track on a Cat to make it turn to the right.

      Let’s put supposition on top of supposition and consider the atom. The nucleus is rather large compared to the fine grained structure of space. This, along with the electrical charges of the nucleus, force the space crystal to rearrange its structure in the region around the nucleus. This new structure must be concentric to the nucleus and as you move away from the neucleus it must transition back to the normal structure of space. It must be that in this transition, every so often, a concentric layer of bonding occurs that tends to trap electrons. The reason that electrons accelerating around the nucleus do not radiate energy as per Maxwell is that in that region the space structure has been rearranged such that they are traveling in a straight line vis-a-vis the local structure.

      The “randomness” of the whereabouts of particles per QM is simply due to the particles having to rattle their way through the crystal structure of space. Quantized light is due to not normally seeing generators capable of generating continuous waves at that frequency. I’m sure that experiments in the microwave range, where you can generate waves both by vibrating electrons (cavity magnatron) and jumping electrons (atomic source), would show that you can have both continuous and quantized waves at the same frequency. While not proof this would lende creedence to this theory.

      The problem I see is with some of the predictions of Special Relativity. I have long suspected that some of these effects are illusion. A Mohammad Shafiq Kahn posted a comment under “Could Simple Experiments Reveal the Quantum Nature of Spacetime” by George Musser. He posted a link to a peer reviewed article purporting to refute Special Relativity. I don’t understand why Mr. Musser did not comment on this, but some of you qualified physicists should check it out and report back.

    • Dan Deprez

      There are a significant number of blue shifted galaxies, including outliers. Couldn’t every visible part of the universe be in a huge gradational well? An item further in the well would appear to be accelerating away, as would items not as far into the well. The effect of being accelerated into the well could only be noticed by observing that almost every visible item would be accelerating away relative to the observer. Only items at the same potential would appear to be red-shifted.

    • Anonymous

      I am a layman. Pbarnes – I think the model you propose is sublime and elegant. It just really seems to make sense to me. The only problem I have is when you state the following in regard to E=mc2: “that C (the speed that vibrations travel through space) is going to be proportional to the internal pressure of our space crystal, as is the released energy.” “C” – the speed of light – by definitition is a constant and not a relativistic value. So how can it be proportional to anything that does not also remain constant? Are you implying that the internal pressure of the space crystal remains constant? If so then the non-constant “E” cannot be proportional to the internal pressure of the space crystal. E-mc2 means that energy is proporitional to the value of the speed of light only as attentuated by the amount of mass (non-space crystal mass) and vice~versa. Could you explain?

      Cody Reisdorf – I share your concern that many hypotheses that may be grandly debunked are entering the public sphere and that this could weaken an already fragile confidence the public has in science. As a layman, I know how much I thirst for every shread of new ideas and proof of theories, so I am thankful for all of the sites like Nova and shows that are produced regarding these ideas. But I see your point.

      On another note. I don’t know if my next question is truly related to the concept of dark energy or more specifically to how we observe it, but here it goes. I have always heard that if you could travel thtough space indefinitey in what you perceive is a straight line that you will actually return to your starting point because space-time is curved in three dimensions akin to the two-dimensional curve of the surface of a sphere. Now, this idea obviously assumes a smooth and uniform three-dimensional space-time surface. But, if there is some truth to it, when we say that we are observing galaxies near the end of the known universe – or put another away – that we are able to look back in time 13+ billion years to near the big bang – isn’t it possible that we are peering into our own backside and perhaps just seeing the Milky Way as it appeared 13 billion years ago and not a distant galaxy? If so, could the galaxies we observe as accelerating away from one another simply be our observation of how nearby galaxies and ours were moving in the earliest times? Just an idea. Would love to hear thoughts. Thanks.

      • Pbarnes

        Thanks for reading and commenting on my post. What I did not explain is that time will also be slower when the internal pressure is less. The speed of anything has a time component (distance/time).
        If you are in that region and measured the speed of light it would be “C”, but if you could somehow compare that “C” with “C” from another region with different internal pressure it would be different. Time ,as we experience it, is measured by change and the more tightly the space particles are bound the quicker they translate change to their linked partners.

        I, like you, am an amature, but when I came up with this idea it seemed so like what we know about Ma Nature and offers physical explanations of things like right-handedness and the atom that it seems to me it almost has to be correct, or at least near to being right.

    • This was my search. “In galaxies with so many worm holes why isn’t the universe ever going to collapse due to perfect balance?” Before that I looked at this

      I always wanted to believe in a power that was good. These are the reasons why. And it’s just an element of my psyche. Go live and learn. Become the scientist you are.

      • Ok this is the one most mistaken assumption the entire “If we accounted for how the universe is clumpy instead of smooth, it might turn out that the gravitational pull of clusters of galaxies and other large agglomerations of matter alter spacetime more than
        previously appreciated.” This is the account. If we forgot about the shape and just thought it didn’t matter, pardon the pun, The whole argument makes my google search more probable.

        “There is something wrong with gravity?” I love this quote.

    • Ed Yablecki

      Armchair physicist here, thinking about dark energy. Have they accounted on light being the dark energy? Here we have pure energy, and plenty of it. Massless, so as not to have gravitational effect. Where does all this energy go if not absorbed and converted to another energy. Imagine all this energy building up on the outskirts of space, what a variance. Is this variance significant enough to overtake gravity, stretching space out at a higher rate then supposed.

    • Chris U

      I believe the “observance” of the universe rushing apart faster and faster is incorrect. Redshift and the stars they use to measure light are most likely where their error is. Dark energy and matter, are just like believing “there is a big guy in the sky who controls everything and can read your mind”.