- 3319 - DARK ENERGY - what is causing it? What is causing the accelerating expansion of the universe? It requires energy to accelerate anything. Let’s call it “dark energy”. It has never been directly observed or measured. Instead, scientists can only make inferences about it from its effects on the space and matter that we can see.
--------------------- 3319 - DARK ENERGY - what is causing it?
- Here on Earth researchers claim that hints of dark energy were detected at the “Gran Sasso National Laboratory” in Italy during an experiment designed to detect dark matter.
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- Looking at data from the “XENON1T“, an experiment designed to detect rare interactions between hypothetical dark matter particles and components of the noble gas xenon held in a special detector.
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- Statistically, there is a 5 percent chance the detection was an anomaly. The detection of the 2012 discovery Higgs Boson, by comparison, was much more certain, there was only a chance in about 3.5 million that detection was anomalous.
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- Dark energy repels instead of attracts, meaning it’s the energy that is expanding the universe. Physicists have known the universe is expanding for years, but in the late 1990s, observations made it clear that the universe was not just growing larger but doing so at an “accelerating rate“. A constant velocity needs no energy expended, but to accelerate something an additional energy is required. That is physics as we know it.
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- Why it's a strange result is that since normal gravity is attractive, we would expect all the galaxies to be pulling on each other and slowing down the expansion of the universe. Its accelerating expansion was a giant surprise. And one we can not explain.
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- All the known stuff should be pulling in on the universe not pushing outward. So astrophysicists dreamed up new stuff to explain the strange behavior: “dark energy“. Filling all of space, dark energy’s negative pressure is inflating the cosmos like a balloon.
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- It’s believed to be about 68 percent of the mass of the universe, though the ratio grows with the expansion of the universe. Dark energy seems to interact very little with gravity. Remember mass and energy are two different forms of the same thing, E = mc^2.
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- Around 27 percent of the mass is dark matter, which is unrelated to dark energy. Dark matter may be objects we can’t easily detect or matter made out of exotic particles. Normal matter consisting of the remaining 5 percent. Both dark matter and normal matter interact with gravity.
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- Finding out the nature of dark energy is essential for finding the fate of our universe, but here are a few theories on what might happen. In June, 2020, a XENON team reported their experiment had recorded an excessive number of particle interactions with electrons in the detector compared with their predictions.
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- The XENON1T experiment may have incidentally detected dark energy “chameleon” particles, a hypothetical form of dark energy that could be created in the Sun and, under the conditions of the XENON1T experiment, interact with normal matter in much the same way as solar axions, but without contradicting observations of other stars.
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- “Chameleon theories” are just one of many frameworks for understanding dark energy. Scientists are exploring the prospects for direct detection of dark energy by current and upcoming terrestrial dark matter direct detection experiments.
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- If dark energy is driven by a new light degree of freedom coupled to matter and photons then dark energy quanta are predicted to be produced in the Sun. These quanta free-stream toward Earth where they can interact with Standard Model particles in the detection chambers of direct detection experiments, presenting the possibility that these experiments could be used to test dark energy.
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- Screening mechanisms, which suppress fifth forces associated with new light particles, and are a necessary feature of many dark energy models, prevent production processes from occurring in the core of the Sun, and similarly, in the cores of red giant, horizontal branch, and white dwarf stars.
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- Instead, the coupling of dark energy to photons leads to production in the strong magnetic field of the “solar tachocline” via a mechanism analogous to the “Primakoff process“.
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- Solar tachocline is the transition region of stars of more than 0.3 solar masses, between the radiative interior and the differentially rotating outer convective zone.
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- This difference causes the region to have a very large shear as the rotation rate changes very rapidly. The convective exterior rotates as a normal fluid with differential rotation with the poles rotating slowly and the equator rotating quickly.
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- The radiative interior exhibits solid-body rotation, possibly due to a fossil field. The rotation rate through the interior is roughly equal to the rotation rate at mid-latitudes, in-between the rate at the slow poles and the fast equator.
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- Recent results from helioseismology indicate that the tachocline is located at a radius of at most 0.70 times the solar radius (measured from the core, with the surface at 1 solar radius), with a thickness of 0.04 times the solar radius. This would mean the area has a very large shear profile that is one way that large scale magnetic fields can be formed.
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- The geometry and width of the tachocline are thought to play an important role in models of the stellar dynamos by winding up the weaker poloidal field to create a much stronger toroidal field.
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- Recent radio observations of cooler stars and brown dwarfs, which do not have a radiative core and only have a convective zone, demonstrate that they maintain large-scale, solar-strength magnetic fields and display solar-like activity despite the absence of tachoclines. This suggests that the convective zone alone may be responsible for the function of the solar dynamo.
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- The Primakoff effect, named after Henry Primakoff, is the resonant production of neutral pseudoscalar mesons by high-energy photons interacting with an atomic nucleus. It can be viewed as the reverse process of the decay of the meson into two photons and has been used for the measurement of the decay width of neutral mesons.
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- It could also take place in stars and be a production mechanism of certain hypothetical particles, such as the axion. The Primakoff effect is the conversion of axions into photons in the presence of very strong electromagnetic field.
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- The XENON dark matter experiment is installed underground at the Laboratory Nazionali del Gran Sasso of INFN, Italy. A 62 kilogram liquid xenon target is operated as a dual phase (liquid/gas) time projection chamber to search for interactions of dark matter particles.
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- In the XENON experiment any particle interaction in the liquid xenon yields two signals: a prompt flash of light, and a delayed charge signal. Together, these two signals give away the energy and position of the interaction as well as the type of the interacting particle.
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- An interaction in the target generates scintillation light which is recorded as a prompt signal by two arrays of photomultiplier tubes at the top and bottom of the chamber. In addition, each interaction liberates electrons, which are drifted by an electric field to the liquid-gas interface with a speed of about 2 mm/μs. (2 millimeters per microsecond)
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- There, a strong electric field extracts the electrons and generates proportional scintillation which is recorded by the same photomultiplier arrays as a delayed signal.
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- The time difference between these two signals gives the depth of the interaction in the time-projection chamber with a resolution of a few millimeters. The hit pattern of the signal on the top array allows to reconstruct the horizontal position of the interaction vertex also with a resolution of a few mm.
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- This experiment is able to precisely localize events in all three coordinates. This enables the fiducialization of the target, yielding a dramatic reduction of external radioactive backgrounds due to the self-shielding capability of liquid xenon.
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- The ratio of the two signals allows to discriminate electronic recoils, which are the dominant background, from nuclear recoils, which are expected from Dark Matter interactions.
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- The more energy a particle deposits in the detector, the brighter both signals are, hence allowing us to reconstruct the particle’s deposited energy as well.
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- This allows for detectable signals on Earth while evading the strong constraints that would typically result from stellar probes of new light particles.
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- By eexamining whether the electron recoil excess recently reported by the XENON1T collaboration can be explained by chameleon-screened dark energy, and finding that such a model is preferred over the background-only hypothesis at the 2.0 sigma level, in a large range of parameter space not excluded by stellar probes.
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- This raises the tantalizing possibility that XENON1T may have achieved the first direct detection of dark energy. Possibility, but it’s still dark.
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- October 29, 2021 DARK ENERGY - what is causing it? 3319
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