The Search For Dark Energy Chameleons
The latest experiments in the search for the source of Dark Energy peered 1,000 times more closely than ever for particles called Chameleons. Their theoretical ability to hide themselves may explain why they’ve never been detected.
That day in 1998 may never be equalled in your lifetime in terms of the raw impact to our conception of the universe. It was then that scientists and engineers discovered that the expansion of the universe was not slowing down as expected but was actually and mind-blowingly expanding faster and faster. To explain this would require the existence of an unknown energy (Dark Energy) that acted like an anti-gravitational force that accounts for 68% of all the energy of the universe. If you add that to the 27% of the universe’s energy consumed by Dark Matter, then our old idea of the mass/energy of the entire universe shrank that day down to a paltry 5% of the universe.
This sort of paradigm shift has happened many times. When we realized earth was just one planet in a solar system filled with planets; when the number of galaxies (and thus the size of the universe) changed dramatically from one to many to more than 170 billion; when our reality expanded from one universe to an infinite multiverse (…whoops, we haven’t discovered that yet).
But what is Dark Energy? Unfortunately, the discovery of the nature of Dark Energy has been just as fruitless as the search for Dark Matter. There are many theories and hypotheses of course. Dark Energy could simply be woven into the fabric of the universe. On the other hand it could manifest itself from one of many different hypothetical particles or fields. One of those possible types of Dark Energy particles is called a Chameleon particle.
Chameleon particles were hypothesized in 2004 by theorist Justin Khoury, of the University of Pennsylvania. He proposed that their mass and the reach of their force varied depending on the density of surrounding matter. Chameleons near earth then would be somewhat more massive but they could only influence objects that were about one millimeter away. That same Chameleon however would be very different if it were in deep space where the matter density is quite low. It would then be much lighter and able to exert itself over a distance of not just one millimeter but 92 sextillion millimeters (several thousand parsecs or about 9,800 light years). This force would then have the mojo it needs to power cosmic acceleration.
This chameleonic ability makes these particles quite attractive to researchers. This is because most models paint a picture of Dark Energy that should be detectable whether you’re in an earth-bound lab or in deep space between galaxies. If the particles themselves changed their nature when they were near the earth or your lab equipment in such a way to make themselves almost undetectable, then that would explain why we haven’t spotted them yet.
The recent experiments looking for chameleons were led by Paul Hamilton and Holger Müller at at the University of 
California, Berkeley. Hamilton realized that his detectors could be very well suited for detecting chameleons. Experiments in the past used large machines. Their high mass though, effectively caused any potential chameleon Dark Energy particles to hide themselves even more effectively. Hamilton had machines at his disposal that are among the most sensitive force detectors on the planet and he leveraged them in a clever way. Instead of looking for a force between two large masses, he looked for it between atoms in a vacuum and a larger mass. He did this by dropping cesium atoms and measuring the force of attraction between them and an aluminum sphere using sensitive lasers.
Wanna know what he found?
Nothing……The force acting on the atoms was just what you’d expect from gravity alone. No extra force created by chameleons (or more precisely, a chameleon field) was detected.
Still, it’s not as bad as it sounds. Khoury has stated recently:
“Holger has ruled out chameleons that interact with normal matter more strongly than gravity, but he is now pushing his experiment into areas where chameleons interact on the same scale as gravity, where they are more likely to exist,”
This is one of the cool things about negative experimental results like this. It’s a winnowing process. The candidates for Dark Energy just got smaller because we ruled out a chameleon force stronger than gravity. The next more sensitive experiments may finally find those pesky chameleons. Or they may not . In that case we could then confidently cross off chameleons which would mean that other hypothetical particles just got more likely to be responsible for the Dark Energy …like symmetrons.
Whatever the hell they are
Image Credits:http://news.berkeley.edu/2015/08/20/experiment-attempts-to-snare-a-dark-energy-chameleon/
http://www.ibtimes.com/dark-energy-chameleon-fields-could-explain-what-universe-made-2064314