August 21, 2019, by Dr. Meghan Gray

Looking for dark energy – in the lab

This guest post from Dr. Ben Elder, a postdoctoral fellow in the Particle Theory and Cosmology Group, reporting on the very exciting publication of a research project. The aim? Trying to to understand nothing less than one of the biggest mysteries in cosmology by looking for experimental evidence in the lab!

When we look at the universe at the largest scales, we find that space is expanding. In fact, not only is it expanding, the rate of expansion is accelerating. This shocking discovery earned a Nobel Prize in physics just 20 years ago.

Einstein’s equations relate the contents of the universe to its expansion. Everything we know about (mostly, matter and photons) would cause the universe’s expansion to slow, and eventually to contract. So something else must be responsible for the universe’s acceleration. The nature of this phenomenon is a subject of much debate. Is it a fundamental property of spacetime? Is it something we don’t understand about gravity? Or is it an entirely new type of particle?

This last hypothesis is termed dark energy. If they exist, dark energy particles would generically couple to ordinary matter — the stuff we are made from — and would therefore mediate a gravity-like “fifth force.” But if this force exists, why haven’t we seen it already?

One possibility is that it hides. The prototypical example of this is the hypothetical chameleon particle, which has a very peculiar set of interactions that allows its mass to vary depending on its environment. In the vacuum of space, the mass of chameleon particles is very small, and therefore the fifth force they mediate is long-ranged. But in dense environments, like on Earth, chameleons interact with matter in such a way that they become very heavy. This makes them sluggish, and the fifth force becomes too short-ranged to detect.

In 2014, here in Nottingham, particle theorists Profs. Clare Burrage and Ed Copeland put their heads together with cold atom physicist Prof. Ed Hinds, who was visiting from Imperial College London. Together, they thought up a new way to search for chameleons. The idea is to place a small, marble-sized metal sphere inside a vacuum chamber, and then to drop an atom nearby. The acceleration of the atom is closely monitored as it passes by the marble, allowing one to be sensitive to any potential fifth forces between the atom and marble. Since this test is done inside a vacuum chamber with very small objects, the chameleon force remain long-ranged, and the fifth force could be detectable.

Principle of the experiment. Vacuum chamber walls are at ± Z. Solid red curve: scalar field ϕ is small at ±Z, rising to ϕ_bg at the center of the empty chamber. Dashed(dotted) blue curve: ball in position 1(2) perturbs ϕ to produce a gradient ∇ϕ. Atoms at the center of the chamber have acceleration a_ϕ∝∇ϕ toward the ball, which we measure by atom interferometry.

Figure and caption taken from Sabulsky et al., 2019, DOI:

This experiment has now been done. In a paper that recently appeared in Physical Review Letters, a collaboration between theorists (including myself!) at Nottingham and experimentalists at Imperial report their findings. Although we were able to measure the ordinary (and incredibly tiny) gravitational force between the marble and atom, we’ve seen no evidence of fifth forces so far. In the paper, we used this null result to place powerful constraints on chameleon-like theories of dark energy. My own role in this was to develop pen-and-paper approximations, as well as numerical solutions, to solve the complicated equations of motion for different types of dark energy particles inside the vacuum chamber.

Just because we didn’t see chameleons yet doesn’t mean we should stop looking. There remains a particular region of parameter space where chameleons are perhaps most likely to exist, which remained tantalizingly just out of the current experiment’s reach. The experimentalists are currently working hard to boost their experimental sensitivity, while we theorists are refining our calculations in order to extract as much information as possible from future measurements.

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