Justin Khoury, physicist at the University of Pennsylvania, has proposed one possible reason why dark energy particles have yet to be detected: “they’re hiding from us.”
One of the great known unknowns of the universe is the nature of dark energy, the force field making the universe expand faster. Current theories range from end-of-the universe scenarios to dark energy as the manifestation of advanced alien life.
Cosmologists are now exploring the possibility that the vast majority of the energy in the universe is in the form of a hitherto undiscovered substance called “quintessence” that it causes the expansion of the universe to speed up. Most forms of energy, such as matter or radiation, cause the expansion to slow down due to the attractive force of gravity. For quintessence, however, the gravitational force is repulsive, and this causes the expansion of the universe to accelerate.
Dark energy is a sort of “repulsive gravity” that pushes matter and space-time away instead of pulling it closer. Rather than gathering around he regions of dense matter of stars or galaxies, dark energy hides out in the most isolated neighborhoods of the universe in the vast regions of empty interstellar space.
But what sort of matter or energy field would act in this reclusive manner? If an unknown particle was responsible for the acceleration of the universe’s expansion, it would be unlike anything even CERN’s cutting-edge particle physicists had ever seen before.
Physicists at Durham University, UK, simulated the cosmos using an alternative model for gravity—f(R)-gravity, a so called Chameleon Theory –a hypothetical particle proposed by Khoury that couples to matter more weakly than gravity–a dark energy candidate that drives drive the currently observed acceleration of the universe’s expansion.
Supercomputer simulations of galaxies have shown that Einstein’s theory of General Relativity might not be the only way to explain how gravity works or how galaxies form. The resulting images produced by the simulation show that galaxies like our Milky Way could still form in the universe even with different laws of gravity. The findings show the viability of Chameleon Theory—so called because it changes behavior according to the environment—as an alternative to General Relativity in explaining the formation of structures in the universe.
The research could also help further understanding of dark energy—the mysterious substance that is accelerating the expansion rate 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. It is the foundation of modern cosmology but also plays a role in everyday life, for example in calculating GPS positions in smartphones.
Scientists already know from theoretical calculations that Chameleon Theory can reproduce the success of General Relativity in the solar system. The Durham team has now shown that this theory allows realistic galaxies like our Milky Way to form and can be distinguished from General Relativity on very large cosmological scales.
“Chameleon Theory allows for the laws of gravity to be modified so we can test the effect of changes in gravity on galaxy formation,” said research co-lead author Christian Arnold, in Durham University’s Institute for Computational Cosmology.
Computer generated images showing a disk galaxy from a modified gravity simulation are available. Images show (right side of image, in red-blue color) the gas density within the disk of the galaxy with the stars shown as bright dots. The left side of the images show the force changes in the gas within the disk, where the dark central regions correspond to standard, General Relativity-like forces and the bright yellow regions correspond to enhanced (modified forces). Images show views of the simulated galaxy from above and the side. Credit: Christian Arnold/Baojiu Li/Durham University.
“Through our simulations we have shown for the first time that even if you change gravity, it would not prevent disc galaxies with spiral arms from forming.
“Our research definitely does not mean that General Relativity is wrong, but it does show that it does not have to be the only way to explain gravity’s role in the evolution of the universe.”
The researchers looked at the interaction between gravity in Chameleon Theory and supermassive black holes that sit at the center of galaxies. Black holes play a key role in galaxy formation because the heat and material they eject when swallowing surrounding matter can burn away the gas needed to form stars, effectively stopping star formation.
The amount of heat spewed out by black holes is altered by changing gravity, affecting how galaxies form. However, the new simulations showed that even accounting for the change in gravity caused by applying Chameleon Theory, galaxies were still be able to form.
General Relativity also has consequences for understanding the accelerating expansion of the universe. Scientists believe this expansion is being driven by dark energy and the Durham researchers say their findings could be a small step towards explaining the properties of this substance.
Research co-lead author Professor Baojiu Li, of Durham University’s Institute for Computational Cosmology, said: “In General Relativity, scientists account for the accelerated expansion of the universe by introducing a mysterious form of matter called dark energy—the simplest form of which may be 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 weakly than gravity–a dark energy candidate that drives the currently observed acceleration of the universe’s expansion. It was 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 the cosmological observations, and unlike many other theories it doesn’t contradict existing theories,” Holger Müller from the University of California, Berkeley summarized to IFLScience.
“The chameleon theory introduces a new ‘fifth’ force into our understanding of physics,” Clare Burrage, a physicist from the University of Nottingham, reports IFLScience. “This force’s strength varies depending on how much matter is in the vicinity. The force gets weaker as the amount of matter gets denser, so it wouldn’t be easily detectable on Earth. However, in the empty voids of space, the force extends to a massive and powerful range, pushing the matter in the universe apart – the opposite effect of gravity.
“However, alternatives to a cosmological constant which explain the accelerated expansion by modifying the law of gravity, like f(R) gravity, are also widely considered given how little is known about dark energy.”
Theorists have proposed numerous theories to explain the still mysterious energy. It could be simply woven into the fabric of the universe, a cosmological constant that Albert Einstein proposed in the equations of general relativity and then disavowed. Or it could be quintessence, represented by any number of hypothetical particles, including offspring of the Higgs boson.
In 2004, Khoury proposed that dark energy particles, which he dubbed chameleons, vary in mass depending on the density of surrounding matter.
In the emptiness of space, chameleons would have a small mass and exert force over long distances, able to push space apart. In a laboratory, however, with matter all around, they would have a large mass and extremely small reach. In physics, a low mass implies a long-range force, while a high mass implies a short-range force.
This would be one way to explain why the energy that dominates the universe is hard to detect in a lab.
“The chameleon field is light in empty space but as soon as it enters an object it becomes very heavy and so couples only to the outermost layer of a big object, and not to the internal parts,” said Müller, who is also a faculty scientist at Lawrence Berkeley National Laboratory. “It would pull only on the outermost nanometer.”
When UC Berkeley post-doctoral fellow Paul Hamilton read an article by theorist Clare Burrage outlining a way to detect such a particle, he suspected that the atom interferometer he and Müller had built at UC Berkeley would be able to detect chameleons if they existed.
Müller and his team have built some of the most sensitive detectors of forces anywhere, using them to search for slight gravitational anomalies that would indicate a problem with Einstein’s General Theory of Relativity. While the most sensitive of these are physically too large to sense the short-range chameleon force, the team immediately realized that one of their less sensitive atom interferometers would be ideal.
That’s what Hamilton, Müller and his team did. They dropped cesium atoms above an inch-diameter aluminum sphere and used sensitive lasers to measure the forces on the atoms as they were in free fall for about 10 to 20 milliseconds. They detected no force other than Earth’s gravity, which rules out chameleon-induced forces a million times weaker than gravity. This eliminates a large 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 suppress the chameleon field to the point of being undetectable.
Experiments at CERN in Geneva and the Fermi National Accelerator Laboratory in Illinois, as well as other tests using neutron interferometers, also are searching for evidence of chameleons, so far without luck. Müller and his team are currently improving their experiment to rule out all other possible particle energies or, in the best-case scenario, discover evidence that chameleons really do exist.
New particles associated with dark energy typically imply a fifth force beyond the known strong, weak, electromagnetic and gravitational forces in the universe. In order not to conflict with known bounds on such fifth forces, a hypothetical new force would have to be camouflaged or “screened” by the matter around it – hence the name chameleon field.
“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,” Khoury said.
Their experiments may also help narrow the search for other hypothetical screened dark energy fields, such as symmetrons and forms of modified gravity, such as so-called f(R) gravity.
“In the worst case, we will learn more of what dark energy is not. Hopefully, that gives us a better idea of what it might be,” Müller said. “One day, someone will be lucky and find it.”
The Durham researchers expect their findings can be tested through observations using the Square Kilometer Array (SKA) telescope, based in Australia and South Africa, which is due to begin observations in 2020. SKA will be the world’s largest radio telescope and aims to challenge Einstein’s theory of General Relativity, look at how the first stars and galaxies formed after the Big Bang, and help scientists to understand the nature or dark energy.
So, at the end of the day, one thing is certain: there’s something out there we don’t yet know. For years now scientists have been looking for “dark matter” or “dark energy” – with our current inventory of particles and forces in nature we just can’t explain major cosmological phenomena, such as why the universe is expanding at an ever faster rate.
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