Thursday, December 29, 2022

3798 - DARK MATTER - mysteries yet to be discovered?

 

 -  3798  -  DARK  MATTER -  mysteries yet to be discovered?   Despite recent advances in astrophysics and astronomy, scientists still don't understand exactly how galaxies can exist. The most common explanation for this is a so-far undiscovered form of matter: “dark matter”. 


---------------------  3798  -  DARK  MATTER -  mysteries yet to be discovered?

-   Astronomers understand how stars form, burn, and die, and they are improving their understanding of how planets assemble themselves into planetary systems like our own, but they don’t understand how galaxies can exist.

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-  Galaxies are collections of stars held together by gravity. Like our solar system, they rotate, with stars marching in paths, orbiting the galactic center. At any fixed distance from the center of the galaxy, stars moving faster require stronger gravity to hold them in that orbit.

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-  When astronomers measure the orbital speed of stars in galaxies at a range of distances from the center, they find that the stars are moving lunt of hydrogen gas in galaxies and, while there’s a lot of it out there, there’s not enough to explain the galaxy rotation mystery.

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-  Other explanations that have been proposed include things like burned out stars, black holes, and other objects that are known to exist within galaxies but don’t emit light.  However, astronomers searched for such objects (called MACHOs, short for MAssive Compact Halo Objects) in the 1990s and, again, while they found examples of MACHOs, there weren’t enough to explain the motion of stars in galaxies.

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-  Perhaps dark matter exists as a kind of a “gas,” or as never-before-seen particles. These particles are generically called “WIMPs,” short for “Weakly Interacting Massive Particles.” WIMPs, if they exist, are basically stable subatomic particles, with a mass somewhere in the range of the mass of a proton up to 10,000 protons, or even more. 

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-  Like all dark matter particle candidates, WIMPs interact gravitationally, but that “W” in the name means that they also interact via the weak nuclear force. The weak nuclear force is involved in some forms of radioactivity. much stronger than gravity, but unlike gravity’s infinite range, the weak nuclear force only acts over tiny distances, distances much smaller than a proton.

 

-   If WIMPs exist, they pervade galaxies, including our Milky Way, and even our own solar system. Depending on the mass of the WIMPs, astronomers estimate that if you make a fist, one dark matter particle could be found inside it.

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-  Scientists have been looking for direct evidence for the existence of WIMPs for many decades. They do this in several ways. For example, some WIMP theories suggest that WIMPs can be made in particle accelerators, like the Large Hadron Collider in Europe. Particle physicists look at their data, hoping to see the signature of WIMP production. No evidence has been observed so far.

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-  Another way in which researchers look for WIMPs is directly observing dark matter particles that pass through the solar system. Scientists build very large detectors and cool them to very cold temperatures so the atoms of the detectors are moving slowly.  They then put these detectors a half-mile or more underground to shield them from radiation from space. Then they wait, hoping that a dark matter particle will interact in their detector, disturbing one of the nearly stationary atoms. 

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-  But despite decades of efforts, no WIMPs have been observed. Predictions in the 1980s suggested researchers could expect to detect WIMPs at a particular rate. When no WIMPs were detected, researchers built a series of detectors with much greater sensitivity, all of which failed to find WIMPs.

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-   Current detectors are 100 million times more sensitive than the ones of the 1980s, and no definitive observation of WIMPs have occurred, including a very recent measurement by the LZ experiment, which employs 10 tons of xenon to achieve unparalleled sensitivity to WIMPs.

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-  After decades of failing to detect dark matter, the scientific community is reexamining the situation.  Astronomers are certain that galaxies rotate faster than can be accounted for using the known laws of motion and gravity and the observed amount of matter. The dark matter hypothesis is a solution for a matter deficit, but perhaps it’s not the answer. Maybe the actual explanation is that the laws of motion and gravity need to be reexamined.

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-  The name for such an approach is called MOND, short for “MOdifications of Newtonian Dynamics.” The laws of motion worked out by Isaac Newton back in the 1600s work just fine. But for very small forces and very small accelerations (like in the outskirts of galaxies), these laws needed to be adjusted. After making those adjustments, he could correctly predict the rotation of galaxies. 

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-   In order to test the MOND theory, researchers needed to compare its predictions in other situations, such as applying it to the motion of large clusters of galaxies held together by their mutual gravitational attraction. MOND theory struggles to make a prediction of this motion that agrees with theory, and it also disagrees with other observations.

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-  If dark matter exists, it is five times more prevalent than ordinary atomic matter. If we need to revisit our laws of motion and gravity, this will have significant consequences for our modeling of the history of the universe.

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-  Scientists identified a data signature for dark matter that can potentially be detected by experiments. The effect they found is a daily "diurnal modulation" in the scattering of particles.  Dark matter, a type of matter that is predicted to make up around 27 percent of the known universe, has never been detected experimentally.

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-  Astronomers propose a new type of effect that relates to the so-called “sub-GeV dark matter” which is boosted by cosmic rays. Looking for this effect can potentially allow direct detection of dark matter using nuclear recoil techniques.

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-  Astronomers are looking for a prominent signature of accelerated dark matter particles that come from the galaxy’s center, where dark matter and cosmic rays are at high density. They found that these particles have a “diurnal modulation”, a scattering pattern that is linked to the time of day.

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-  At periods when the Galaxy Center faces the side of the planet that’s opposite the location of the detector, the Earth shadows a large amount of these particles. At other times, they come in as a signal with “higher recoil energy.”

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-  The conventional diurnal effect is only for slow moving (nonrelativistic) Dark Matter particles in our galaxy (so-called standard DM halo).  The effect is negligibly small either from direct experimental constraints, or due to the detection threshold. For light DM particles the DM-nucleus interaction is much less constrained, which leaves room for strong diurnal modulation.

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December 24, 2022     DARK  MATTER - yet to be discovered?    3798                                                                                                                               

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