Scientists at the University of California, Berkeley have begun investigating the properties of dark energy by allowing it to manifest in the form of hypothetical particles known as chameleons.

In a study featured in the journal Science, UC Berkeley researcher Holger Müller and his colleagues searched for chameleon particles or other similar ultralight units in order to prove if these particles are the true manifestation of dark energy, or if they are merely impossible to find.

The concept behind dark energy was first established in 1998 when researchers observed the expansion of the universe at a rate that is ever increasing. They saw that the objects were being pushed apart by an invisible force permeating all of space and consisting of around 68 percent of cosmic energy in the universe.

One of the teams that made the Nobel Prize-winning find was comprised of scientists from UC Berkley, and the award was shared with physicist Saul Perlmutter.

Numerous theories regarding the properties of dark energy have been proposed since its discovery. Some theorists suggest that dark energy could be woven into the universe's fabric, a cosmological constant that the famous German theoretical physicist Albert Einstein introduced in his general relativity equations only to have it disavowed. It could also be quintessence, composed of hypothetical particles such as offspring of the Higgs boson.

In 2004, University of Pennsylvania theorist Justin Khoury suggested that the reason why particles of dark energy remain relatively unseen is because they are hiding.

Khoury said that these specific particles, which he named chameleons, differ in their mass, depending on the density of the matter that surrounds them.

It is generally believed that in the emptiness of space, chameleon particles would have a relatively small mass. These particles are capable of exerting force over vast distances and can even push apart space.

In a laboratory setting, however, where matter is present all around, chameleon particles would have a relatively larger mass with a significantly limited reach.

In the field of physics, an object that has low mass has a force capable of reaching long ranges, while one that has a high mass has a force that can only reach a short range.

This aspect is viewed as one possible explanation as to why the energy that virtually surrounds the universe is so difficult to detect in a laboratory.

Müller said that the field of chameleon is light when it is in an empty space, but it becomes relatively heavy as soon as it enters a particular object. It attaches only to the object's outermost layer, and not to its internal parts.

Müller added that chameleon would only pull on the outermost nanometer.

To determine the existence of chameleon particles, the UC Berkley researchers made use of an atom interferometer, which Müller and fellow scientist Paul Hamilton designed and built. They also followed suggestion made by theorist Clare Burrage on how to possibly detect such particles.

Burrage proposed that chameleon particles can be detected by measuring the attraction produced by its field between a large mass and an atom. Measuring the attraction produced between two large masses would have suppressed the chameleon field to the extent that it would be undetectable.

The researchers carried out Burrage's suggestion by dropping atoms of cesium on an aluminum sphere one inch in diameter. They then used sensitive lasers to identify the forces on the atoms during their 10- to 20-millisecond free fall.

Müller and his team did not detect any other force aside from the gravity of the Earth, which removes the possibility of forces induced by chameleon particles that are a million times weaker compared to gravity. This result also removes a wide range of potential energies for the chameleon particle.

The UC Berkley researchers are now trying to enhance their experiment in order to eliminate all other possible energies for the particle, or even discover proof that the chameleon particles exist in the best-case scenario.

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