Snaring a dark energy 'chameleon': Is dark energy hiding from us? Matter may be screening it from our view

August 20, 2015
UC Berkeley physicists used an atom interferometer to try to detect anomalous forces from a chameleon field, a hypothetical dark energy particle. If chameleons exist, they would have a very small effect on the interaction between cesium atoms and an aluminum sphere. The group's failure to detect this effect narrows the search for the particle. Credit: Holger Muller/UC Berkeley

If dark energy is hiding in our midst in the form of hypothetical particles called "chameleons," Holger Müller and his team at the University of California, Berkeley, plan to flush them out.

The results of an experiment reported in this week's issue of Science narrows the search for chameleons a thousand times compared to previous tests, and Müller, an assistant professor of physics, hopes that his next experiment will either expose chameleons or similar ultralight particles as the real dark energy, or prove they were a will-o'-the-wisp after all.

Dark energy was first discovered in 1998 when scientists observed that the universe was expanding at an ever increasing rate, apparently pushed apart by an unseen pressure permeating all of space and making up about 68 percent of the energy in the cosmos. Several UC Berkeley scientists were members of the two teams that made that Nobel Prize-winning discovery, and physicist Saul Perlmutter shared the prize.

Since then, theorists have proposed numerous theories to explain the still mysterious energy. It could be simply woven into the fabric of the universe, a cosmological constant that Albert Einstein proposed in the equations of general relativity and then disavowed. Or it could be quintessence, represented by any number of , including offspring of the Higgs boson.

In 2004, theorist and co-author Justin Khoury of the University of Pennsylvania proposed one possible reason why dark energy particles haven't been detected: they're hiding from us.

Specifically, Khoury proposed that dark energy particles, which he dubbed chameleons, vary in mass depending on the density of surrounding matter.

In the emptiness of space, chameleons would have a small mass and exert force over long distances, able to push space apart. In a laboratory, however, with matter all around, they would have a large mass and extremely small reach. In physics, a low mass implies a long-range force, while a high mass implies a short-range force.

The vacuum chamber of the atom interferometer contains a one-inch diameter aluminum sphere. If chameleons exist, cesium atoms would fall toward the sphere with a slightly greater acceleration than their gravitational attraction would predict. Credit: Holger Muller

This would be one way to explain why the energy that dominates the universe is hard to detect in a lab.

"The chameleon field is light in empty space but as soon as it enters an object it becomes very heavy and so couples only to the outermost layer of a big object, and not to the internal parts," said Müller, who is also a faculty scientist at Lawrence Berkeley National Laboratory. "It would pull only on the outermost nanometer."

Lifting the camouflage

When UC Berkeley post-doctoral fellow Paul Hamilton read an article by theorist Clare Burrage last August outlining a way to detect such a particle, he suspected that the atom interferometer he and Müller had built at UC Berkeley would be able to detect chameleons if they existed. Müller and his team have built some of the most sensitive detectors of forces anywhere, using them to search for slight gravitational anomalies that would indicate a problem with Einstein's General Theory of Relativity. While the most sensitive of these are physically too large to sense the short-range chameleon force, the team immediately realized that one of their less sensitive atom interferometers would be ideal.

Burrage suggested measuring the attraction caused by the chameleon field between an atom and a larger mass, instead of the attraction between two large masses, which would suppress the chameleon field to the point of being undetectable.

That's what Hamilton, Müller and his team did. They dropped cesium atoms above an inch-diameter aluminum sphere and used sensitive lasers to measure the forces on the atoms as they were in free fall for about 10 to 20 milliseconds. They detected no force other than Earth's gravity, which rules out chameleon-induced forces a million times weaker than gravity. This eliminates a large range of possible energies for the particle.

What about symmetrons?

Experiments at CERN in Geneva and the Fermi National Accelerator Laboratory in Illinois, as well as other tests using neutron interferometers, also are searching for evidence of chameleons, so far without luck. Müller and his team are currently improving their experiment to rule out all other possible particle energies or, in the best-case scenario, discover evidence that chameleons really do exist.

"Holger has ruled out chameleons that interact with normal matter more strongly than gravity, but he is now pushing his experiment into areas where chameleons interact on the same scale as gravity, where they are more likely to exist," Khoury said.

Their experiments may also help narrow the search for other hypothetical screened dark energy fields, such as symmetrons and forms of modified gravity, such as so-called f(R) gravity.

"In the worst case, we will learn more of what is not. Hopefully, that gives us a better idea of what it might be," Müller said. "One day, someone will be lucky and find it."

Explore further: Chameleon model tries to explain the origin of dark energy

More information: "Atom-interferometry constraints on dark energy," by P. Hamilton et al:

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3 / 5 (11) Aug 20, 2015
I find it amazing that something discovered in 1998 has eluded all tests to verify it. I would call that proposed instead of discovered. Since dark matter and energy are only around to balance the equations on what we see versus what we think is happening, maybe they haven't been verified because they never existed in the first place. Occam's Razor tells us that neither ever existed and we need to come up with better models.
Aug 20, 2015
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not rated yet Aug 20, 2015
2.1 / 5 (9) Aug 20, 2015
This increasingly starting to look like a parade of idiocy. Some main stream scientists completely lost their self respect and sense of reality fot doubtful transitional goodies.It seems that the official imposed by consensus theory is sacred cow for them and it is unthinkable to questioning it. Therefore thay are looking for imaginary physical phenomena to explain our reality wit such politicaly correct trheories.
1 / 5 (3) Aug 20, 2015
Re: "Since then, theorists have proposed numerous theories to explain the still mysterious energy. It could be simply woven into the fabric of the universe, a cosmological constant that Albert Einstein proposed in the equations of general relativity and then disavowed. Or it could be quintessence, represented by any number of hypothetical particles, including offspring of the Higgs boson."

It's interesting that of all of the imaginative could-be's, Halton Arp being right is NEVER even mentioned as a possibility in any of these articles. The idea of a collective mistake having been made by the scientific community on allegations that quasars are in fact closer than redshift suggests is basically so absurd that a hypothetical form of repulsive gravity is considered far more likely.
2.5 / 5 (14) Aug 20, 2015
Jeffhans1 claims
Occam's Razor tells us that neither ever existed and we need to come up with better models
Since O.Razor has NIL/0 metric or evidence base or has ANY sort of Provenance whatsoever, then PLEASE advise us casual observers who might be awoken observing the linguistic problems inherent within intelligence just how a static non conscious (dumb) cliche has import & is NOT a resort to unintelligent laziness for the rednecks who imagine they have any useful Physics ?

IOW: Is a cliche the very best you can do ?

FFS, Where did you study; Philosophy, Maths, Physics even ?


ie. Please start by learning (core) Physics & appreciating that all Maths can only describe it cannot explain but, it is the gestalt of Physics <-> Maths that offers foundational means to progress essential cognition to arrive at experimental methodology ie Empiricism, whilst appreciating the asymptotic nature of Science progress to essential Truths...

Learn Physics !
3.7 / 5 (3) Aug 20, 2015
Based on my limited understanding, Dark Matter was theorized to exist based on celestial observations/movements that couldn't be accounted for by visible matter. So as one poster suggests above, maybe the observations were wrong to begin with? Another possibility is that gravity doesn't work on a large scale the way we believe it should, thus eliminating the need to create magical forces and mass. Lastly, what if dark matter is not matter at all? How does science treat the space in between particles of observable matter? What is it "made" of? For empty space to exist, something has to be there right? Something has to hold open up a space in nothingness in order for us to have 3-dimensions. Perhaps Dark Matter is that framework?
Aug 20, 2015
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Da Schneib
2 / 5 (4) Aug 21, 2015
It's important to keep in mind that all of the theories of dark energy that propose additional particles or fifth forces are less likely than that it is vacuum pressure, which has been observed in the laboratory in Casimir experiments since 1947, and these experiments confirm a factor in General Relativity Theory's field equations for gravity called "cosmological constant," proposed by Einstein in 1917. Most likely, we have known about what dark energy is for nearly a hundred years, and actually detected it in the laboratory over fifty years ago.

Cosmological constant, embodied and demonstrated by the Casimir Effect, is accepted as the explanation for dark energy by the majority of the physics community at this time. It is a feature of the current "standard model" of cosmology, called the "Lambda-CDM theory," by the majority of astrophysicists and cosmologists.

Da Schneib
5 / 5 (1) Aug 21, 2015

However, the more of these alternative theories that can be eliminated by experiments such as this one, the more certain we can become that we have the right answer.

Physorg should be more careful to note when hypotheses that experiments like this one seek to support are minority positions within the scientific community, particularly when they're up against something with as strong support and wide acceptance as the highly successful and so-far uncontroverted Lambda-CDM theory. Should such an experiment actually succeed and appear to support one of these alternatives, *that* would be major news; but it does no service to non-scientists' understanding of the current state of science to fail to mention that there is a well-known, uncontroversial, widely accepted theory that already appears to sufficiently explain all substantiated observations when presenting material of this kind.
5 / 5 (1) Aug 21, 2015
LHC Dark Matter Experiments
Aug 21, 2015
This comment has been removed by a moderator.
1 / 5 (2) Aug 21, 2015
Here is another possible explanation for the missing mass:

- Most of the matter of giant black holes is contained the their skin.

- Our universe is inside a black hole

- The 'big bang' is a big bounce

What might be the cause of such a scenario? That gravity is a quantum effect and breaks down over distances much greater than the Planck length.
Aug 21, 2015
This comment has been removed by a moderator.
Aug 21, 2015
This comment has been removed by a moderator.
not rated yet Aug 21, 2015
"a low mass implies a long-range force, while a high mass implies a short-range force" ???

Dark matter exists all around us. Even a vacuum contains dark matter. This is not long range.
1.4 / 5 (5) Aug 21, 2015
"What is the deal with PHYS.ORG today? The first 3 articles on the website all have to do with Dark Matter. Please get back to real science."

It is simple, dark cult. Fictional black holes, dark matter and energy. It seems that some people love darkness more than lite whose existence present as fact again and again without any evidence, because such can get such evidence.
not rated yet Aug 24, 2015
the possibilities extend beyond imagination - failing is just a way not to succeed. I tend to subscribe to the idea that black holes have something to do with it. perhaps the fabric of space-time itself being wrapped on itself generates some unseen force. perhaps there are white holes from a parallel universe dumping particles that do not subscribe to the four fundamental forces. whatver the cause, I love the pure science of this and kinda wish I could peak into the future about 1000 years.

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