Monday, July 26, 2010

Detection of Dark Energy on Earth? - Improbable

I recently came across this paper
    Exploring the possibility of detecting dark energy in a terrestrial experiment using atom interferometry
    By Martin L. Perl and Holger Mueller
    arXiv:1001.4061v1 [physics.ins-det]
The topic sounded interesting, so I read it, and here's my conclusions.


Dark energy is according to today's most widely accepted model for the evolution of the universe the main constituent filling space-time around us. It has the peculiar property that it accelerates the expansion of the universe, a feature that has been confirmed by an increasing number of experiments. In the simplest case, the dark energy component is just constant in time and space and is identical to the Cosmological Constant, commonly denoted Λ. There are however many alternative proposals for dark energy that are not just constant, but so far experimental tests for their specific behavior are lacking.

Due to its peculiar properties, as well as the specific value of its measured density, the existence of dark energy is one of the biggest puzzles in theoretical physics today. While it is easily possible to incorporate it into our models as a source for gravity, its microscopic origin is not understood. For more details, see my post on the Cosmological Constant.

The other puzzle that modern cosmology has given to us is that of dark matter. Dark matter is another constituent of the universe whose existence has been confirmed by many experiments, but it's microscopic origin is still unclear. In contrast to dark energy however, dark matter behaves pretty much like normal matter, the stuff that we are made of. Except that, well, it's dark, meaning it does not or hardly emit any detectable radiation. In other words, we can't see it like we see galaxies on our night sky. The most widely accepted explanation for our observations is that dark matter is made of particles that just happen to couple very weakly to those that we are made of. Many candidates for dark matter particles are presently discussed and experiments for direct detection are under way in different places, though results are inconclusive so far. For more details, see my post on Dark Matter.

Detecting Dark Energy with Atom Interferometry

The idea of Perl and Mueller's proposal is the following. Beams of atoms are brought to interfere after having traveled two different paths. These paths are of the same length, but differ in their location within the Earth's gravitational field such that one beam feels more, the other less, of the gravitational force. With the method of interferometry one can detect even tiny shifts in the phase of the wavelengths of the beam. The phase of the beam does depend on the force exerted on it, which means that the interferometry allows to measure the difference in the gravitational force that the atoms have been subject to on the two paths. These experiments have previously been successfully performed. Note that the experiment is sensitive not only to the gravitational force, but in fact to any force that would act on the particles. For a nice explanation of such experiments, see eg. arXiv:0905.1929v2 [gr-qc].

Perl and Mueller now suggest to use two interferometers, both with the same setup as above, and measure the difference in the phase shift between both. With this one would be able to measure even tiny changes in the gravitational force between the locations of the both interferometers, or any other unknown force acting on the atoms. They have estimated the precision that can be reached with this experiment to be about 10-17 of the gravitational force on the surface of the Earth. That I think is quite remarkable indeed.

What does this have to do with dark energy? In their paper, they suggest that this experiment would be able to detect fluctuations in the dark energy density.

Can this work?

Let us note that in case the dark energy was just a Cosmological Constant, it wouldn't have such fluctuations and thus not be detectable by this method. Nevertheless, if it was possible what Perl and Mueller claim, this would give us a grip on the microscopic origin of dark energy and be quite exciting indeed.

I was puzzled however by two things.

First, it is entirely unclear why the detection method proposed would be sensitive to dark energy in particular. It simply measures tiny fluctuations in the forces on the particles. On the timescales that the authors are interested in, one can pretty much exclude fluctuations due to motion of stellar objects or shifts in the Earth's matter. But it seems more plausible to me that such fluctuations, should they exist, would be caused by drifting clumps of dark matter rather than dark energy. In any case, one wouldn't actually know what the origin was.

Second, and more important, I was very suspicious that atom interferometry would suffice to measure something as dilute as the density of dark energy. Putting in some numbers, I estimated the gravitational force that a cubic meter of dark energy stuff would have at its surface. I was guessing that a cubic meter would be the typical size of the experiment and thus the scale that the density fluctuations should occur on. It turns out that this gravitational force of the dark energy clump would be be 38 orders of magnitude smaller than the gravitational force of the Earth. That's more than 20 orders of magnitude below the precision of the proposed experiment. If one takes clumps of dark matter instead one gains 5 orders of magnitude, but still way off. If one considers larger clumps or overdensities (possibly moving very quickly through the experiment), one can gain some more orders of magnitude, but it becomes increasingly implausible at this point.

So I wrote an email to the authors...

... asking for a clarification. I got a fast and very useful reply from Holger Mueller. He explains that Martin Perl is the creator of the idea, and Mueller is helping out on the experimental site. Mueller agreed on my reservations about the experiment's suitability to detect dark energy:
“You are right that the only way we can detect dark energy (or dark matter) is if it has a nongravitational interaction with ordinary matter that is much stronger than the gravitational one (and the dark energy or dark matter must be inhomogeneous).”
This is because, as previously mentioned, their proposed experiment does not distinguish between the sort of force acting on the atom beams. I do however not know of any sort of dark energy that would have this property needed for detection.

Mueller explained his point of view as follows:
“To me as an experimentalist, it is not my primary concern whether there is a theory suggesting that there should be a signal, but whether our experiment will probe some region of the parameter space wherein signals have not been ruled out by previous experiments.”
And I agree that it is a worthwhile experiment to be done. After all one never knows what one might find! However, and unfortunately as I want to add, it seems extremely implausible to me that this experiment would detect dark energy.


  1. the experiment is sensitive not only to the gravitational force, but in fact to any force that would act on the particles A property dipole must be a small fraction of total mass, a differential epsilon, for no prior anomaly has been observed.

    If said epsilon is shared by both sets of test masses, only the difference of the difference is active, (epsilon)^2. If theory or prior observation constrains violation, (epsilon)^3.

    The experiment seeks a chained differential in a background that is homogeneous by its non-interative nature. Brownian motion is lumpy. Thermal photon gas slightly self-correlates. Dark energy will be less lumpy than the CMB.

    Three small "identical" solid spherical balls are coated with superconductor. Two are single crystal alpha-quartz, enantiomorphic space groups P3(1)21 and P3(2)21. The third is amorphous fused silica. Meissner levitate in hard vacuum then observe for 24 hours.

    Absent interactive background, the three balls are diurnally indistinguishable. Given any interactive background, fused silica will be inert. The left- versus right-handed quartz balls will rotate in opposite directions, stop, rotate in opposite opposite directions, stop, over 24 hours. Given the Earths's rotation, solar orbit, and the sun's galactic orbit there is great opportunity to detect a "vacuum" wind with chiral propellers.

  2. Dear Bee,

    thanks for the post and contacting the authors!

    Even if such an experiment might not reveal Dark Energy, atom interferometry and what can be measured with it is just awesome!

    Cheers, Stefan

  3. I second Stefan. I am really impressed by Dr. Mueller's experimental work. It is emblematic of the best of scientific discipline.

    Are they going to have long enough runs and sensitivity to pick up the diurnal variation in the gravitation redshift due to rotation withing the suns field?

  4. It sounds like the classic case of making reference to $current_hot_topic in your worthwhile science, in an attempt to raise its profile.

    Ten years ago, they might have been able to add profile by stating how this experiment related to astrobiology.

  5. Hi Aaron,

    I don't know. The papers have been a little sketchy on the precision when it comes to timescales, and I don't know enough about atom interferometry to be able to tell. Best,


  6. Hi Rob,

    You wrote:
    It sounds like the classic case of making reference to $current_hot_topic in your worthwhile science, in an attempt to raise its profile.

    Hip, hip, and Hooray! I'm all for raising Science's profile. Good for them.

    As far as "current issues" go, it sure is. "Dark Matter Phenomenology" seems to be THE career track for newly minted PhD.s in Physics to pursue in the US, especially if they want a tenure track position at one of America's top research institutions, according to a recent post by Peter Woit. A track, I might add, will only come to one in a hundred of them, again according to that post.

    25 years ago, that track would have been "String Theory."

    It's nice to see Marty Perl, NPP, still publishing at age 83. The rest of us should live so long, let alone publish at such an age.

    I'm glad to see this proposal is about Dark Energy, not Dark Matter for a change. I'm 99% sure Dark Energy will give up its secrets before Dark Matter does, as Dark Energy seems to be a geometrical consequence of our Universe, therefore less complicated and less complex than Dark Matter.

    Bee's conclusion is a bit sad, but hopeful. It probably won't be able to distinguish DE from other factors, but it looks like a fine start in an interesting direction. I'm curious how much such an experiment would cost to run?

  7. Hi Bee,

    A nice description of one of the ongoing attempts to sort out these dark energy/dark matter issues. However as you have demonstrated it won’t serve to be much help in either case. Also this type of experimentation is a long way from being as definitive of purpose as the type which Michelson and Mor;ey performed more then a century ago which was to support or deny the presence of the aether or rather the need to consider the existence of a rest frame. That is because they really don’t know or care what effect they should have noticed or not. I’m not saying this experiment is useless as it will as they point out explore things at a level never done before, yet if anything is detected it will only serve to create yet another mystery rather than having one solved. I wonder what they will call this one if anything is revealed, perhaps “dark phenomena”:-)

    On a similar note I noticed on Woit’s blog yesterday they announced new Tevetron results ruling out even higher energy levels as unlikely to have the Higgs found in. It seems the general reaction is not to start looking for alternatives yet to now go back to concentrate the search at lower energy levels thinking that’s where they missed it. One thing this tells me is in this respect science hasn’t changed much as some ideas just aren’t given up very easily. However it does have one wonder where doubt plays in here which is to doubt what we think we know or doubt what we have demonstrated or have failed to.



  8. Hey, thanks for looking at this.

    Interesting information on Atom Interferometer

    Also,neutron interferometer


  9. But we know about dark energy just because it was observed on Earth originally..

  10. From a June 10, 2009 New Scientist article:

    '"We don't know that gravity is strictly an attractive force," cautions Paul Wesson of the University of Waterloo in Ontario, Canada. He points to the "dark energy" that seems to be accelerating the expansion of the universe, and suggests it may indicate that gravity can work both ways. Some physicists speculate that dark energy could be a repulsive gravitational force that only acts over large scales. "There is precedent for such behaviour in a fundamental force," Wesson says. "The strong nuclear force is attractive at some distances and repulsive at others.'

    So if you drop an apple, you detect "dark energy" which causes it to accelerate downwards; the mechanism for this dual role of the gravitational field was predicted and published in 1996 including the a ~ Hv cosmological acceleration of the universe and thus the "effective" value of the small positive CC, two years before it was observed. (Feel free to delete this comment or make a politically-correct caustic remark about crackpotism.)

  11. Well, I agree with the first part of your comment, though I find it very misleading to talk about dark energy as a repulsive gravitational force. It's not the same thing. You want to talk about repulsive gravity, you'll first have to come up with a consistent theory that realizes this. I have no clue what the second part of your comment is supposed to mean. You trying to say you can detect the CC in watching a falling apple? Arguably, if the CC was locally present it would slightly influence the fall of your apple, but I can't see how this would even be remotely detectable. You run into the same problem as Perl and Mueller, it's way too small compared to the gravitational forces we deal with on Earth/on small distances. Best,


  12. Hi Bee,

    Pauli and Fierz in 1939 found that for two masses to attract by exchange of gravitons, the gravitons need to be spin-2:

    ‘In the particular case of spin 2, rest-mass zero, the equations agree in the force-free case with Einstein’s equations for gravitational waves in general relativity in first approximation ...’

    – Markus Fierz and Wolfgang Pauli, ‘On relativistic wave equations for particles of arbitrary spin in an electromagnetic field’, Proc. Roy. Soc. London., v. A173, pp. 211-232 (1939).

    What I predicted in 1996 is that two masses don't attract; they repel. Hence spin-1 gravitons. This predicts the cosmological acceleration accurately when you use the same model for gravitation.

    So why does an apple appear to be attracted to the Earth, instead of being repelled?

    Answer: the exchange of spin-1 gravitons between apple and Earth does cause a repulsion.

    However, that repulsion is trivial and is simply not significant compared to the effects of the exchange of gravitons with distant masses, with gravitons converging inward fro great masses (galaxy clusters etc) isotropically distributed all around us.

    The Pauli-Fierz theory of spin-2 gravitons necessitates hand-wavingly ignoring the exchange of gravitons with all the mass in the universe, and just pretending that the only exchange of gravitons is between the two masses you are concentrating on. It's the old reductionist fallacy; quantum gravity requires holistic thinking in that there is no mechanism suggested by anyone to stop gravitons being exchanged between all of the masses in the universe.

    Just like the Casimir force where virtual radiation pushes two conducting metal plates together in a vacuum, gravity can operate with spin-1 field quanta!

    Where masses are so distant from one another that they begin to approach each other's cosmological "horizon" distance, the gravitational mechanism of converging gravitons from greater distances no longer applies to the geometry of the situation, so repulsion predominates. Hence "dark energy" causing accelerating expansion!

    Kind regards,
    Nigel Cook

  13. For all I know, just from the attraction you get that for charges of equal sign the spin has to be integer. (Gravity is not universally attractive. It is attractive between positive masses. However, we haven't seen any negative masses and they are generically problematic.) You want spin 2 because it couples universally. Masses around us *do* affect the gravitational attraction between bodies on Earth, you can measure it, you can even see it. But point is that in most situations this influence is luckily negligible. (You don't have to know today's motion of Venus to get on the highway.) In any case, if you'd consider an isotropic distribution the effect would null out. (There's no gravitational field inside a sphere.) We have plenty of observational evidence that Einstein's spin-2 description of gravity is extremely accurate. Instead of making new predictions, maybe better first reproduce the old ones. Best,


  14. "Masses around us *do* affect the gravitational attraction between bodies on Earth, you can measure it, you can even see it. But point is that in most situations this influence is luckily negligible."

    Take F=ma. Stick in the observed Hubble mass of the accelerating universe and the observed cosmological acceleration of the universe, ~6 x 10^(-10) ms^(-2). This gives something like 10^43 Newtons radial outward force. Newton's 3rd law then suggests equal and opposite reaction force. This 10^43 Newtons inward is hardly negligible, and my paper in 1996 showed that it predicts gravitation (e.g. the value of G) correctly within the input errors in the data, with particles introducing asymmetry and being pushed together. This was discussed by Feynman in 1964: it not only predicts gravity but also explains the radial contraction effect of general relativity which causes the departures from Newtonian gravitation. Feynman raised the point that it then in 1964 predicted nothing observed. The cosmological acceleration was discovered in 1998. Feynman also pointed out that on-shell particles would slow down moving objects, so the original LeSage version of this gravity idea is wrong. However, there is no true link between rank-2 tensors (general relativity) and spin-2 gravitons and between rank-1 tensors (Maxwell) and spin-1 field quanta (photons), that proves spin-2 gravitons in the Pauli-Fierz paper as often misleadingly claimed. The rank is simply the order of the differential equations. General relativity describes gravitational fields in terms of curved spacetime, i.e. acceleration (rank-2 tensors), while Maxwell's equations describe the field in terms of diverging and curling spatial field lines (rank-1 tensors). If you wrote a rank-2 tensor for Coulomb's law (which isn't needed but is possible), you would then have a rank-2 tensor equation for a spin-1 field. Thus, there is no real rank-spin connection.


  15. It doesn't give any outward force, see above. It's one of the things that you can wreck with modifications of GR, has a name, but forgot, sorry. The FRW solution doesn't apply on sub-galactic scales anyway. Besides that, may I kindly ask you to either come back to topic of this post or drop your elaborations. Best,


  16. "It doesn't give any outward force, see above."

    I don't see anything above about this. The cosmological acceleration outward is on the order a~Hc. The simplest way to approach this to to look for outward force and the inward reaction force.

    "The FRW solution doesn't apply on sub-galactic scales anyway."

    The FRW solution is not referred to. Curvature applies on all scales and the contraction of Earth's radius (see Feynman's lectures) is (1/3)MG/c^2 = 1.5 mm.

    "Besides that, may I kindly ask you to either come back to topic of this post or drop your elaborations."

    I thought the post was about detection of dark energy. You can predict the acceleration of the universe and thus dark energy from the fall of an apple!

    However, thanks for being kind. I won't make any more comments on this post as I can see you're bored by this approach which predicted the acceleration accurately in 1996.


  17. I said above the gravitational force inside a spherical cavity is zero. If your theory doesn't have this feature, you have a grave problem with cosmological perturbations and structure formation.

    "The FRW solution is not referred to."

    Then what is H?

    This post is about the paper by Perl and Mueller, read first sentence. Best,


  18. "Then what is H?"

    Hubble's parameter, as in v = HR, which Hubble discovered observationally in 1929.

    It's observational and is not predicted by general relativity, in which metrics can be found to represent anything.


  19. Gravitational force inside a spherical cavity will be zero: in the middle there is no asymmetry, and thus no gravity (which is an asymmetry in graviton exchange). Away from the middle you just have Newton's "hollow shell" theorem, where a proximity of an observer to the shell at one side means that most of the mass is on the opposite side, with the inverse-square law compensating and eliminating gravitational force. For weak field (little contraction) this gives Newton's law with all the usual implications.



  20. Nige,

    My question for H was rhetoric. It's a parameter in the FRW metric, yet you say you make no reference to the FRW metric. It doesn't fit. How does one compute the gravitational force: you compute the gravitational field. There is, on subgalactic scales, no H in the solution. Galaxies don't expand. It's the space between them that does. Consequently, there's no H in your field and I don't know what you're even talking about. Correct, the gravitational field inside a sphere is zero. So why care what's outside the sphere? Last warning: your next comment is not about the Mueller & Perl paper, and it goes into the garbage bin. Best,


  21. Bee, you know your "Recent comments" section is messed up, right? It's frozen in 2008, for some reason.

    Is Nigel "nige" Cook the "LeSage Gravity" guy? If so, I'd rather read Randall Mills about "Hydrinos", and in fact I don't want to do that. Just read the Wiki entry on "LeSage Gravity", and pay attention to Poincare's view on same. That's good enough for me.

    Life is too short to read BAD Science, other than for entertainment purposes. Heck, it takes half a lifetime just to be caught up on the GOOD Science, and that was beFORE the Information Overload Age.

  22. Yes, I know the comment feed doesn't work. It's a blogger bug, can't do nothing about it, sorry. Best,


  23. Newton's 1/r^2 is accurate to 60 and to 10 microns centers of mass separation.
    50 microns macroscopic paper
    10 microns nanofab paper

    Big G is measured as an attraction by a torsion pendulum apparatus that nulls external gravitation,

    Large scale gravitation deviating from local gravitation (MOND) is unsatisfying. Perhaps classical gravitation (as with Bohr's QM) is operational but fundamentally wrong, thus failure of quantum gravitations that share with it a weak assumption.

    No Equivalence Principle violation exceeding one part in 20 trillion relative has appeared beginning with Galileo Galilei and Simon Stevin in the 1500s: lab bench; astronomic including Nordtvedt effect, pulsars, and black holes. Luboš waxes eloquent about EP violation being a disaster for theory.

    A choice candidate for EP violation exists outside physics' illusion of knowledge: Do opposite shoes vacuum free fall differently?
    The worst it can do is succeed.

    Europeans knew all swans were white. Australia was ever so naughty in counterpoint.

  24. Hi Bee,

    You might want to burst some bubbles for people? :)

    Observations have confirmed that 95 percent of the universe is made of dark energy and dark matter unlike any we have seen or touched in our most advanced experiments. Theorists have found a way to reconcile gravity with quantum physics, but at the price of postulating extra dimensions beyond the familiar four dimensions of space and time. As the magnitude of the current revolution becomes apparent, the science of particle physics has a clear path forward. Quantum Universe

  25. Thought you might be interested Bee.

    It took about six weeks for CHASE’s 10 scientists to collect the 30 hours of data they needed to search for a dark energy discovery. According to the preliminary results presented at ICHEP, the subsequent data analysis didn’t reveal any chameleons, which allowed the experiment to rule out a wide range of dark energy models that predict such particles.

    “This was the first experiment employing this particular technology that was sensitive to all chameleon dark energy models,” notes Steffen.

    The experiment, which took less than 18 months from design to the end of data taking, is already being dismantled. Although the CHASE team doesn’t plan a successor right now, the search continues at the ADMX experiment in Washington, which is analyzing chameleon search data gathered using a different method.

    “It would have been great to find something, but I can’t say that I was really sitting on the edge of my chair expecting to see chameleons,” adds Steffen. “It would take new developments in how we approach the problem for us to make serious plans to continue to search for them.”
    Lighting up the dark


  26. Theorists have found a way to reconcile gravity with quantum physics...

    They have ?! Hooboy, I guess Bee and a whole bunch of people can retire now. Lookout Naxos, here we come.

  27. Steven,

    I believe it's a reference to string theory. The price you have to pay is however somewhat more than just introducing extra dimensions. (Just adding extra dimensions does not only not help with gravity, it also ruins some nice features of quantum field theories.) Arguably, LQG is also a way to reconcile gravity with quantum theory, and it's considerably more straightforwardly so than string theory. In both cases there's numerous unsolved theoretical issues, and the question remains whether the theory has anything to do with reality. But you knew that.

    Comment feed works again. Best,


  28. 1) Neutrinos are left-handed (spin antiparallel to momentum). Antineutrinos are right-handed.

    (With rest mass not exceeding 0.2 eV, only Big Bang neutrinos red-shifted by cosmic inflation can be non-relativistic, possessing helicity but not chirality.)

    2) Massed neutrinos are Majorana. They are their own antiparticles, validated by observing neutrinoless double beta-decay.

    (1) and (2) cannot both be true.

    SUSY partners are ridiculous; the Higgs is a phantasm. High temp supercons resist theory. MgB2 mocks BCS (that explains but cannot predict). Lenny Suskind opened a Pandora's box without hope crushed at the bottom, for no quantum gravitation is predictive. Nothing is resolved.

    The Shroud of Turin projects a positive curvature face upon a zero curvature plane without distortion. Making a Shroud of Turin forgery is trivially easy. Religion or physics, do you believe your lying eyes or Official Truth?


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