Thursday, February 19, 2015

New experiment doesn’t see fifth force, rules out class of dark energy models

Sketch of new experiment.
Fig 1 from arXiv:1502.03888

Three months ago, I told you about a paper that suggested a new way to look for certain types of dark matter fields, called “chameleon fields”. Chameleon fields can explain the observed accelerated expansion of the universe without the necessity to introduce a cosmological constant. Their defining feature is a “screening mechanism” that suppresses the field in the vicinity of matter. The Chameleon fields become noticeable between galaxies, where the average energy density is very thin, but the field is tiny and unmeasurable nearby massive objects, such as the Earth.

Or so we thought, until it was pointed out by Burrage, Copeland and Hinds that the fields should be observable in vacuum chambers, when measured not with massive probes but with light ones, such as for example single atoms. The idea is that the Chameleon field inside a vacuum chamber would not be suppressed, or not very much suppressed, and then atoms in the chamber are subject to a modified gravitational field, that is the usual gravitational field, plus the extra force from the Chameleon.

You might not believe it, but half a year after they proposed the experiment, it’s been done already, by a group of researchers from Berkeley and the University of Pennsylvania
    Atom-interferometry constraints on dark energy
    Paul Hamilton, Matt Jaffe, Philipp Haslinger, Quinn Simmons, Holger Müller, Justin Khoury
    arXiv:1502.03888 [physics.atom-ph]
I am stunned to say the least! I’m used to experiments taking a decade or two from the idea to planning. If they come into existence at all. So how awesome is this?

Here is what they did in the experiment.

They used a vacuum chamber in which there is an atom interferometer for a cloud of about 10 million Cesium atoms. The vacuum chamber also contains a massive sphere. The sphere serves to suppress the field on one arm of the interferometer, so that a phase difference resulting from the Chameleon field should become measurable. The atoms are each brought into superpositions somewhere above the sphere, and split into two wave-packages. They are directed with laser pulses, that make one wave-package go up – away from the sphere – and down again, and the other wave-package go down – towards the sphere – and up again. Then the phase-shift between the different wave-packages is measured. This phase-shift contains information about the gravitational field on each path.

They also make a measurement in which the sphere is moved aside entirely, so as to figure out what is the offset from the gravitational field of the Earth alone, which allows them to extract the (potential) influence of the Chameleon field by itself.

Their data doesn’t contain any evidence for an usual fifth force, so they can exclude the presence of the Chameleon field to some precision which derives from their data. Thanks to using atoms instead of more massive probes, their experiment is the first of which the precision is high enough to rule out part of the parameter space in which a dark energy field could have been found. The models for Chameleon fields have a second parameter, and part of this space isn’t excluded yet. However, if the experiment can be improved by some orders of magnitude, it might be possible to rule it out completely. This would mean then that we could discard of these models entirely.

It is always hard to explain how one can get excited about null results, but see: ruling out certain models frees up mental space for other ideas. Of course the people who have worked on it won’t be happy, but such is science. (Though Justin Khoury, who is originator of the idea, co-authored the paper and so seems content contributing to its demise.) The Chaemeleon isn’t quite dead yet, but I’m happy to report it’s on the way to the nirvana of deceased dark energy models.


  1. Is there any basic motivation for such chameleon fields, or are they just an ad-hoc deus ex machina in order to avoid the cosmological constant?

  2. For all I know, no, there's no basic motivation for this type of field in particular, unless you want to count the usual 'scalar fields are everywhere and easy to make' argument that is also mentioned in the paper (first paragraph).

  3. "It is always hard to explain how one can get excited about null results, but see: ruling out certain models frees up mental space for other ideas."

    The Michelson-Morley experiment comes to mind. Possible "other ideas" were:

    A. The speed of light is variable (depends on the speed of the light source).

    B. Moving things contract in the direction of motion.

    They chose B but the correct idea was A.

  4. "I’m used to experiments taking a decade or two from the idea to planning. If they come into existence at all. So how awesome is this?"

    The speed of the experiment is probably because this is a small tweak on experiments they already had going on. Interferometry with cold cesium is what the Muller group does, and adding a sphere in the vacuum chamber is pretty straightforward.

  5. Yes, yes. I'm a theoretician, I'm easy to impress. Show me a vacuum pump and I think it's the coolest thing ever ;)

  6. "Show me a vacuum pump and I think it's the coolest thing ever ;)"

    On the other hand, it's sometimes the experimentalists who are too easy too impress. :-)

  7. I propose that Nobel Prizes should be awarded for null results experiments such as this and the LUX Dark Matter experiment.

  8. I remember being impressed by two simple things: That try as I could I could not remove all the air out of a bottle, yet with a little force one can open a bellows under water even if its opening is sealed.
    OK the teachers say nature abhors a vacuum and that little bubble atop the column of mercury when I asked they said there would always be a molecule or two.
    Even in general science why might a
    student not think that if you raised a filled hose above 40 feet that the vacuum there might somehow be made to pump water? Later I actually tried it as an experiment which of course I was pretty sure of a null result, but along the way I found a remarkable siphon that allowed me to pour ten thousand candles in ten seconds accurate to the drop.
    One way to pump water from very deep is to send shock waves thru the column. Anything less depends on how well we can prime the pump or apply ram jets - that is how we teach the young the beginnings of science.

  9. Bee..
    Titanium sublimation pump, 12,000 liter/s hydrogen, 2×10^(-8) torr, no moving parts.

    Rigorously derived but flawed postulate theory (e.g., quantum gravitation, SUSY, dark matter) workarounds obtain null results, here "ruling out chameleons that reproduce the acceleration of the cosmos".

    In kind interesting tests are DOI: 10.5281/zenodo.15107 10.5281/zenodo.15439 10.5281/zenodo.15441 10.5281/zenodo.15445 Look. If a postulate could be defended it would not be postulated. Falsify exact vacuum mirror symmetry toward hadrons, then heal physics.

  10. Glad to see one dark energy fall, but how many models are now left standing?

  11. Tanks for this, what a fantastic experiment!

    I am still putting my pennies on MOND :))

  12. Don't put your pennies on MOND, but do look at page 5 of where Milgrom refers to the strength of space. Then think back to the balloon analogy for the expanding universe. You have a balloon in a vacuum. The pressure inside is balanced by the tension in the skin, and there's two ways to make it bigger. One way is to increase the pressure, but that's like creating energy ex nihilo. The other way is to reduce the tension of the skin. Make it a bubble-gum balloon. The skin gets thinner and weaker, so it expands, so the skin gets thinner and weaker, and so on.

  13. @johnduffield

    Universe models require boojums outside derived theory to make visible things large scale work. Milgrom acceleration is a one value universal fit. It must be testably sourced.

    Massless boson photon exact spacetime symmetries are observably not exact toward massed fermions (quarks, hadrons see trace chiral anisotropic vacuum). Postulate it. Unending chiral corrections vanish, baryogenesis is natural. Noether's theorems exclude abrupt discontinuous symmetries (chirality). Milgrom acceleration is trace momentum conservation leakage. Nothing forbids it.

    Detecting vacuum trace chiral anisotropy toward matter in existing equipment is trivial - for chemistry. Four examples: DOI: 10.5281/zenodo.15445. Don't look at what cannot be there. Dark matter and chameleon fields clothe the Emperor.

  14. UncleAI,

    I agree with you to a point, in that the question as chemistry is trivial, but so trivial nature gives us the evidence for which we still cannot predict with certainty its experiment. The balloon expansion model is rather simplistic, neutrinos cannot spin the other way reaching the speed of light then going beyond it yet they seem to have mass.
    In the DNA, at T T in a sequence there is new evidence that cyclobutane, four carbons not only can form disrupted by uv but by electrons that are delayed after the exposure thus in the dark.
    Perhaps it is time we take another look at the foundations of what we mean when we try to resolve the ultraviolet catastrophe.

  15. Uncle Al: IMHO dark matter isn't mysterious if you remember that space has its vacuum energy, and this has a mass equivalence. Space is dark, and there's a lot of it about.

  16. arXiv:1502.03888, Fig 3. Absence of performance never ends. Falsification is different.

    Non-empirical postulates' theory cannot recover. Anomalies' unending curve fits are diagnostic. Elegant theory denies falsifying postulates. Progress is bloody: GR and QM, Yang and Lee, Bell. Quantum gravitation and SUSY.

    Equivalence Principle violation must accommodate Einstein's elevator. An Eötvös experiment opposing single crystal alpha-quartz enantiomers succeeds or fails. Success ends all physics' chiral curve fits. Do empirically better.

    Stop plucking belly button lint.

  17. « I am still putting my pennies on MOND »

    MOND is an empirical relationship having no logical-deductive link with the rest of physics, it's really strange to believe that this theory could have a great interest.

    It is the same for the "chameleon fields"; replacing an arbitrary constant by an additional field does not seem very promising.

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  19. It what way would such a field rule out a cosmological constant. You still need to describe it and isn't that what the cosmological constant is? Why not try to prove that space/time is stretching i.e. expanding in the lab just like Hubble first discovered at galactic scales and then try to figure out if it's a constant. If it's not a constant then that would be the dark energy wouldn't it?


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