How to Build a Dark Energy Detector
The notion of dark energy is peculiar, even by cosmological standards.
Cosmologists have foisted the idea upon us to explain the apparent accelerating expansion of the Universe. They say that this acceleration is caused by energy that fills space at a density of 10^-10 joules per cubic metre.
What’s strange about this idea is that as space expands, so too does the amount of energy. If you’ve spotted the flaw in this argument, you’re not alone. Forgetting the law of conservation of energy is no small oversight.
What we need is another way of studying dark energy, ideally in a lab on Earth. Today, Martin Perl at Stanford University and Holger Mueller down the road at the University of California, Berkeley, suggest just such an experiment.
The dark energy density might sound small but Perl and Mueller point out that physicists routinely measure fields with much smaller energy densities. For example an electric field of 1 Volt per metre has an energy density of 10^-12 joules per cubic metre. That’s easy to measure on Earth.
Of course there are some important differences between an electric field and the dark energy field that make measurements tricky. Not least of these is that you can’t turn off dark energy. Another is that there is no known reference against which to measure it.
That leaves the possibility of a gradient in the dark energy field. If there is such a gradient, then it ought to be possible to measure its effect and the best way to do this is with atom interferometry, say Perl and Mueller.
Atom interferometry measures the phase change caused by the difference in two trajectories of an atom in space. So if a gradient in this field exists it should be possible to spot it by cancelling out the effects of all other forces. Perl and Mueller suggest screening out electromagnetic forces with conventional shields and using two atom interferometers to cancel out the the effect of gravitational forces.
That should allow measurements with unprecedented accuracy. Experiments with single atom interferometers have already measured the Earth’s gravitational pull to an accuracy of 10^-9. The double interferometer technique should increase this to at least 10^-17.
That’s a very exciting experiment which looks to be within reach with today’s technology.
There are two potential flies in Perl and Mueller’s ointment. The first is that the nature of dark energy is entirely unknown. If it exists and if there is a gradient, it is by no means certain that dark energy will exert a force on atoms at all. That will leave them the endless task of trying to place tighter and tighter limits on the size of a non-existent force.
The second is that some other unknown force will rear its head in this regime and swamp the measurements. If that happens, it’s hard to imagine Perl and Mueller being too upset. That’s the kind of discovery that ought to put a smile on any physicists face.
Ref:arxiv.org/abs/1001.4061: Exploring The Possibility Of Detecting Dark Energy In A Terrestrial Experiment Using Atom Interferometry
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