- Exploring the possibility of detecting dark energy in a terrestrial experiment using atom interferometry
By Martin L. Perl and Holger Mueller
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.