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The Chameleon – "The dark energy is hiding from us"

Chameleon Theory of Gravity

Justin Khoury, a physicist at the University of Pennsylvania, has proposed one of the reasons why dark energy particles have not yet been detected: "they hide us".

One of the big known unknowns of the universe is the nature of black energy, the force field that accelerates the expansion of the universe. Current theories range from end-of-universe scenarios to dark energy as a manifestation of advanced extraterrestrial life.

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Cosmologists are now exploring the possibility that the vast majority of energy in the universe comes in the form of a yet unknown substance, called "quintessence," which accelerates the expansion of the universe. # 39; universe. Most forms of energy, such as matter or radiation, slow down the expansion because of the gravitational pull force. For quintessence, however, the gravitational force is repulsive, accelerating the expansion of the universe.

Black energy is a kind of "repulsive gravity" that distances matter and space-time instead of bringing it closer together. Rather than congregating around regions of dense matter of stars or galaxies, dark energy is hiding in the most isolated neighborhoods of the universe, in the vast regions of the world. empty interstellar space.

But what kind of matter or field of energy would act in this solitary way? If an unknown particle were responsible for accelerating the expansion of the universe, it would not look like anything even CERN's state-of-the-art particle physicists had ever seen before.

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Physicists from Durham University, UK, simulated the cosmos using an alternative model of gravitation – f (R) -, a so-called Chameleon theory – a hypothetical particle proposed by Khoury that couples with matter more weakly that gravity – a candidate for dark energy that Orders lead to the currently observed acceleration of the expansion of the universe.

Supercomputer simulations of galaxies have shown that Einstein's theory of general relativity may not be the only way to explain how gravity or galaxy formation works. The resulting images generated by the simulation show that galaxies such as our Milky Way could still form in the universe, even with different laws of gravity. The results show the viability of Chameleon's theory – so named because it modifies behavior according to the environment – as an alternative to general relativity to explain the formation of structures in the universe.

The research could also help better understand the dark energy, the mysterious substance that accelerates the rate of expansion of the universe.

General relativity was developed by Albert Einstein in the early 1900s to explain the gravitational effect of large objects in space, for example to explain the orbit of Mercury in the solar system. This is the foundation of modern cosmology, but it also plays a role in everyday life, for example in calculating GPS positions in smartphones.

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Scientists already know from theoretical calculations that Chameleon's theory can replicate the success of general relativity in the solar system. The Durham team has now shown that this theory allows the formation of realistic galaxies such as the Milky Way and could be distinguished from general relativity at very large cosmological scales.

"Chameleon Theory makes it possible to modify the laws of gravity in order to be able to test the effect of gravity changes on the formation of galaxies," said co-lead author of the research, Christian Arnold, at the same time. Institute of Computational Cosmology at the University of Durham.

Chameleon Theory of Gravity

Computer generated images showing a galaxy of discs from a modified gravimetric simulation are available. The images show (on the right of the image, in red-blue) the density of gas in the disk of the galaxy with the stars represented by luminous points. The left side of the images shows the force changes in the gas inside the disc, where the dark central regions correspond to the standard forces of the general relativity type and the bright yellow regions correspond to the reinforced (modified) forces. The images show views of the simulated galaxy from above and from the side. Credit: Christian Arnold / Baojiu Li University / Durham.

"With our simulations, we showed for the first time that even if you change the gravity, it would not prevent disk galaxies with spiral arms from forming.

"Our researches certainly do not mean that general relativity is wrong, but they show that it is not necessarily the only way to explain the role of gravity in the evolution of the world." universe."

The researchers examined the interaction between gravity in Chameleon Theory and supermassive black holes in the center of the galaxies. Black holes play a key role in the formation of galaxies because the heat and materials that they eject by swallowing the surrounding material can burn the gases needed for star formation, thus blocking their formation.

The amount of heat released by black holes is changed according to gravity, which affects the formation of galaxies. However, new simulations have shown that even taking into account the change in gravity caused by the application of Chameleon's theory, galaxies were still able to form.

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The general relativity also has consequences to understand the acceleration of the expansion of the universe. Scientists believe this energy is driven by dark energy and Durham researchers believe their discoveries could be a small step forward in explaining the properties of this substance.

Professor Baojiu Li, co-researcher at the Computational Cosmology Institute of Durham University, said, "In general relativity, scientists explain the accelerated expansion of the 39 universe by introducing a mysterious form of matter called black energy – the simplest form being a cosmological constant, whose density is a constant in space and time.

The Chameleon Field – A New Fifth Force

Enter the chameleon, a hypothetical particle that couples to matter more feebly than gravity – a candidate for dark energy that drives the currently observed acceleration of the expansion of the universe. It is these strange properties that gave physicists the idea of ​​the chameleon field.

"The chameleon particle is a particle that has all the required properties: it can explain cosmological observations and, unlike many other theories, it does not contradict existing theories," Holger Müller summed up. University of California at Berkeley.

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"The theory of chameleons introduces a new" fifth "force in our understanding of physics," says Clare Burrage, a physicist at the University of Nottingham, IFLScience. "The strength of this force varies according to the amount of matter nearby. The force weakens as the amount of matter becomes denser, so that it is not easily detectable on Earth. However, in the empty voids of space, the force extends over a massive and powerful range, thus moving matter away from the universe – the opposite effect of gravity.

"However, the alternatives to a cosmological constant that explain the accelerated expansion by modifying the law of gravity, such as gravity f (R), are also largely taken into account given the limited knowledge we have about the Black energy. "

Theorists have proposed many theories to explain this still mysterious energy. This could be simply woven into the fabric of the universe, a cosmological constant proposed by Albert Einstein in the equations of general relativity and then disavowed. Or it could be the quintessence, represented by any number of hypothetical particles, including the offspring of the Higgs boson.

In 2004, Khoury proposed that black energy particles, which he calls chameleons, vary in mass according to the density of the surrounding matter.

In the void of space, the chameleons would have a small mass and exert a force over long distances, able to separate the space. In a laboratory, however, with matter all around, they would have a large mass and an extremely reduced range. In physics, a low mass implies a long range force, while a high mass implies a short range force.

It would be a way to explain why it is difficult to detect the energy that dominates the universe in a laboratory.

"The chameleon field is light in an empty space, but as soon as it enters an object, it becomes very heavy and thus only couples to the outermost layer of a large object, and not to the internal parts, "said Müller, who is also a faculty. scientist at the Lawrence Berkeley National Laboratory. "He would shoot only at the outermost nanometer."

When Berkeley University Postdoctoral Fellow Paul Hamilton read an article by theorist Clare Burrage describing a way to detect such a particle, he suspected that the atomic interferometer he and Müller had built at the University of California. Berkeley would be able to detect chameleons if they existed.

Müller and his team built some of the most sensitive force detectors in the world, using them to look for slight gravitational anomalies indicating a problem with Einstein's general theory of relativity. Although the most sensitive of them were physically too large to detect the strength of a short-range chameleon, the team immediately realized that one of their less sensitive atomic interferometers would be ideal .

That's what Hamilton, Müller and his team did. They dropped cesium atoms onto an aluminum sphere one inch in diameter and used sensitive lasers to measure the forces exerted on free-falling atoms for about 10 to 20 milliseconds. They have detected no force other than Earth's gravity, which excludes the forces induced by chameleons a million times weaker than gravity. This eliminates a wide range of possible energies for the particle.

Burrage suggested measuring the attraction caused by the chameleon field between an atom and a larger mass, instead of the attraction between two large masses, which would remove the chameleon field to the point of becoming undetectable.

Experiments conducted at CERN in Geneva and at the Fermilab National Laboratory of Accelerators in Illinois, as well as other tests using neutron interferometers, are also looking for traces of chameleons, with no success for the future. 39; instant. Müller and his team are currently improving their experiments to eliminate all other possible particle energies or, at best, to find evidence of the true existence of chameleons.

The new particles associated with dark energy generally involve a fifth force beyond the known strong, weak, electromagnetic, and gravitational forces of the universe. In order not to conflict with the known limits of these fifth forces, a hypothetical new force should be camouflaged or "masked" by the surrounding matter – hence the chameleon field name.

"Holger has excluded chameleons that interact with normal matter more strongly than gravity, but he is now pushing his experience into areas where chameleons interact on the same scale as gravity, where they are more likely to exist" Khoury said.

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Their experiments can also help narrow the search for other preselected black energy fields, such as symmetrons and modified gravity forms, such as gravity f (R).

"In the worst case, we will learn more about what dark energy is not. Hopefully this gives us a better idea of ​​what this might be, "said Müller. "One day, someone will have luck and find it."

Durham researchers are waiting for their discoveries to be tested using observations made with the Square Kilometer Array (SKA) telescope based in Australia and South Africa. South, which should begin its observations in 2020. The SKA will be the largest radio telescope in the world and aims to challenge the theory of general relativity, examine how the first stars and galaxies formed after the Big Bang and help scientists understand nature or dark energy.

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So, at the end of the day, one thing is certain: there is something we do not know yet. For years, scientists have been searching for "dark matter" or "black energy" – with our current inventory of particles and forces of nature, we simply can not explain major cosmological phenomena, such as the reason for which the universe is growing faster and faster. .

The Daily Galaxy via the University of Durham, IFL Science and UC Berkeley

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